linux/Documentation/trace/ftrace.txt
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   1                ftrace - Function Tracer
   2                ========================
   3
   4Copyright 2008 Red Hat Inc.
   5   Author:   Steven Rostedt <srostedt@redhat.com>
   6  License:   The GNU Free Documentation License, Version 1.2
   7               (dual licensed under the GPL v2)
   8Reviewers:   Elias Oltmanns, Randy Dunlap, Andrew Morton,
   9             John Kacur, and David Teigland.
  10
  11Written for: 2.6.28-rc2
  12
  13Introduction
  14------------
  15
  16Ftrace is an internal tracer designed to help out developers and
  17designers of systems to find what is going on inside the kernel.
  18It can be used for debugging or analyzing latencies and
  19performance issues that take place outside of user-space.
  20
  21Although ftrace is the function tracer, it also includes an
  22infrastructure that allows for other types of tracing. Some of
  23the tracers that are currently in ftrace include a tracer to
  24trace context switches, the time it takes for a high priority
  25task to run after it was woken up, the time interrupts are
  26disabled, and more (ftrace allows for tracer plugins, which
  27means that the list of tracers can always grow).
  28
  29
  30The File System
  31---------------
  32
  33Ftrace uses the debugfs file system to hold the control files as
  34well as the files to display output.
  35
  36To mount the debugfs system:
  37
  38  # mkdir /debug
  39  # mount -t debugfs nodev /debug
  40
  41( Note: it is more common to mount at /sys/kernel/debug, but for
  42  simplicity this document will use /debug)
  43
  44That's it! (assuming that you have ftrace configured into your kernel)
  45
  46After mounting the debugfs, you can see a directory called
  47"tracing".  This directory contains the control and output files
  48of ftrace. Here is a list of some of the key files:
  49
  50
  51 Note: all time values are in microseconds.
  52
  53  current_tracer:
  54
  55        This is used to set or display the current tracer
  56        that is configured.
  57
  58  available_tracers:
  59
  60        This holds the different types of tracers that
  61        have been compiled into the kernel. The
  62        tracers listed here can be configured by
  63        echoing their name into current_tracer.
  64
  65  tracing_enabled:
  66
  67        This sets or displays whether the current_tracer
  68        is activated and tracing or not. Echo 0 into this
  69        file to disable the tracer or 1 to enable it.
  70
  71  trace:
  72
  73        This file holds the output of the trace in a human
  74        readable format (described below).
  75
  76  latency_trace:
  77
  78        This file shows the same trace but the information
  79        is organized more to display possible latencies
  80        in the system (described below).
  81
  82  trace_pipe:
  83
  84        The output is the same as the "trace" file but this
  85        file is meant to be streamed with live tracing.
  86        Reads from this file will block until new data
  87        is retrieved. Unlike the "trace" and "latency_trace"
  88        files, this file is a consumer. This means reading
  89        from this file causes sequential reads to display
  90        more current data. Once data is read from this
  91        file, it is consumed, and will not be read
  92        again with a sequential read. The "trace" and
  93        "latency_trace" files are static, and if the
  94        tracer is not adding more data, they will display
  95        the same information every time they are read.
  96
  97  trace_options:
  98
  99        This file lets the user control the amount of data
 100        that is displayed in one of the above output
 101        files.
 102
 103  tracing_max_latency:
 104
 105        Some of the tracers record the max latency.
 106        For example, the time interrupts are disabled.
 107        This time is saved in this file. The max trace
 108        will also be stored, and displayed by either
 109        "trace" or "latency_trace".  A new max trace will
 110        only be recorded if the latency is greater than
 111        the value in this file. (in microseconds)
 112
 113  buffer_size_kb:
 114
 115        This sets or displays the number of kilobytes each CPU
 116        buffer can hold. The tracer buffers are the same size
 117        for each CPU. The displayed number is the size of the
 118        CPU buffer and not total size of all buffers. The
 119        trace buffers are allocated in pages (blocks of memory
 120        that the kernel uses for allocation, usually 4 KB in size).
 121        If the last page allocated has room for more bytes
 122        than requested, the rest of the page will be used,
 123        making the actual allocation bigger than requested.
 124        ( Note, the size may not be a multiple of the page size
 125          due to buffer managment overhead. )
 126
 127        This can only be updated when the current_tracer
 128        is set to "nop".
 129
 130  tracing_cpumask:
 131
 132        This is a mask that lets the user only trace
 133        on specified CPUS. The format is a hex string
 134        representing the CPUS.
 135
 136  set_ftrace_filter:
 137
 138        When dynamic ftrace is configured in (see the
 139        section below "dynamic ftrace"), the code is dynamically
 140        modified (code text rewrite) to disable calling of the
 141        function profiler (mcount). This lets tracing be configured
 142        in with practically no overhead in performance.  This also
 143        has a side effect of enabling or disabling specific functions
 144        to be traced. Echoing names of functions into this file
 145        will limit the trace to only those functions.
 146
 147  set_ftrace_notrace:
 148
 149        This has an effect opposite to that of
 150        set_ftrace_filter. Any function that is added here will not
 151        be traced. If a function exists in both set_ftrace_filter
 152        and set_ftrace_notrace, the function will _not_ be traced.
 153
 154  set_ftrace_pid:
 155
 156        Have the function tracer only trace a single thread.
 157
 158  set_graph_function:
 159
 160        Set a "trigger" function where tracing should start
 161        with the function graph tracer (See the section
 162        "dynamic ftrace" for more details).
 163
 164  available_filter_functions:
 165
 166        This lists the functions that ftrace
 167        has processed and can trace. These are the function
 168        names that you can pass to "set_ftrace_filter" or
 169        "set_ftrace_notrace". (See the section "dynamic ftrace"
 170        below for more details.)
 171
 172
 173The Tracers
 174-----------
 175
 176Here is the list of current tracers that may be configured.
 177
 178  "function"
 179
 180        Function call tracer to trace all kernel functions.
 181
 182  "function_graph_tracer"
 183
 184        Similar to the function tracer except that the
 185        function tracer probes the functions on their entry
 186        whereas the function graph tracer traces on both entry
 187        and exit of the functions. It then provides the ability
 188        to draw a graph of function calls similar to C code
 189        source.
 190
 191  "sched_switch"
 192
 193        Traces the context switches and wakeups between tasks.
 194
 195  "irqsoff"
 196
 197        Traces the areas that disable interrupts and saves
 198        the trace with the longest max latency.
 199        See tracing_max_latency. When a new max is recorded,
 200        it replaces the old trace. It is best to view this
 201        trace via the latency_trace file.
 202
 203  "preemptoff"
 204
 205        Similar to irqsoff but traces and records the amount of
 206        time for which preemption is disabled.
 207
 208  "preemptirqsoff"
 209
 210        Similar to irqsoff and preemptoff, but traces and
 211        records the largest time for which irqs and/or preemption
 212        is disabled.
 213
 214  "wakeup"
 215
 216        Traces and records the max latency that it takes for
 217        the highest priority task to get scheduled after
 218        it has been woken up.
 219
 220  "hw-branch-tracer"
 221
 222        Uses the BTS CPU feature on x86 CPUs to traces all
 223        branches executed.
 224
 225  "nop"
 226
 227        This is the "trace nothing" tracer. To remove all
 228        tracers from tracing simply echo "nop" into
 229        current_tracer.
 230
 231
 232Examples of using the tracer
 233----------------------------
 234
 235Here are typical examples of using the tracers when controlling
 236them only with the debugfs interface (without using any
 237user-land utilities).
 238
 239Output format:
 240--------------
 241
 242Here is an example of the output format of the file "trace"
 243
 244                             --------
 245# tracer: function
 246#
 247#           TASK-PID   CPU#    TIMESTAMP  FUNCTION
 248#              | |      |          |         |
 249            bash-4251  [01] 10152.583854: path_put <-path_walk
 250            bash-4251  [01] 10152.583855: dput <-path_put
 251            bash-4251  [01] 10152.583855: _atomic_dec_and_lock <-dput
 252                             --------
 253
 254A header is printed with the tracer name that is represented by
 255the trace. In this case the tracer is "function". Then a header
 256showing the format. Task name "bash", the task PID "4251", the
 257CPU that it was running on "01", the timestamp in <secs>.<usecs>
 258format, the function name that was traced "path_put" and the
 259parent function that called this function "path_walk". The
 260timestamp is the time at which the function was entered.
 261
 262The sched_switch tracer also includes tracing of task wakeups
 263and context switches.
 264
 265     ksoftirqd/1-7     [01]  1453.070013:      7:115:R   +  2916:115:S
 266     ksoftirqd/1-7     [01]  1453.070013:      7:115:R   +    10:115:S
 267     ksoftirqd/1-7     [01]  1453.070013:      7:115:R ==>    10:115:R
 268        events/1-10    [01]  1453.070013:     10:115:S ==>  2916:115:R
 269     kondemand/1-2916  [01]  1453.070013:   2916:115:S ==>     7:115:R
 270     ksoftirqd/1-7     [01]  1453.070013:      7:115:S ==>     0:140:R
 271
 272Wake ups are represented by a "+" and the context switches are
 273shown as "==>".  The format is:
 274
 275 Context switches:
 276
 277       Previous task              Next Task
 278
 279  <pid>:<prio>:<state>  ==>  <pid>:<prio>:<state>
 280
 281 Wake ups:
 282
 283       Current task               Task waking up
 284
 285  <pid>:<prio>:<state>    +  <pid>:<prio>:<state>
 286
 287The prio is the internal kernel priority, which is the inverse
 288of the priority that is usually displayed by user-space tools.
 289Zero represents the highest priority (99). Prio 100 starts the
 290"nice" priorities with 100 being equal to nice -20 and 139 being
 291nice 19. The prio "140" is reserved for the idle task which is
 292the lowest priority thread (pid 0).
 293
 294
 295Latency trace format
 296--------------------
 297
 298For traces that display latency times, the latency_trace file
 299gives somewhat more information to see why a latency happened.
 300Here is a typical trace.
 301
 302# tracer: irqsoff
 303#
 304irqsoff latency trace v1.1.5 on 2.6.26-rc8
 305--------------------------------------------------------------------
 306 latency: 97 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
 307    -----------------
 308    | task: swapper-0 (uid:0 nice:0 policy:0 rt_prio:0)
 309    -----------------
 310 => started at: apic_timer_interrupt
 311 => ended at:   do_softirq
 312
 313#                _------=> CPU#
 314#               / _-----=> irqs-off
 315#              | / _----=> need-resched
 316#              || / _---=> hardirq/softirq
 317#              ||| / _--=> preempt-depth
 318#              |||| /
 319#              |||||     delay
 320#  cmd     pid ||||| time  |   caller
 321#     \   /    |||||   \   |   /
 322  <idle>-0     0d..1    0us+: trace_hardirqs_off_thunk (apic_timer_interrupt)
 323  <idle>-0     0d.s.   97us : __do_softirq (do_softirq)
 324  <idle>-0     0d.s1   98us : trace_hardirqs_on (do_softirq)
 325
 326
 327This shows that the current tracer is "irqsoff" tracing the time
 328for which interrupts were disabled. It gives the trace version
 329and the version of the kernel upon which this was executed on
 330(2.6.26-rc8). Then it displays the max latency in microsecs (97
 331us). The number of trace entries displayed and the total number
 332recorded (both are three: #3/3). The type of preemption that was
 333used (PREEMPT). VP, KP, SP, and HP are always zero and are
 334reserved for later use. #P is the number of online CPUS (#P:2).
 335
 336The task is the process that was running when the latency
 337occurred. (swapper pid: 0).
 338
 339The start and stop (the functions in which the interrupts were
 340disabled and enabled respectively) that caused the latencies:
 341
 342  apic_timer_interrupt is where the interrupts were disabled.
 343  do_softirq is where they were enabled again.
 344
 345The next lines after the header are the trace itself. The header
 346explains which is which.
 347
 348  cmd: The name of the process in the trace.
 349
 350  pid: The PID of that process.
 351
 352  CPU#: The CPU which the process was running on.
 353
 354  irqs-off: 'd' interrupts are disabled. '.' otherwise.
 355            Note: If the architecture does not support a way to
 356                  read the irq flags variable, an 'X' will always
 357                  be printed here.
 358
 359  need-resched: 'N' task need_resched is set, '.' otherwise.
 360
 361  hardirq/softirq:
 362        'H' - hard irq occurred inside a softirq.
 363        'h' - hard irq is running
 364        's' - soft irq is running
 365        '.' - normal context.
 366
 367  preempt-depth: The level of preempt_disabled
 368
 369The above is mostly meaningful for kernel developers.
 370
 371  time: This differs from the trace file output. The trace file output
 372        includes an absolute timestamp. The timestamp used by the
 373        latency_trace file is relative to the start of the trace.
 374
 375  delay: This is just to help catch your eye a bit better. And
 376         needs to be fixed to be only relative to the same CPU.
 377         The marks are determined by the difference between this
 378         current trace and the next trace.
 379          '!' - greater than preempt_mark_thresh (default 100)
 380          '+' - greater than 1 microsecond
 381          ' ' - less than or equal to 1 microsecond.
 382
 383  The rest is the same as the 'trace' file.
 384
 385
 386trace_options
 387-------------
 388
 389The trace_options file is used to control what gets printed in
 390the trace output. To see what is available, simply cat the file:
 391
 392  cat /debug/tracing/trace_options
 393  print-parent nosym-offset nosym-addr noverbose noraw nohex nobin \
 394  noblock nostacktrace nosched-tree nouserstacktrace nosym-userobj
 395
 396To disable one of the options, echo in the option prepended with
 397"no".
 398
 399  echo noprint-parent > /debug/tracing/trace_options
 400
 401To enable an option, leave off the "no".
 402
 403  echo sym-offset > /debug/tracing/trace_options
 404
 405Here are the available options:
 406
 407  print-parent - On function traces, display the calling (parent)
 408                 function as well as the function being traced.
 409
 410  print-parent:
 411   bash-4000  [01]  1477.606694: simple_strtoul <-strict_strtoul
 412
 413  noprint-parent:
 414   bash-4000  [01]  1477.606694: simple_strtoul
 415
 416
 417  sym-offset - Display not only the function name, but also the
 418               offset in the function. For example, instead of
 419               seeing just "ktime_get", you will see
 420               "ktime_get+0xb/0x20".
 421
 422  sym-offset:
 423   bash-4000  [01]  1477.606694: simple_strtoul+0x6/0xa0
 424
 425  sym-addr - this will also display the function address as well
 426             as the function name.
 427
 428  sym-addr:
 429   bash-4000  [01]  1477.606694: simple_strtoul <c0339346>
 430
 431  verbose - This deals with the latency_trace file.
 432
 433    bash  4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \
 434    (+0.000ms): simple_strtoul (strict_strtoul)
 435
 436  raw - This will display raw numbers. This option is best for
 437        use with user applications that can translate the raw
 438        numbers better than having it done in the kernel.
 439
 440  hex - Similar to raw, but the numbers will be in a hexadecimal
 441        format.
 442
 443  bin - This will print out the formats in raw binary.
 444
 445  block - TBD (needs update)
 446
 447  stacktrace - This is one of the options that changes the trace
 448               itself. When a trace is recorded, so is the stack
 449               of functions. This allows for back traces of
 450               trace sites.
 451
 452  userstacktrace - This option changes the trace. It records a
 453                   stacktrace of the current userspace thread.
 454
 455  sym-userobj - when user stacktrace are enabled, look up which
 456                object the address belongs to, and print a
 457                relative address. This is especially useful when
 458                ASLR is on, otherwise you don't get a chance to
 459                resolve the address to object/file/line after
 460                the app is no longer running
 461
 462                The lookup is performed when you read
 463                trace,trace_pipe,latency_trace. Example:
 464
 465                a.out-1623  [000] 40874.465068: /root/a.out[+0x480] <-/root/a.out[+0
 466x494] <- /root/a.out[+0x4a8] <- /lib/libc-2.7.so[+0x1e1a6]
 467
 468  sched-tree - trace all tasks that are on the runqueue, at
 469               every scheduling event. Will add overhead if
 470               there's a lot of tasks running at once.
 471
 472
 473sched_switch
 474------------
 475
 476This tracer simply records schedule switches. Here is an example
 477of how to use it.
 478
 479 # echo sched_switch > /debug/tracing/current_tracer
 480 # echo 1 > /debug/tracing/tracing_enabled
 481 # sleep 1
 482 # echo 0 > /debug/tracing/tracing_enabled
 483 # cat /debug/tracing/trace
 484
 485# tracer: sched_switch
 486#
 487#           TASK-PID   CPU#    TIMESTAMP  FUNCTION
 488#              | |      |          |         |
 489            bash-3997  [01]   240.132281:   3997:120:R   +  4055:120:R
 490            bash-3997  [01]   240.132284:   3997:120:R ==>  4055:120:R
 491           sleep-4055  [01]   240.132371:   4055:120:S ==>  3997:120:R
 492            bash-3997  [01]   240.132454:   3997:120:R   +  4055:120:S
 493            bash-3997  [01]   240.132457:   3997:120:R ==>  4055:120:R
 494           sleep-4055  [01]   240.132460:   4055:120:D ==>  3997:120:R
 495            bash-3997  [01]   240.132463:   3997:120:R   +  4055:120:D
 496            bash-3997  [01]   240.132465:   3997:120:R ==>  4055:120:R
 497          <idle>-0     [00]   240.132589:      0:140:R   +     4:115:S
 498          <idle>-0     [00]   240.132591:      0:140:R ==>     4:115:R
 499     ksoftirqd/0-4     [00]   240.132595:      4:115:S ==>     0:140:R
 500          <idle>-0     [00]   240.132598:      0:140:R   +     4:115:S
 501          <idle>-0     [00]   240.132599:      0:140:R ==>     4:115:R
 502     ksoftirqd/0-4     [00]   240.132603:      4:115:S ==>     0:140:R
 503           sleep-4055  [01]   240.133058:   4055:120:S ==>  3997:120:R
 504 [...]
 505
 506
 507As we have discussed previously about this format, the header
 508shows the name of the trace and points to the options. The
 509"FUNCTION" is a misnomer since here it represents the wake ups
 510and context switches.
 511
 512The sched_switch file only lists the wake ups (represented with
 513'+') and context switches ('==>') with the previous task or
 514current task first followed by the next task or task waking up.
 515The format for both of these is PID:KERNEL-PRIO:TASK-STATE.
 516Remember that the KERNEL-PRIO is the inverse of the actual
 517priority with zero (0) being the highest priority and the nice
 518values starting at 100 (nice -20). Below is a quick chart to map
 519the kernel priority to user land priorities.
 520
 521  Kernel priority: 0 to 99    ==> user RT priority 99 to 0
 522  Kernel priority: 100 to 139 ==> user nice -20 to 19
 523  Kernel priority: 140        ==> idle task priority
 524
 525The task states are:
 526
 527 R - running : wants to run, may not actually be running
 528 S - sleep   : process is waiting to be woken up (handles signals)
 529 D - disk sleep (uninterruptible sleep) : process must be woken up
 530                                        (ignores signals)
 531 T - stopped : process suspended
 532 t - traced  : process is being traced (with something like gdb)
 533 Z - zombie  : process waiting to be cleaned up
 534 X - unknown
 535
 536
 537ftrace_enabled
 538--------------
 539
 540The following tracers (listed below) give different output
 541depending on whether or not the sysctl ftrace_enabled is set. To
 542set ftrace_enabled, one can either use the sysctl function or
 543set it via the proc file system interface.
 544
 545  sysctl kernel.ftrace_enabled=1
 546
 547 or
 548
 549  echo 1 > /proc/sys/kernel/ftrace_enabled
 550
 551To disable ftrace_enabled simply replace the '1' with '0' in the
 552above commands.
 553
 554When ftrace_enabled is set the tracers will also record the
 555functions that are within the trace. The descriptions of the
 556tracers will also show an example with ftrace enabled.
 557
 558
 559irqsoff
 560-------
 561
 562When interrupts are disabled, the CPU can not react to any other
 563external event (besides NMIs and SMIs). This prevents the timer
 564interrupt from triggering or the mouse interrupt from letting
 565the kernel know of a new mouse event. The result is a latency
 566with the reaction time.
 567
 568The irqsoff tracer tracks the time for which interrupts are
 569disabled. When a new maximum latency is hit, the tracer saves
 570the trace leading up to that latency point so that every time a
 571new maximum is reached, the old saved trace is discarded and the
 572new trace is saved.
 573
 574To reset the maximum, echo 0 into tracing_max_latency. Here is
 575an example:
 576
 577 # echo irqsoff > /debug/tracing/current_tracer
 578 # echo 0 > /debug/tracing/tracing_max_latency
 579 # echo 1 > /debug/tracing/tracing_enabled
 580 # ls -ltr
 581 [...]
 582 # echo 0 > /debug/tracing/tracing_enabled
 583 # cat /debug/tracing/latency_trace
 584# tracer: irqsoff
 585#
 586irqsoff latency trace v1.1.5 on 2.6.26
 587--------------------------------------------------------------------
 588 latency: 12 us, #3/3, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
 589    -----------------
 590    | task: bash-3730 (uid:0 nice:0 policy:0 rt_prio:0)
 591    -----------------
 592 => started at: sys_setpgid
 593 => ended at:   sys_setpgid
 594
 595#                _------=> CPU#
 596#               / _-----=> irqs-off
 597#              | / _----=> need-resched
 598#              || / _---=> hardirq/softirq
 599#              ||| / _--=> preempt-depth
 600#              |||| /
 601#              |||||     delay
 602#  cmd     pid ||||| time  |   caller
 603#     \   /    |||||   \   |   /
 604    bash-3730  1d...    0us : _write_lock_irq (sys_setpgid)
 605    bash-3730  1d..1    1us+: _write_unlock_irq (sys_setpgid)
 606    bash-3730  1d..2   14us : trace_hardirqs_on (sys_setpgid)
 607
 608
 609Here we see that that we had a latency of 12 microsecs (which is
 610very good). The _write_lock_irq in sys_setpgid disabled
 611interrupts. The difference between the 12 and the displayed
 612timestamp 14us occurred because the clock was incremented
 613between the time of recording the max latency and the time of
 614recording the function that had that latency.
 615
 616Note the above example had ftrace_enabled not set. If we set the
 617ftrace_enabled, we get a much larger output:
 618
 619# tracer: irqsoff
 620#
 621irqsoff latency trace v1.1.5 on 2.6.26-rc8
 622--------------------------------------------------------------------
 623 latency: 50 us, #101/101, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
 624    -----------------
 625    | task: ls-4339 (uid:0 nice:0 policy:0 rt_prio:0)
 626    -----------------
 627 => started at: __alloc_pages_internal
 628 => ended at:   __alloc_pages_internal
 629
 630#                _------=> CPU#
 631#               / _-----=> irqs-off
 632#              | / _----=> need-resched
 633#              || / _---=> hardirq/softirq
 634#              ||| / _--=> preempt-depth
 635#              |||| /
 636#              |||||     delay
 637#  cmd     pid ||||| time  |   caller
 638#     \   /    |||||   \   |   /
 639      ls-4339  0...1    0us+: get_page_from_freelist (__alloc_pages_internal)
 640      ls-4339  0d..1    3us : rmqueue_bulk (get_page_from_freelist)
 641      ls-4339  0d..1    3us : _spin_lock (rmqueue_bulk)
 642      ls-4339  0d..1    4us : add_preempt_count (_spin_lock)
 643      ls-4339  0d..2    4us : __rmqueue (rmqueue_bulk)
 644      ls-4339  0d..2    5us : __rmqueue_smallest (__rmqueue)
 645      ls-4339  0d..2    5us : __mod_zone_page_state (__rmqueue_smallest)
 646      ls-4339  0d..2    6us : __rmqueue (rmqueue_bulk)
 647      ls-4339  0d..2    6us : __rmqueue_smallest (__rmqueue)
 648      ls-4339  0d..2    7us : __mod_zone_page_state (__rmqueue_smallest)
 649      ls-4339  0d..2    7us : __rmqueue (rmqueue_bulk)
 650      ls-4339  0d..2    8us : __rmqueue_smallest (__rmqueue)
 651[...]
 652      ls-4339  0d..2   46us : __rmqueue_smallest (__rmqueue)
 653      ls-4339  0d..2   47us : __mod_zone_page_state (__rmqueue_smallest)
 654      ls-4339  0d..2   47us : __rmqueue (rmqueue_bulk)
 655      ls-4339  0d..2   48us : __rmqueue_smallest (__rmqueue)
 656      ls-4339  0d..2   48us : __mod_zone_page_state (__rmqueue_smallest)
 657      ls-4339  0d..2   49us : _spin_unlock (rmqueue_bulk)
 658      ls-4339  0d..2   49us : sub_preempt_count (_spin_unlock)
 659      ls-4339  0d..1   50us : get_page_from_freelist (__alloc_pages_internal)
 660      ls-4339  0d..2   51us : trace_hardirqs_on (__alloc_pages_internal)
 661
 662
 663
 664Here we traced a 50 microsecond latency. But we also see all the
 665functions that were called during that time. Note that by
 666enabling function tracing, we incur an added overhead. This
 667overhead may extend the latency times. But nevertheless, this
 668trace has provided some very helpful debugging information.
 669
 670
 671preemptoff
 672----------
 673
 674When preemption is disabled, we may be able to receive
 675interrupts but the task cannot be preempted and a higher
 676priority task must wait for preemption to be enabled again
 677before it can preempt a lower priority task.
 678
 679The preemptoff tracer traces the places that disable preemption.
 680Like the irqsoff tracer, it records the maximum latency for
 681which preemption was disabled. The control of preemptoff tracer
 682is much like the irqsoff tracer.
 683
 684 # echo preemptoff > /debug/tracing/current_tracer
 685 # echo 0 > /debug/tracing/tracing_max_latency
 686 # echo 1 > /debug/tracing/tracing_enabled
 687 # ls -ltr
 688 [...]
 689 # echo 0 > /debug/tracing/tracing_enabled
 690 # cat /debug/tracing/latency_trace
 691# tracer: preemptoff
 692#
 693preemptoff latency trace v1.1.5 on 2.6.26-rc8
 694--------------------------------------------------------------------
 695 latency: 29 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
 696    -----------------
 697    | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0)
 698    -----------------
 699 => started at: do_IRQ
 700 => ended at:   __do_softirq
 701
 702#                _------=> CPU#
 703#               / _-----=> irqs-off
 704#              | / _----=> need-resched
 705#              || / _---=> hardirq/softirq
 706#              ||| / _--=> preempt-depth
 707#              |||| /
 708#              |||||     delay
 709#  cmd     pid ||||| time  |   caller
 710#     \   /    |||||   \   |   /
 711    sshd-4261  0d.h.    0us+: irq_enter (do_IRQ)
 712    sshd-4261  0d.s.   29us : _local_bh_enable (__do_softirq)
 713    sshd-4261  0d.s1   30us : trace_preempt_on (__do_softirq)
 714
 715
 716This has some more changes. Preemption was disabled when an
 717interrupt came in (notice the 'h'), and was enabled while doing
 718a softirq. (notice the 's'). But we also see that interrupts
 719have been disabled when entering the preempt off section and
 720leaving it (the 'd'). We do not know if interrupts were enabled
 721in the mean time.
 722
 723# tracer: preemptoff
 724#
 725preemptoff latency trace v1.1.5 on 2.6.26-rc8
 726--------------------------------------------------------------------
 727 latency: 63 us, #87/87, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
 728    -----------------
 729    | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0)
 730    -----------------
 731 => started at: remove_wait_queue
 732 => ended at:   __do_softirq
 733
 734#                _------=> CPU#
 735#               / _-----=> irqs-off
 736#              | / _----=> need-resched
 737#              || / _---=> hardirq/softirq
 738#              ||| / _--=> preempt-depth
 739#              |||| /
 740#              |||||     delay
 741#  cmd     pid ||||| time  |   caller
 742#     \   /    |||||   \   |   /
 743    sshd-4261  0d..1    0us : _spin_lock_irqsave (remove_wait_queue)
 744    sshd-4261  0d..1    1us : _spin_unlock_irqrestore (remove_wait_queue)
 745    sshd-4261  0d..1    2us : do_IRQ (common_interrupt)
 746    sshd-4261  0d..1    2us : irq_enter (do_IRQ)
 747    sshd-4261  0d..1    2us : idle_cpu (irq_enter)
 748    sshd-4261  0d..1    3us : add_preempt_count (irq_enter)
 749    sshd-4261  0d.h1    3us : idle_cpu (irq_enter)
 750    sshd-4261  0d.h.    4us : handle_fasteoi_irq (do_IRQ)
 751[...]
 752    sshd-4261  0d.h.   12us : add_preempt_count (_spin_lock)
 753    sshd-4261  0d.h1   12us : ack_ioapic_quirk_irq (handle_fasteoi_irq)
 754    sshd-4261  0d.h1   13us : move_native_irq (ack_ioapic_quirk_irq)
 755    sshd-4261  0d.h1   13us : _spin_unlock (handle_fasteoi_irq)
 756    sshd-4261  0d.h1   14us : sub_preempt_count (_spin_unlock)
 757    sshd-4261  0d.h1   14us : irq_exit (do_IRQ)
 758    sshd-4261  0d.h1   15us : sub_preempt_count (irq_exit)
 759    sshd-4261  0d..2   15us : do_softirq (irq_exit)
 760    sshd-4261  0d...   15us : __do_softirq (do_softirq)
 761    sshd-4261  0d...   16us : __local_bh_disable (__do_softirq)
 762    sshd-4261  0d...   16us+: add_preempt_count (__local_bh_disable)
 763    sshd-4261  0d.s4   20us : add_preempt_count (__local_bh_disable)
 764    sshd-4261  0d.s4   21us : sub_preempt_count (local_bh_enable)
 765    sshd-4261  0d.s5   21us : sub_preempt_count (local_bh_enable)
 766[...]
 767    sshd-4261  0d.s6   41us : add_preempt_count (__local_bh_disable)
 768    sshd-4261  0d.s6   42us : sub_preempt_count (local_bh_enable)
 769    sshd-4261  0d.s7   42us : sub_preempt_count (local_bh_enable)
 770    sshd-4261  0d.s5   43us : add_preempt_count (__local_bh_disable)
 771    sshd-4261  0d.s5   43us : sub_preempt_count (local_bh_enable_ip)
 772    sshd-4261  0d.s6   44us : sub_preempt_count (local_bh_enable_ip)
 773    sshd-4261  0d.s5   44us : add_preempt_count (__local_bh_disable)
 774    sshd-4261  0d.s5   45us : sub_preempt_count (local_bh_enable)
 775[...]
 776    sshd-4261  0d.s.   63us : _local_bh_enable (__do_softirq)
 777    sshd-4261  0d.s1   64us : trace_preempt_on (__do_softirq)
 778
 779
 780The above is an example of the preemptoff trace with
 781ftrace_enabled set. Here we see that interrupts were disabled
 782the entire time. The irq_enter code lets us know that we entered
 783an interrupt 'h'. Before that, the functions being traced still
 784show that it is not in an interrupt, but we can see from the
 785functions themselves that this is not the case.
 786
 787Notice that __do_softirq when called does not have a
 788preempt_count. It may seem that we missed a preempt enabling.
 789What really happened is that the preempt count is held on the
 790thread's stack and we switched to the softirq stack (4K stacks
 791in effect). The code does not copy the preempt count, but
 792because interrupts are disabled, we do not need to worry about
 793it. Having a tracer like this is good for letting people know
 794what really happens inside the kernel.
 795
 796
 797preemptirqsoff
 798--------------
 799
 800Knowing the locations that have interrupts disabled or
 801preemption disabled for the longest times is helpful. But
 802sometimes we would like to know when either preemption and/or
 803interrupts are disabled.
 804
 805Consider the following code:
 806
 807    local_irq_disable();
 808    call_function_with_irqs_off();
 809    preempt_disable();
 810    call_function_with_irqs_and_preemption_off();
 811    local_irq_enable();
 812    call_function_with_preemption_off();
 813    preempt_enable();
 814
 815The irqsoff tracer will record the total length of
 816call_function_with_irqs_off() and
 817call_function_with_irqs_and_preemption_off().
 818
 819The preemptoff tracer will record the total length of
 820call_function_with_irqs_and_preemption_off() and
 821call_function_with_preemption_off().
 822
 823But neither will trace the time that interrupts and/or
 824preemption is disabled. This total time is the time that we can
 825not schedule. To record this time, use the preemptirqsoff
 826tracer.
 827
 828Again, using this trace is much like the irqsoff and preemptoff
 829tracers.
 830
 831 # echo preemptirqsoff > /debug/tracing/current_tracer
 832 # echo 0 > /debug/tracing/tracing_max_latency
 833 # echo 1 > /debug/tracing/tracing_enabled
 834 # ls -ltr
 835 [...]
 836 # echo 0 > /debug/tracing/tracing_enabled
 837 # cat /debug/tracing/latency_trace
 838# tracer: preemptirqsoff
 839#
 840preemptirqsoff latency trace v1.1.5 on 2.6.26-rc8
 841--------------------------------------------------------------------
 842 latency: 293 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
 843    -----------------
 844    | task: ls-4860 (uid:0 nice:0 policy:0 rt_prio:0)
 845    -----------------
 846 => started at: apic_timer_interrupt
 847 => ended at:   __do_softirq
 848
 849#                _------=> CPU#
 850#               / _-----=> irqs-off
 851#              | / _----=> need-resched
 852#              || / _---=> hardirq/softirq
 853#              ||| / _--=> preempt-depth
 854#              |||| /
 855#              |||||     delay
 856#  cmd     pid ||||| time  |   caller
 857#     \   /    |||||   \   |   /
 858      ls-4860  0d...    0us!: trace_hardirqs_off_thunk (apic_timer_interrupt)
 859      ls-4860  0d.s.  294us : _local_bh_enable (__do_softirq)
 860      ls-4860  0d.s1  294us : trace_preempt_on (__do_softirq)
 861
 862
 863
 864The trace_hardirqs_off_thunk is called from assembly on x86 when
 865interrupts are disabled in the assembly code. Without the
 866function tracing, we do not know if interrupts were enabled
 867within the preemption points. We do see that it started with
 868preemption enabled.
 869
 870Here is a trace with ftrace_enabled set:
 871
 872
 873# tracer: preemptirqsoff
 874#
 875preemptirqsoff latency trace v1.1.5 on 2.6.26-rc8
 876--------------------------------------------------------------------
 877 latency: 105 us, #183/183, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
 878    -----------------
 879    | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0)
 880    -----------------
 881 => started at: write_chan
 882 => ended at:   __do_softirq
 883
 884#                _------=> CPU#
 885#               / _-----=> irqs-off
 886#              | / _----=> need-resched
 887#              || / _---=> hardirq/softirq
 888#              ||| / _--=> preempt-depth
 889#              |||| /
 890#              |||||     delay
 891#  cmd     pid ||||| time  |   caller
 892#     \   /    |||||   \   |   /
 893      ls-4473  0.N..    0us : preempt_schedule (write_chan)
 894      ls-4473  0dN.1    1us : _spin_lock (schedule)
 895      ls-4473  0dN.1    2us : add_preempt_count (_spin_lock)
 896      ls-4473  0d..2    2us : put_prev_task_fair (schedule)
 897[...]
 898      ls-4473  0d..2   13us : set_normalized_timespec (ktime_get_ts)
 899      ls-4473  0d..2   13us : __switch_to (schedule)
 900    sshd-4261  0d..2   14us : finish_task_switch (schedule)
 901    sshd-4261  0d..2   14us : _spin_unlock_irq (finish_task_switch)
 902    sshd-4261  0d..1   15us : add_preempt_count (_spin_lock_irqsave)
 903    sshd-4261  0d..2   16us : _spin_unlock_irqrestore (hrtick_set)
 904    sshd-4261  0d..2   16us : do_IRQ (common_interrupt)
 905    sshd-4261  0d..2   17us : irq_enter (do_IRQ)
 906    sshd-4261  0d..2   17us : idle_cpu (irq_enter)
 907    sshd-4261  0d..2   18us : add_preempt_count (irq_enter)
 908    sshd-4261  0d.h2   18us : idle_cpu (irq_enter)
 909    sshd-4261  0d.h.   18us : handle_fasteoi_irq (do_IRQ)
 910    sshd-4261  0d.h.   19us : _spin_lock (handle_fasteoi_irq)
 911    sshd-4261  0d.h.   19us : add_preempt_count (_spin_lock)
 912    sshd-4261  0d.h1   20us : _spin_unlock (handle_fasteoi_irq)
 913    sshd-4261  0d.h1   20us : sub_preempt_count (_spin_unlock)
 914[...]
 915    sshd-4261  0d.h1   28us : _spin_unlock (handle_fasteoi_irq)
 916    sshd-4261  0d.h1   29us : sub_preempt_count (_spin_unlock)
 917    sshd-4261  0d.h2   29us : irq_exit (do_IRQ)
 918    sshd-4261  0d.h2   29us : sub_preempt_count (irq_exit)
 919    sshd-4261  0d..3   30us : do_softirq (irq_exit)
 920    sshd-4261  0d...   30us : __do_softirq (do_softirq)
 921    sshd-4261  0d...   31us : __local_bh_disable (__do_softirq)
 922    sshd-4261  0d...   31us+: add_preempt_count (__local_bh_disable)
 923    sshd-4261  0d.s4   34us : add_preempt_count (__local_bh_disable)
 924[...]
 925    sshd-4261  0d.s3   43us : sub_preempt_count (local_bh_enable_ip)
 926    sshd-4261  0d.s4   44us : sub_preempt_count (local_bh_enable_ip)
 927    sshd-4261  0d.s3   44us : smp_apic_timer_interrupt (apic_timer_interrupt)
 928    sshd-4261  0d.s3   45us : irq_enter (smp_apic_timer_interrupt)
 929    sshd-4261  0d.s3   45us : idle_cpu (irq_enter)
 930    sshd-4261  0d.s3   46us : add_preempt_count (irq_enter)
 931    sshd-4261  0d.H3   46us : idle_cpu (irq_enter)
 932    sshd-4261  0d.H3   47us : hrtimer_interrupt (smp_apic_timer_interrupt)
 933    sshd-4261  0d.H3   47us : ktime_get (hrtimer_interrupt)
 934[...]
 935    sshd-4261  0d.H3   81us : tick_program_event (hrtimer_interrupt)
 936    sshd-4261  0d.H3   82us : ktime_get (tick_program_event)
 937    sshd-4261  0d.H3   82us : ktime_get_ts (ktime_get)
 938    sshd-4261  0d.H3   83us : getnstimeofday (ktime_get_ts)
 939    sshd-4261  0d.H3   83us : set_normalized_timespec (ktime_get_ts)
 940    sshd-4261  0d.H3   84us : clockevents_program_event (tick_program_event)
 941    sshd-4261  0d.H3   84us : lapic_next_event (clockevents_program_event)
 942    sshd-4261  0d.H3   85us : irq_exit (smp_apic_timer_interrupt)
 943    sshd-4261  0d.H3   85us : sub_preempt_count (irq_exit)
 944    sshd-4261  0d.s4   86us : sub_preempt_count (irq_exit)
 945    sshd-4261  0d.s3   86us : add_preempt_count (__local_bh_disable)
 946[...]
 947    sshd-4261  0d.s1   98us : sub_preempt_count (net_rx_action)
 948    sshd-4261  0d.s.   99us : add_preempt_count (_spin_lock_irq)
 949    sshd-4261  0d.s1   99us+: _spin_unlock_irq (run_timer_softirq)
 950    sshd-4261  0d.s.  104us : _local_bh_enable (__do_softirq)
 951    sshd-4261  0d.s.  104us : sub_preempt_count (_local_bh_enable)
 952    sshd-4261  0d.s.  105us : _local_bh_enable (__do_softirq)
 953    sshd-4261  0d.s1  105us : trace_preempt_on (__do_softirq)
 954
 955
 956This is a very interesting trace. It started with the preemption
 957of the ls task. We see that the task had the "need_resched" bit
 958set via the 'N' in the trace.  Interrupts were disabled before
 959the spin_lock at the beginning of the trace. We see that a
 960schedule took place to run sshd.  When the interrupts were
 961enabled, we took an interrupt. On return from the interrupt
 962handler, the softirq ran. We took another interrupt while
 963running the softirq as we see from the capital 'H'.
 964
 965
 966wakeup
 967------
 968
 969In a Real-Time environment it is very important to know the
 970wakeup time it takes for the highest priority task that is woken
 971up to the time that it executes. This is also known as "schedule
 972latency". I stress the point that this is about RT tasks. It is
 973also important to know the scheduling latency of non-RT tasks,
 974but the average schedule latency is better for non-RT tasks.
 975Tools like LatencyTop are more appropriate for such
 976measurements.
 977
 978Real-Time environments are interested in the worst case latency.
 979That is the longest latency it takes for something to happen,
 980and not the average. We can have a very fast scheduler that may
 981only have a large latency once in a while, but that would not
 982work well with Real-Time tasks.  The wakeup tracer was designed
 983to record the worst case wakeups of RT tasks. Non-RT tasks are
 984not recorded because the tracer only records one worst case and
 985tracing non-RT tasks that are unpredictable will overwrite the
 986worst case latency of RT tasks.
 987
 988Since this tracer only deals with RT tasks, we will run this
 989slightly differently than we did with the previous tracers.
 990Instead of performing an 'ls', we will run 'sleep 1' under
 991'chrt' which changes the priority of the task.
 992
 993 # echo wakeup > /debug/tracing/current_tracer
 994 # echo 0 > /debug/tracing/tracing_max_latency
 995 # echo 1 > /debug/tracing/tracing_enabled
 996 # chrt -f 5 sleep 1
 997 # echo 0 > /debug/tracing/tracing_enabled
 998 # cat /debug/tracing/latency_trace
 999# tracer: wakeup
1000#
1001wakeup latency trace v1.1.5 on 2.6.26-rc8
1002--------------------------------------------------------------------
1003 latency: 4 us, #2/2, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
1004    -----------------
1005    | task: sleep-4901 (uid:0 nice:0 policy:1 rt_prio:5)
1006    -----------------
1007
1008#                _------=> CPU#
1009#               / _-----=> irqs-off
1010#              | / _----=> need-resched
1011#              || / _---=> hardirq/softirq
1012#              ||| / _--=> preempt-depth
1013#              |||| /
1014#              |||||     delay
1015#  cmd     pid ||||| time  |   caller
1016#     \   /    |||||   \   |   /
1017  <idle>-0     1d.h4    0us+: try_to_wake_up (wake_up_process)
1018  <idle>-0     1d..4    4us : schedule (cpu_idle)
1019
1020
1021Running this on an idle system, we see that it only took 4
1022microseconds to perform the task switch.  Note, since the trace
1023marker in the schedule is before the actual "switch", we stop
1024the tracing when the recorded task is about to schedule in. This
1025may change if we add a new marker at the end of the scheduler.
1026
1027Notice that the recorded task is 'sleep' with the PID of 4901
1028and it has an rt_prio of 5. This priority is user-space priority
1029and not the internal kernel priority. The policy is 1 for
1030SCHED_FIFO and 2 for SCHED_RR.
1031
1032Doing the same with chrt -r 5 and ftrace_enabled set.
1033
1034# tracer: wakeup
1035#
1036wakeup latency trace v1.1.5 on 2.6.26-rc8
1037--------------------------------------------------------------------
1038 latency: 50 us, #60/60, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
1039    -----------------
1040    | task: sleep-4068 (uid:0 nice:0 policy:2 rt_prio:5)
1041    -----------------
1042
1043#                _------=> CPU#
1044#               / _-----=> irqs-off
1045#              | / _----=> need-resched
1046#              || / _---=> hardirq/softirq
1047#              ||| / _--=> preempt-depth
1048#              |||| /
1049#              |||||     delay
1050#  cmd     pid ||||| time  |   caller
1051#     \   /    |||||   \   |   /
1052ksoftirq-7     1d.H3    0us : try_to_wake_up (wake_up_process)
1053ksoftirq-7     1d.H4    1us : sub_preempt_count (marker_probe_cb)
1054ksoftirq-7     1d.H3    2us : check_preempt_wakeup (try_to_wake_up)
1055ksoftirq-7     1d.H3    3us : update_curr (check_preempt_wakeup)
1056ksoftirq-7     1d.H3    4us : calc_delta_mine (update_curr)
1057ksoftirq-7     1d.H3    5us : __resched_task (check_preempt_wakeup)
1058ksoftirq-7     1d.H3    6us : task_wake_up_rt (try_to_wake_up)
1059ksoftirq-7     1d.H3    7us : _spin_unlock_irqrestore (try_to_wake_up)
1060[...]
1061ksoftirq-7     1d.H2   17us : irq_exit (smp_apic_timer_interrupt)
1062ksoftirq-7     1d.H2   18us : sub_preempt_count (irq_exit)
1063ksoftirq-7     1d.s3   19us : sub_preempt_count (irq_exit)
1064ksoftirq-7     1..s2   20us : rcu_process_callbacks (__do_softirq)
1065[...]
1066ksoftirq-7     1..s2   26us : __rcu_process_callbacks (rcu_process_callbacks)
1067ksoftirq-7     1d.s2   27us : _local_bh_enable (__do_softirq)
1068ksoftirq-7     1d.s2   28us : sub_preempt_count (_local_bh_enable)
1069ksoftirq-7     1.N.3   29us : sub_preempt_count (ksoftirqd)
1070ksoftirq-7     1.N.2   30us : _cond_resched (ksoftirqd)
1071ksoftirq-7     1.N.2   31us : __cond_resched (_cond_resched)
1072ksoftirq-7     1.N.2   32us : add_preempt_count (__cond_resched)
1073ksoftirq-7     1.N.2   33us : schedule (__cond_resched)
1074ksoftirq-7     1.N.2   33us : add_preempt_count (schedule)
1075ksoftirq-7     1.N.3   34us : hrtick_clear (schedule)
1076ksoftirq-7     1dN.3   35us : _spin_lock (schedule)
1077ksoftirq-7     1dN.3   36us : add_preempt_count (_spin_lock)
1078ksoftirq-7     1d..4   37us : put_prev_task_fair (schedule)
1079ksoftirq-7     1d..4   38us : update_curr (put_prev_task_fair)
1080[...]
1081ksoftirq-7     1d..5   47us : _spin_trylock (tracing_record_cmdline)
1082ksoftirq-7     1d..5   48us : add_preempt_count (_spin_trylock)
1083ksoftirq-7     1d..6   49us : _spin_unlock (tracing_record_cmdline)
1084ksoftirq-7     1d..6   49us : sub_preempt_count (_spin_unlock)
1085ksoftirq-7     1d..4   50us : schedule (__cond_resched)
1086
1087The interrupt went off while running ksoftirqd. This task runs
1088at SCHED_OTHER. Why did not we see the 'N' set early? This may
1089be a harmless bug with x86_32 and 4K stacks. On x86_32 with 4K
1090stacks configured, the interrupt and softirq run with their own
1091stack. Some information is held on the top of the task's stack
1092(need_resched and preempt_count are both stored there). The
1093setting of the NEED_RESCHED bit is done directly to the task's
1094stack, but the reading of the NEED_RESCHED is done by looking at
1095the current stack, which in this case is the stack for the hard
1096interrupt. This hides the fact that NEED_RESCHED has been set.
1097We do not see the 'N' until we switch back to the task's
1098assigned stack.
1099
1100function
1101--------
1102
1103This tracer is the function tracer. Enabling the function tracer
1104can be done from the debug file system. Make sure the
1105ftrace_enabled is set; otherwise this tracer is a nop.
1106
1107 # sysctl kernel.ftrace_enabled=1
1108 # echo function > /debug/tracing/current_tracer
1109 # echo 1 > /debug/tracing/tracing_enabled
1110 # usleep 1
1111 # echo 0 > /debug/tracing/tracing_enabled
1112 # cat /debug/tracing/trace
1113# tracer: function
1114#
1115#           TASK-PID   CPU#    TIMESTAMP  FUNCTION
1116#              | |      |          |         |
1117            bash-4003  [00]   123.638713: finish_task_switch <-schedule
1118            bash-4003  [00]   123.638714: _spin_unlock_irq <-finish_task_switch
1119            bash-4003  [00]   123.638714: sub_preempt_count <-_spin_unlock_irq
1120            bash-4003  [00]   123.638715: hrtick_set <-schedule
1121            bash-4003  [00]   123.638715: _spin_lock_irqsave <-hrtick_set
1122            bash-4003  [00]   123.638716: add_preempt_count <-_spin_lock_irqsave
1123            bash-4003  [00]   123.638716: _spin_unlock_irqrestore <-hrtick_set
1124            bash-4003  [00]   123.638717: sub_preempt_count <-_spin_unlock_irqrestore
1125            bash-4003  [00]   123.638717: hrtick_clear <-hrtick_set
1126            bash-4003  [00]   123.638718: sub_preempt_count <-schedule
1127            bash-4003  [00]   123.638718: sub_preempt_count <-preempt_schedule
1128            bash-4003  [00]   123.638719: wait_for_completion <-__stop_machine_run
1129            bash-4003  [00]   123.638719: wait_for_common <-wait_for_completion
1130            bash-4003  [00]   123.638720: _spin_lock_irq <-wait_for_common
1131            bash-4003  [00]   123.638720: add_preempt_count <-_spin_lock_irq
1132[...]
1133
1134
1135Note: function tracer uses ring buffers to store the above
1136entries. The newest data may overwrite the oldest data.
1137Sometimes using echo to stop the trace is not sufficient because
1138the tracing could have overwritten the data that you wanted to
1139record. For this reason, it is sometimes better to disable
1140tracing directly from a program. This allows you to stop the
1141tracing at the point that you hit the part that you are
1142interested in. To disable the tracing directly from a C program,
1143something like following code snippet can be used:
1144
1145int trace_fd;
1146[...]
1147int main(int argc, char *argv[]) {
1148        [...]
1149        trace_fd = open("/debug/tracing/tracing_enabled", O_WRONLY);
1150        [...]
1151        if (condition_hit()) {
1152                write(trace_fd, "0", 1);
1153        }
1154        [...]
1155}
1156
1157Note: Here we hard coded the path name. The debugfs mount is not
1158guaranteed to be at /debug (and is more commonly at
1159/sys/kernel/debug). For simple one time traces, the above is
1160sufficent. For anything else, a search through /proc/mounts may
1161be needed to find where the debugfs file-system is mounted.
1162
1163
1164Single thread tracing
1165---------------------
1166
1167By writing into /debug/tracing/set_ftrace_pid you can trace a
1168single thread. For example:
1169
1170# cat /debug/tracing/set_ftrace_pid
1171no pid
1172# echo 3111 > /debug/tracing/set_ftrace_pid
1173# cat /debug/tracing/set_ftrace_pid
11743111
1175# echo function > /debug/tracing/current_tracer
1176# cat /debug/tracing/trace | head
1177 # tracer: function
1178 #
1179 #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
1180 #              | |       |          |         |
1181     yum-updatesd-3111  [003]  1637.254676: finish_task_switch <-thread_return
1182     yum-updatesd-3111  [003]  1637.254681: hrtimer_cancel <-schedule_hrtimeout_range
1183     yum-updatesd-3111  [003]  1637.254682: hrtimer_try_to_cancel <-hrtimer_cancel
1184     yum-updatesd-3111  [003]  1637.254683: lock_hrtimer_base <-hrtimer_try_to_cancel
1185     yum-updatesd-3111  [003]  1637.254685: fget_light <-do_sys_poll
1186     yum-updatesd-3111  [003]  1637.254686: pipe_poll <-do_sys_poll
1187# echo -1 > /debug/tracing/set_ftrace_pid
1188# cat /debug/tracing/trace |head
1189 # tracer: function
1190 #
1191 #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
1192 #              | |       |          |         |
1193 ##### CPU 3 buffer started ####
1194     yum-updatesd-3111  [003]  1701.957688: free_poll_entry <-poll_freewait
1195     yum-updatesd-3111  [003]  1701.957689: remove_wait_queue <-free_poll_entry
1196     yum-updatesd-3111  [003]  1701.957691: fput <-free_poll_entry
1197     yum-updatesd-3111  [003]  1701.957692: audit_syscall_exit <-sysret_audit
1198     yum-updatesd-3111  [003]  1701.957693: path_put <-audit_syscall_exit
1199
1200If you want to trace a function when executing, you could use
1201something like this simple program:
1202
1203#include <stdio.h>
1204#include <stdlib.h>
1205#include <sys/types.h>
1206#include <sys/stat.h>
1207#include <fcntl.h>
1208#include <unistd.h>
1209
1210int main (int argc, char **argv)
1211{
1212        if (argc < 1)
1213                exit(-1);
1214
1215        if (fork() > 0) {
1216                int fd, ffd;
1217                char line[64];
1218                int s;
1219
1220                ffd = open("/debug/tracing/current_tracer", O_WRONLY);
1221                if (ffd < 0)
1222                        exit(-1);
1223                write(ffd, "nop", 3);
1224
1225                fd = open("/debug/tracing/set_ftrace_pid", O_WRONLY);
1226                s = sprintf(line, "%d\n", getpid());
1227                write(fd, line, s);
1228
1229                write(ffd, "function", 8);
1230
1231                close(fd);
1232                close(ffd);
1233
1234                execvp(argv[1], argv+1);
1235        }
1236
1237        return 0;
1238}
1239
1240
1241hw-branch-tracer (x86 only)
1242---------------------------
1243
1244This tracer uses the x86 last branch tracing hardware feature to
1245collect a branch trace on all cpus with relatively low overhead.
1246
1247The tracer uses a fixed-size circular buffer per cpu and only
1248traces ring 0 branches. The trace file dumps that buffer in the
1249following format:
1250
1251# tracer: hw-branch-tracer
1252#
1253# CPU#        TO  <-  FROM
1254   0  scheduler_tick+0xb5/0x1bf   <-  task_tick_idle+0x5/0x6
1255   2  run_posix_cpu_timers+0x2b/0x72a     <-  run_posix_cpu_timers+0x25/0x72a
1256   0  scheduler_tick+0x139/0x1bf          <-  scheduler_tick+0xed/0x1bf
1257   0  scheduler_tick+0x17c/0x1bf          <-  scheduler_tick+0x148/0x1bf
1258   2  run_posix_cpu_timers+0x9e/0x72a     <-  run_posix_cpu_timers+0x5e/0x72a
1259   0  scheduler_tick+0x1b6/0x1bf          <-  scheduler_tick+0x1aa/0x1bf
1260
1261
1262The tracer may be used to dump the trace for the oops'ing cpu on
1263a kernel oops into the system log. To enable this,
1264ftrace_dump_on_oops must be set. To set ftrace_dump_on_oops, one
1265can either use the sysctl function or set it via the proc system
1266interface.
1267
1268  sysctl kernel.ftrace_dump_on_oops=1
1269
1270or
1271
1272  echo 1 > /proc/sys/kernel/ftrace_dump_on_oops
1273
1274
1275Here's an example of such a dump after a null pointer
1276dereference in a kernel module:
1277
1278[57848.105921] BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
1279[57848.106019] IP: [<ffffffffa0000006>] open+0x6/0x14 [oops]
1280[57848.106019] PGD 2354e9067 PUD 2375e7067 PMD 0
1281[57848.106019] Oops: 0002 [#1] SMP
1282[57848.106019] last sysfs file: /sys/devices/pci0000:00/0000:00:1e.0/0000:20:05.0/local_cpus
1283[57848.106019] Dumping ftrace buffer:
1284[57848.106019] ---------------------------------
1285[...]
1286[57848.106019]    0  chrdev_open+0xe6/0x165       <-  cdev_put+0x23/0x24
1287[57848.106019]    0  chrdev_open+0x117/0x165      <-  chrdev_open+0xfa/0x165
1288[57848.106019]    0  chrdev_open+0x120/0x165      <-  chrdev_open+0x11c/0x165
1289[57848.106019]    0  chrdev_open+0x134/0x165      <-  chrdev_open+0x12b/0x165
1290[57848.106019]    0  open+0x0/0x14 [oops]         <-  chrdev_open+0x144/0x165
1291[57848.106019]    0  page_fault+0x0/0x30          <-  open+0x6/0x14 [oops]
1292[57848.106019]    0  error_entry+0x0/0x5b         <-  page_fault+0x4/0x30
1293[57848.106019]    0  error_kernelspace+0x0/0x31   <-  error_entry+0x59/0x5b
1294[57848.106019]    0  error_sti+0x0/0x1    <-  error_kernelspace+0x2d/0x31
1295[57848.106019]    0  page_fault+0x9/0x30          <-  error_sti+0x0/0x1
1296[57848.106019]    0  do_page_fault+0x0/0x881      <-  page_fault+0x1a/0x30
1297[...]
1298[57848.106019]    0  do_page_fault+0x66b/0x881    <-  is_prefetch+0x1ee/0x1f2
1299[57848.106019]    0  do_page_fault+0x6e0/0x881    <-  do_page_fault+0x67a/0x881
1300[57848.106019]    0  oops_begin+0x0/0x96          <-  do_page_fault+0x6e0/0x881
1301[57848.106019]    0  trace_hw_branch_oops+0x0/0x2d        <-  oops_begin+0x9/0x96
1302[...]
1303[57848.106019]    0  ds_suspend_bts+0x2a/0xe3     <-  ds_suspend_bts+0x1a/0xe3
1304[57848.106019] ---------------------------------
1305[57848.106019] CPU 0
1306[57848.106019] Modules linked in: oops
1307[57848.106019] Pid: 5542, comm: cat Tainted: G        W  2.6.28 #23
1308[57848.106019] RIP: 0010:[<ffffffffa0000006>]  [<ffffffffa0000006>] open+0x6/0x14 [oops]
1309[57848.106019] RSP: 0018:ffff880235457d48  EFLAGS: 00010246
1310[...]
1311
1312
1313function graph tracer
1314---------------------------
1315
1316This tracer is similar to the function tracer except that it
1317probes a function on its entry and its exit. This is done by
1318using a dynamically allocated stack of return addresses in each
1319task_struct. On function entry the tracer overwrites the return
1320address of each function traced to set a custom probe. Thus the
1321original return address is stored on the stack of return address
1322in the task_struct.
1323
1324Probing on both ends of a function leads to special features
1325such as:
1326
1327- measure of a function's time execution
1328- having a reliable call stack to draw function calls graph
1329
1330This tracer is useful in several situations:
1331
1332- you want to find the reason of a strange kernel behavior and
1333  need to see what happens in detail on any areas (or specific
1334  ones).
1335
1336- you are experiencing weird latencies but it's difficult to
1337  find its origin.
1338
1339- you want to find quickly which path is taken by a specific
1340  function
1341
1342- you just want to peek inside a working kernel and want to see
1343  what happens there.
1344
1345# tracer: function_graph
1346#
1347# CPU  DURATION                  FUNCTION CALLS
1348# |     |   |                     |   |   |   |
1349
1350 0)               |  sys_open() {
1351 0)               |    do_sys_open() {
1352 0)               |      getname() {
1353 0)               |        kmem_cache_alloc() {
1354 0)   1.382 us    |          __might_sleep();
1355 0)   2.478 us    |        }
1356 0)               |        strncpy_from_user() {
1357 0)               |          might_fault() {
1358 0)   1.389 us    |            __might_sleep();
1359 0)   2.553 us    |          }
1360 0)   3.807 us    |        }
1361 0)   7.876 us    |      }
1362 0)               |      alloc_fd() {
1363 0)   0.668 us    |        _spin_lock();
1364 0)   0.570 us    |        expand_files();
1365 0)   0.586 us    |        _spin_unlock();
1366
1367
1368There are several columns that can be dynamically
1369enabled/disabled. You can use every combination of options you
1370want, depending on your needs.
1371
1372- The cpu number on which the function executed is default
1373  enabled.  It is sometimes better to only trace one cpu (see
1374  tracing_cpu_mask file) or you might sometimes see unordered
1375  function calls while cpu tracing switch.
1376
1377        hide: echo nofuncgraph-cpu > /debug/tracing/trace_options
1378        show: echo funcgraph-cpu > /debug/tracing/trace_options
1379
1380- The duration (function's time of execution) is displayed on
1381  the closing bracket line of a function or on the same line
1382  than the current function in case of a leaf one. It is default
1383  enabled.
1384
1385        hide: echo nofuncgraph-duration > /debug/tracing/trace_options
1386        show: echo funcgraph-duration > /debug/tracing/trace_options
1387
1388- The overhead field precedes the duration field in case of
1389  reached duration thresholds.
1390
1391        hide: echo nofuncgraph-overhead > /debug/tracing/trace_options
1392        show: echo funcgraph-overhead > /debug/tracing/trace_options
1393        depends on: funcgraph-duration
1394
1395  ie:
1396
1397  0)               |    up_write() {
1398  0)   0.646 us    |      _spin_lock_irqsave();
1399  0)   0.684 us    |      _spin_unlock_irqrestore();
1400  0)   3.123 us    |    }
1401  0)   0.548 us    |    fput();
1402  0) + 58.628 us   |  }
1403
1404  [...]
1405
1406  0)               |      putname() {
1407  0)               |        kmem_cache_free() {
1408  0)   0.518 us    |          __phys_addr();
1409  0)   1.757 us    |        }
1410  0)   2.861 us    |      }
1411  0) ! 115.305 us  |    }
1412  0) ! 116.402 us  |  }
1413
1414  + means that the function exceeded 10 usecs.
1415  ! means that the function exceeded 100 usecs.
1416
1417
1418- The task/pid field displays the thread cmdline and pid which
1419  executed the function. It is default disabled.
1420
1421        hide: echo nofuncgraph-proc > /debug/tracing/trace_options
1422        show: echo funcgraph-proc > /debug/tracing/trace_options
1423
1424  ie:
1425
1426  # tracer: function_graph
1427  #
1428  # CPU  TASK/PID        DURATION                  FUNCTION CALLS
1429  # |    |    |           |   |                     |   |   |   |
1430  0)    sh-4802     |               |                  d_free() {
1431  0)    sh-4802     |               |                    call_rcu() {
1432  0)    sh-4802     |               |                      __call_rcu() {
1433  0)    sh-4802     |   0.616 us    |                        rcu_process_gp_end();
1434  0)    sh-4802     |   0.586 us    |                        check_for_new_grace_period();
1435  0)    sh-4802     |   2.899 us    |                      }
1436  0)    sh-4802     |   4.040 us    |                    }
1437  0)    sh-4802     |   5.151 us    |                  }
1438  0)    sh-4802     | + 49.370 us   |                }
1439
1440
1441- The absolute time field is an absolute timestamp given by the
1442  system clock since it started. A snapshot of this time is
1443  given on each entry/exit of functions
1444
1445        hide: echo nofuncgraph-abstime > /debug/tracing/trace_options
1446        show: echo funcgraph-abstime > /debug/tracing/trace_options
1447
1448  ie:
1449
1450  #
1451  #      TIME       CPU  DURATION                  FUNCTION CALLS
1452  #       |         |     |   |                     |   |   |   |
1453  360.774522 |   1)   0.541 us    |                                          }
1454  360.774522 |   1)   4.663 us    |                                        }
1455  360.774523 |   1)   0.541 us    |                                        __wake_up_bit();
1456  360.774524 |   1)   6.796 us    |                                      }
1457  360.774524 |   1)   7.952 us    |                                    }
1458  360.774525 |   1)   9.063 us    |                                  }
1459  360.774525 |   1)   0.615 us    |                                  journal_mark_dirty();
1460  360.774527 |   1)   0.578 us    |                                  __brelse();
1461  360.774528 |   1)               |                                  reiserfs_prepare_for_journal() {
1462  360.774528 |   1)               |                                    unlock_buffer() {
1463  360.774529 |   1)               |                                      wake_up_bit() {
1464  360.774529 |   1)               |                                        bit_waitqueue() {
1465  360.774530 |   1)   0.594 us    |                                          __phys_addr();
1466
1467
1468You can put some comments on specific functions by using
1469trace_printk() For example, if you want to put a comment inside
1470the __might_sleep() function, you just have to include
1471<linux/ftrace.h> and call trace_printk() inside __might_sleep()
1472
1473trace_printk("I'm a comment!\n")
1474
1475will produce:
1476
1477 1)               |             __might_sleep() {
1478 1)               |                /* I'm a comment! */
1479 1)   1.449 us    |             }
1480
1481
1482You might find other useful features for this tracer in the
1483following "dynamic ftrace" section such as tracing only specific
1484functions or tasks.
1485
1486dynamic ftrace
1487--------------
1488
1489If CONFIG_DYNAMIC_FTRACE is set, the system will run with
1490virtually no overhead when function tracing is disabled. The way
1491this works is the mcount function call (placed at the start of
1492every kernel function, produced by the -pg switch in gcc),
1493starts of pointing to a simple return. (Enabling FTRACE will
1494include the -pg switch in the compiling of the kernel.)
1495
1496At compile time every C file object is run through the
1497recordmcount.pl script (located in the scripts directory). This
1498script will process the C object using objdump to find all the
1499locations in the .text section that call mcount. (Note, only the
1500.text section is processed, since processing other sections like
1501.init.text may cause races due to those sections being freed).
1502
1503A new section called "__mcount_loc" is created that holds
1504references to all the mcount call sites in the .text section.
1505This section is compiled back into the original object. The
1506final linker will add all these references into a single table.
1507
1508On boot up, before SMP is initialized, the dynamic ftrace code
1509scans this table and updates all the locations into nops. It
1510also records the locations, which are added to the
1511available_filter_functions list.  Modules are processed as they
1512are loaded and before they are executed.  When a module is
1513unloaded, it also removes its functions from the ftrace function
1514list. This is automatic in the module unload code, and the
1515module author does not need to worry about it.
1516
1517When tracing is enabled, kstop_machine is called to prevent
1518races with the CPUS executing code being modified (which can
1519cause the CPU to do undesireable things), and the nops are
1520patched back to calls. But this time, they do not call mcount
1521(which is just a function stub). They now call into the ftrace
1522infrastructure.
1523
1524One special side-effect to the recording of the functions being
1525traced is that we can now selectively choose which functions we
1526wish to trace and which ones we want the mcount calls to remain
1527as nops.
1528
1529Two files are used, one for enabling and one for disabling the
1530tracing of specified functions. They are:
1531
1532  set_ftrace_filter
1533
1534and
1535
1536  set_ftrace_notrace
1537
1538A list of available functions that you can add to these files is
1539listed in:
1540
1541   available_filter_functions
1542
1543 # cat /debug/tracing/available_filter_functions
1544put_prev_task_idle
1545kmem_cache_create
1546pick_next_task_rt
1547get_online_cpus
1548pick_next_task_fair
1549mutex_lock
1550[...]
1551
1552If I am only interested in sys_nanosleep and hrtimer_interrupt:
1553
1554 # echo sys_nanosleep hrtimer_interrupt \
1555                > /debug/tracing/set_ftrace_filter
1556 # echo ftrace > /debug/tracing/current_tracer
1557 # echo 1 > /debug/tracing/tracing_enabled
1558 # usleep 1
1559 # echo 0 > /debug/tracing/tracing_enabled
1560 # cat /debug/tracing/trace
1561# tracer: ftrace
1562#
1563#           TASK-PID   CPU#    TIMESTAMP  FUNCTION
1564#              | |      |          |         |
1565          usleep-4134  [00]  1317.070017: hrtimer_interrupt <-smp_apic_timer_interrupt
1566          usleep-4134  [00]  1317.070111: sys_nanosleep <-syscall_call
1567          <idle>-0     [00]  1317.070115: hrtimer_interrupt <-smp_apic_timer_interrupt
1568
1569To see which functions are being traced, you can cat the file:
1570
1571 # cat /debug/tracing/set_ftrace_filter
1572hrtimer_interrupt
1573sys_nanosleep
1574
1575
1576Perhaps this is not enough. The filters also allow simple wild
1577cards. Only the following are currently available
1578
1579  <match>*  - will match functions that begin with <match>
1580  *<match>  - will match functions that end with <match>
1581  *<match>* - will match functions that have <match> in it
1582
1583These are the only wild cards which are supported.
1584
1585  <match>*<match> will not work.
1586
1587Note: It is better to use quotes to enclose the wild cards,
1588      otherwise the shell may expand the parameters into names
1589      of files in the local directory.
1590
1591 # echo 'hrtimer_*' > /debug/tracing/set_ftrace_filter
1592
1593Produces:
1594
1595# tracer: ftrace
1596#
1597#           TASK-PID   CPU#    TIMESTAMP  FUNCTION
1598#              | |      |          |         |
1599            bash-4003  [00]  1480.611794: hrtimer_init <-copy_process
1600            bash-4003  [00]  1480.611941: hrtimer_start <-hrtick_set
1601            bash-4003  [00]  1480.611956: hrtimer_cancel <-hrtick_clear
1602            bash-4003  [00]  1480.611956: hrtimer_try_to_cancel <-hrtimer_cancel
1603          <idle>-0     [00]  1480.612019: hrtimer_get_next_event <-get_next_timer_interrupt
1604          <idle>-0     [00]  1480.612025: hrtimer_get_next_event <-get_next_timer_interrupt
1605          <idle>-0     [00]  1480.612032: hrtimer_get_next_event <-get_next_timer_interrupt
1606          <idle>-0     [00]  1480.612037: hrtimer_get_next_event <-get_next_timer_interrupt
1607          <idle>-0     [00]  1480.612382: hrtimer_get_next_event <-get_next_timer_interrupt
1608
1609
1610Notice that we lost the sys_nanosleep.
1611
1612 # cat /debug/tracing/set_ftrace_filter
1613hrtimer_run_queues
1614hrtimer_run_pending
1615hrtimer_init
1616hrtimer_cancel
1617hrtimer_try_to_cancel
1618hrtimer_forward
1619hrtimer_start
1620hrtimer_reprogram
1621hrtimer_force_reprogram
1622hrtimer_get_next_event
1623hrtimer_interrupt
1624hrtimer_nanosleep
1625hrtimer_wakeup
1626hrtimer_get_remaining
1627hrtimer_get_res
1628hrtimer_init_sleeper
1629
1630
1631This is because the '>' and '>>' act just like they do in bash.
1632To rewrite the filters, use '>'
1633To append to the filters, use '>>'
1634
1635To clear out a filter so that all functions will be recorded
1636again:
1637
1638 # echo > /debug/tracing/set_ftrace_filter
1639 # cat /debug/tracing/set_ftrace_filter
1640 #
1641
1642Again, now we want to append.
1643
1644 # echo sys_nanosleep > /debug/tracing/set_ftrace_filter
1645 # cat /debug/tracing/set_ftrace_filter
1646sys_nanosleep
1647 # echo 'hrtimer_*' >> /debug/tracing/set_ftrace_filter
1648 # cat /debug/tracing/set_ftrace_filter
1649hrtimer_run_queues
1650hrtimer_run_pending
1651hrtimer_init
1652hrtimer_cancel
1653hrtimer_try_to_cancel
1654hrtimer_forward
1655hrtimer_start
1656hrtimer_reprogram
1657hrtimer_force_reprogram
1658hrtimer_get_next_event
1659hrtimer_interrupt
1660sys_nanosleep
1661hrtimer_nanosleep
1662hrtimer_wakeup
1663hrtimer_get_remaining
1664hrtimer_get_res
1665hrtimer_init_sleeper
1666
1667
1668The set_ftrace_notrace prevents those functions from being
1669traced.
1670
1671 # echo '*preempt*' '*lock*' > /debug/tracing/set_ftrace_notrace
1672
1673Produces:
1674
1675# tracer: ftrace
1676#
1677#           TASK-PID   CPU#    TIMESTAMP  FUNCTION
1678#              | |      |          |         |
1679            bash-4043  [01]   115.281644: finish_task_switch <-schedule
1680            bash-4043  [01]   115.281645: hrtick_set <-schedule
1681            bash-4043  [01]   115.281645: hrtick_clear <-hrtick_set
1682            bash-4043  [01]   115.281646: wait_for_completion <-__stop_machine_run
1683            bash-4043  [01]   115.281647: wait_for_common <-wait_for_completion
1684            bash-4043  [01]   115.281647: kthread_stop <-stop_machine_run
1685            bash-4043  [01]   115.281648: init_waitqueue_head <-kthread_stop
1686            bash-4043  [01]   115.281648: wake_up_process <-kthread_stop
1687            bash-4043  [01]   115.281649: try_to_wake_up <-wake_up_process
1688
1689We can see that there's no more lock or preempt tracing.
1690
1691
1692Dynamic ftrace with the function graph tracer
1693---------------------------------------------
1694
1695Although what has been explained above concerns both the
1696function tracer and the function-graph-tracer, there are some
1697special features only available in the function-graph tracer.
1698
1699If you want to trace only one function and all of its children,
1700you just have to echo its name into set_graph_function:
1701
1702 echo __do_fault > set_graph_function
1703
1704will produce the following "expanded" trace of the __do_fault()
1705function:
1706
1707 0)               |  __do_fault() {
1708 0)               |    filemap_fault() {
1709 0)               |      find_lock_page() {
1710 0)   0.804 us    |        find_get_page();
1711 0)               |        __might_sleep() {
1712 0)   1.329 us    |        }
1713 0)   3.904 us    |      }
1714 0)   4.979 us    |    }
1715 0)   0.653 us    |    _spin_lock();
1716 0)   0.578 us    |    page_add_file_rmap();
1717 0)   0.525 us    |    native_set_pte_at();
1718 0)   0.585 us    |    _spin_unlock();
1719 0)               |    unlock_page() {
1720 0)   0.541 us    |      page_waitqueue();
1721 0)   0.639 us    |      __wake_up_bit();
1722 0)   2.786 us    |    }
1723 0) + 14.237 us   |  }
1724 0)               |  __do_fault() {
1725 0)               |    filemap_fault() {
1726 0)               |      find_lock_page() {
1727 0)   0.698 us    |        find_get_page();
1728 0)               |        __might_sleep() {
1729 0)   1.412 us    |        }
1730 0)   3.950 us    |      }
1731 0)   5.098 us    |    }
1732 0)   0.631 us    |    _spin_lock();
1733 0)   0.571 us    |    page_add_file_rmap();
1734 0)   0.526 us    |    native_set_pte_at();
1735 0)   0.586 us    |    _spin_unlock();
1736 0)               |    unlock_page() {
1737 0)   0.533 us    |      page_waitqueue();
1738 0)   0.638 us    |      __wake_up_bit();
1739 0)   2.793 us    |    }
1740 0) + 14.012 us   |  }
1741
1742You can also expand several functions at once:
1743
1744 echo sys_open > set_graph_function
1745 echo sys_close >> set_graph_function
1746
1747Now if you want to go back to trace all functions you can clear
1748this special filter via:
1749
1750 echo > set_graph_function
1751
1752
1753trace_pipe
1754----------
1755
1756The trace_pipe outputs the same content as the trace file, but
1757the effect on the tracing is different. Every read from
1758trace_pipe is consumed. This means that subsequent reads will be
1759different. The trace is live.
1760
1761 # echo function > /debug/tracing/current_tracer
1762 # cat /debug/tracing/trace_pipe > /tmp/trace.out &
1763[1] 4153
1764 # echo 1 > /debug/tracing/tracing_enabled
1765 # usleep 1
1766 # echo 0 > /debug/tracing/tracing_enabled
1767 # cat /debug/tracing/trace
1768# tracer: function
1769#
1770#           TASK-PID   CPU#    TIMESTAMP  FUNCTION
1771#              | |      |          |         |
1772
1773 #
1774 # cat /tmp/trace.out
1775            bash-4043  [00] 41.267106: finish_task_switch <-schedule
1776            bash-4043  [00] 41.267106: hrtick_set <-schedule
1777            bash-4043  [00] 41.267107: hrtick_clear <-hrtick_set
1778            bash-4043  [00] 41.267108: wait_for_completion <-__stop_machine_run
1779            bash-4043  [00] 41.267108: wait_for_common <-wait_for_completion
1780            bash-4043  [00] 41.267109: kthread_stop <-stop_machine_run
1781            bash-4043  [00] 41.267109: init_waitqueue_head <-kthread_stop
1782            bash-4043  [00] 41.267110: wake_up_process <-kthread_stop
1783            bash-4043  [00] 41.267110: try_to_wake_up <-wake_up_process
1784            bash-4043  [00] 41.267111: select_task_rq_rt <-try_to_wake_up
1785
1786
1787Note, reading the trace_pipe file will block until more input is
1788added. By changing the tracer, trace_pipe will issue an EOF. We
1789needed to set the function tracer _before_ we "cat" the
1790trace_pipe file.
1791
1792
1793trace entries
1794-------------
1795
1796Having too much or not enough data can be troublesome in
1797diagnosing an issue in the kernel. The file buffer_size_kb is
1798used to modify the size of the internal trace buffers. The
1799number listed is the number of entries that can be recorded per
1800CPU. To know the full size, multiply the number of possible CPUS
1801with the number of entries.
1802
1803 # cat /debug/tracing/buffer_size_kb
18041408 (units kilobytes)
1805
1806Note, to modify this, you must have tracing completely disabled.
1807To do that, echo "nop" into the current_tracer. If the
1808current_tracer is not set to "nop", an EINVAL error will be
1809returned.
1810
1811 # echo nop > /debug/tracing/current_tracer
1812 # echo 10000 > /debug/tracing/buffer_size_kb
1813 # cat /debug/tracing/buffer_size_kb
181410000 (units kilobytes)
1815
1816The number of pages which will be allocated is limited to a
1817percentage of available memory. Allocating too much will produce
1818an error.
1819
1820 # echo 1000000000000 > /debug/tracing/buffer_size_kb
1821-bash: echo: write error: Cannot allocate memory
1822 # cat /debug/tracing/buffer_size_kb
182385
1824
1825-----------
1826
1827More details can be found in the source code, in the
1828kernel/tracing/*.c files.
1829
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