linux/Documentation/RCU/trace.txt
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   1CONFIG_RCU_TRACE debugfs Files and Formats
   2
   3
   4The rcutree and rcutiny implementations of RCU provide debugfs trace
   5output that summarizes counters and state.  This information is useful for
   6debugging RCU itself, and can sometimes also help to debug abuses of RCU.
   7The following sections describe the debugfs files and formats, first
   8for rcutree and next for rcutiny.
   9
  10
  11CONFIG_TREE_RCU and CONFIG_TREE_PREEMPT_RCU debugfs Files and Formats
  12
  13These implementations of RCU provide several debugfs directories under the
  14top-level directory "rcu":
  15
  16rcu/rcu_bh
  17rcu/rcu_preempt
  18rcu/rcu_sched
  19
  20Each directory contains files for the corresponding flavor of RCU.
  21Note that rcu/rcu_preempt is only present for CONFIG_TREE_PREEMPT_RCU.
  22For CONFIG_TREE_RCU, the RCU flavor maps onto the RCU-sched flavor,
  23so that activity for both appears in rcu/rcu_sched.
  24
  25In addition, the following file appears in the top-level directory:
  26rcu/rcutorture.  This file displays rcutorture test progress.  The output
  27of "cat rcu/rcutorture" looks as follows:
  28
  29rcutorture test sequence: 0 (test in progress)
  30rcutorture update version number: 615
  31
  32The first line shows the number of rcutorture tests that have completed
  33since boot.  If a test is currently running, the "(test in progress)"
  34string will appear as shown above.  The second line shows the number of
  35update cycles that the current test has started, or zero if there is
  36no test in progress.
  37
  38
  39Within each flavor directory (rcu/rcu_bh, rcu/rcu_sched, and possibly
  40also rcu/rcu_preempt) the following files will be present:
  41
  42rcudata:
  43        Displays fields in struct rcu_data.
  44rcuexp:
  45        Displays statistics for expedited grace periods.
  46rcugp:
  47        Displays grace-period counters.
  48rcuhier:
  49        Displays the struct rcu_node hierarchy.
  50rcu_pending:
  51        Displays counts of the reasons rcu_pending() decided that RCU had
  52        work to do.
  53rcuboost:
  54        Displays RCU boosting statistics.  Only present if
  55        CONFIG_RCU_BOOST=y.
  56
  57The output of "cat rcu/rcu_preempt/rcudata" looks as follows:
  58
  59  0!c=30455 g=30456 pq=1 qp=1 dt=126535/140000000000000/0 df=2002 of=4 ql=0/0 qs=N... b=10 ci=74572 nci=0 co=1131 ca=716
  60  1!c=30719 g=30720 pq=1 qp=0 dt=132007/140000000000000/0 df=1874 of=10 ql=0/0 qs=N... b=10 ci=123209 nci=0 co=685 ca=982
  61  2!c=30150 g=30151 pq=1 qp=1 dt=138537/140000000000000/0 df=1707 of=8 ql=0/0 qs=N... b=10 ci=80132 nci=0 co=1328 ca=1458
  62  3 c=31249 g=31250 pq=1 qp=0 dt=107255/140000000000000/0 df=1749 of=6 ql=0/450 qs=NRW. b=10 ci=151700 nci=0 co=509 ca=622
  63  4!c=29502 g=29503 pq=1 qp=1 dt=83647/140000000000000/0 df=965 of=5 ql=0/0 qs=N... b=10 ci=65643 nci=0 co=1373 ca=1521
  64  5 c=31201 g=31202 pq=1 qp=1 dt=70422/0/0 df=535 of=7 ql=0/0 qs=.... b=10 ci=58500 nci=0 co=764 ca=698
  65  6!c=30253 g=30254 pq=1 qp=1 dt=95363/140000000000000/0 df=780 of=5 ql=0/0 qs=N... b=10 ci=100607 nci=0 co=1414 ca=1353
  66  7 c=31178 g=31178 pq=1 qp=0 dt=91536/0/0 df=547 of=4 ql=0/0 qs=.... b=10 ci=109819 nci=0 co=1115 ca=969
  67
  68This file has one line per CPU, or eight for this 8-CPU system.
  69The fields are as follows:
  70
  71o       The number at the beginning of each line is the CPU number.
  72        CPUs numbers followed by an exclamation mark are offline,
  73        but have been online at least once since boot.  There will be
  74        no output for CPUs that have never been online, which can be
  75        a good thing in the surprisingly common case where NR_CPUS is
  76        substantially larger than the number of actual CPUs.
  77
  78o       "c" is the count of grace periods that this CPU believes have
  79        completed.  Offlined CPUs and CPUs in dynticks idle mode may lag
  80        quite a ways behind, for example, CPU 4 under "rcu_sched" above,
  81        which has been offline through 16 RCU grace periods.  It is not
  82        unusual to see offline CPUs lagging by thousands of grace periods.
  83        Note that although the grace-period number is an unsigned long,
  84        it is printed out as a signed long to allow more human-friendly
  85        representation near boot time.
  86
  87o       "g" is the count of grace periods that this CPU believes have
  88        started.  Again, offlined CPUs and CPUs in dynticks idle mode
  89        may lag behind.  If the "c" and "g" values are equal, this CPU
  90        has already reported a quiescent state for the last RCU grace
  91        period that it is aware of, otherwise, the CPU believes that it
  92        owes RCU a quiescent state.
  93
  94o       "pq" indicates that this CPU has passed through a quiescent state
  95        for the current grace period.  It is possible for "pq" to be
  96        "1" and "c" different than "g", which indicates that although
  97        the CPU has passed through a quiescent state, either (1) this
  98        CPU has not yet reported that fact, (2) some other CPU has not
  99        yet reported for this grace period, or (3) both.
 100
 101o       "qp" indicates that RCU still expects a quiescent state from
 102        this CPU.  Offlined CPUs and CPUs in dyntick idle mode might
 103        well have qp=1, which is OK: RCU is still ignoring them.
 104
 105o       "dt" is the current value of the dyntick counter that is incremented
 106        when entering or leaving idle, either due to a context switch or
 107        due to an interrupt.  This number is even if the CPU is in idle
 108        from RCU's viewpoint and odd otherwise.  The number after the
 109        first "/" is the interrupt nesting depth when in idle state,
 110        or a large number added to the interrupt-nesting depth when
 111        running a non-idle task.  Some architectures do not accurately
 112        count interrupt nesting when running in non-idle kernel context,
 113        which can result in interesting anomalies such as negative
 114        interrupt-nesting levels.  The number after the second "/"
 115        is the NMI nesting depth.
 116
 117o       "df" is the number of times that some other CPU has forced a
 118        quiescent state on behalf of this CPU due to this CPU being in
 119        idle state.
 120
 121o       "of" is the number of times that some other CPU has forced a
 122        quiescent state on behalf of this CPU due to this CPU being
 123        offline.  In a perfect world, this might never happen, but it
 124        turns out that offlining and onlining a CPU can take several grace
 125        periods, and so there is likely to be an extended period of time
 126        when RCU believes that the CPU is online when it really is not.
 127        Please note that erring in the other direction (RCU believing a
 128        CPU is offline when it is really alive and kicking) is a fatal
 129        error, so it makes sense to err conservatively.
 130
 131o       "ql" is the number of RCU callbacks currently residing on
 132        this CPU.  The first number is the number of "lazy" callbacks
 133        that are known to RCU to only be freeing memory, and the number
 134        after the "/" is the total number of callbacks, lazy or not.
 135        These counters count callbacks regardless of what phase of
 136        grace-period processing that they are in (new, waiting for
 137        grace period to start, waiting for grace period to end, ready
 138        to invoke).
 139
 140o       "qs" gives an indication of the state of the callback queue
 141        with four characters:
 142
 143        "N"     Indicates that there are callbacks queued that are not
 144                ready to be handled by the next grace period, and thus
 145                will be handled by the grace period following the next
 146                one.
 147
 148        "R"     Indicates that there are callbacks queued that are
 149                ready to be handled by the next grace period.
 150
 151        "W"     Indicates that there are callbacks queued that are
 152                waiting on the current grace period.
 153
 154        "D"     Indicates that there are callbacks queued that have
 155                already been handled by a prior grace period, and are
 156                thus waiting to be invoked.  Note that callbacks in
 157                the process of being invoked are not counted here.
 158                Callbacks in the process of being invoked are those
 159                that have been removed from the rcu_data structures
 160                queues by rcu_do_batch(), but which have not yet been
 161                invoked.
 162
 163        If there are no callbacks in a given one of the above states,
 164        the corresponding character is replaced by ".".
 165
 166o       "b" is the batch limit for this CPU.  If more than this number
 167        of RCU callbacks is ready to invoke, then the remainder will
 168        be deferred.
 169
 170o       "ci" is the number of RCU callbacks that have been invoked for
 171        this CPU.  Note that ci+nci+ql is the number of callbacks that have
 172        been registered in absence of CPU-hotplug activity.
 173
 174o       "nci" is the number of RCU callbacks that have been offloaded from
 175        this CPU.  This will always be zero unless the kernel was built
 176        with CONFIG_RCU_NOCB_CPU=y and the "rcu_nocbs=" kernel boot
 177        parameter was specified.
 178
 179o       "co" is the number of RCU callbacks that have been orphaned due to
 180        this CPU going offline.  These orphaned callbacks have been moved
 181        to an arbitrarily chosen online CPU.
 182
 183o       "ca" is the number of RCU callbacks that have been adopted by this
 184        CPU due to other CPUs going offline.  Note that ci+co-ca+ql is
 185        the number of RCU callbacks registered on this CPU.
 186
 187
 188Kernels compiled with CONFIG_RCU_BOOST=y display the following from
 189/debug/rcu/rcu_preempt/rcudata:
 190
 191  0!c=12865 g=12866 pq=1 qp=1 dt=83113/140000000000000/0 df=288 of=11 ql=0/0 qs=N... kt=0/O ktl=944 b=10 ci=60709 nci=0 co=748 ca=871
 192  1 c=14407 g=14408 pq=1 qp=0 dt=100679/140000000000000/0 df=378 of=7 ql=0/119 qs=NRW. kt=0/W ktl=9b6 b=10 ci=109740 nci=0 co=589 ca=485
 193  2 c=14407 g=14408 pq=1 qp=0 dt=105486/0/0 df=90 of=9 ql=0/89 qs=NRW. kt=0/W ktl=c0c b=10 ci=83113 nci=0 co=533 ca=490
 194  3 c=14407 g=14408 pq=1 qp=0 dt=107138/0/0 df=142 of=8 ql=0/188 qs=NRW. kt=0/W ktl=b96 b=10 ci=121114 nci=0 co=426 ca=290
 195  4 c=14405 g=14406 pq=1 qp=1 dt=50238/0/0 df=706 of=7 ql=0/0 qs=.... kt=0/W ktl=812 b=10 ci=34929 nci=0 co=643 ca=114
 196  5!c=14168 g=14169 pq=1 qp=0 dt=45465/140000000000000/0 df=161 of=11 ql=0/0 qs=N... kt=0/O ktl=b4d b=10 ci=47712 nci=0 co=677 ca=722
 197  6 c=14404 g=14405 pq=1 qp=0 dt=59454/0/0 df=94 of=6 ql=0/0 qs=.... kt=0/W ktl=e57 b=10 ci=55597 nci=0 co=701 ca=811
 198  7 c=14407 g=14408 pq=1 qp=1 dt=68850/0/0 df=31 of=8 ql=0/0 qs=.... kt=0/W ktl=14bd b=10 ci=77475 nci=0 co=508 ca=1042
 199
 200This is similar to the output discussed above, but contains the following
 201additional fields:
 202
 203o       "kt" is the per-CPU kernel-thread state.  The digit preceding
 204        the first slash is zero if there is no work pending and 1
 205        otherwise.  The character between the first pair of slashes is
 206        as follows:
 207
 208        "S"     The kernel thread is stopped, in other words, all
 209                CPUs corresponding to this rcu_node structure are
 210                offline.
 211
 212        "R"     The kernel thread is running.
 213
 214        "W"     The kernel thread is waiting because there is no work
 215                for it to do.
 216
 217        "O"     The kernel thread is waiting because it has been
 218                forced off of its designated CPU or because its
 219                ->cpus_allowed mask permits it to run on other than
 220                its designated CPU.
 221
 222        "Y"     The kernel thread is yielding to avoid hogging CPU.
 223
 224        "?"     Unknown value, indicates a bug.
 225
 226        The number after the final slash is the CPU that the kthread
 227        is actually running on.
 228
 229        This field is displayed only for CONFIG_RCU_BOOST kernels.
 230
 231o       "ktl" is the low-order 16 bits (in hexadecimal) of the count of
 232        the number of times that this CPU's per-CPU kthread has gone
 233        through its loop servicing invoke_rcu_cpu_kthread() requests.
 234
 235        This field is displayed only for CONFIG_RCU_BOOST kernels.
 236
 237
 238The output of "cat rcu/rcu_preempt/rcuexp" looks as follows:
 239
 240s=21872 d=21872 w=0 tf=0 wd1=0 wd2=0 n=0 sc=21872 dt=21872 dl=0 dx=21872
 241
 242These fields are as follows:
 243
 244o       "s" is the starting sequence number.
 245
 246o       "d" is the ending sequence number.  When the starting and ending
 247        numbers differ, there is an expedited grace period in progress.
 248
 249o       "w" is the number of times that the sequence numbers have been
 250        in danger of wrapping.
 251
 252o       "tf" is the number of times that contention has resulted in a
 253        failure to begin an expedited grace period.
 254
 255o       "wd1" and "wd2" are the number of times that an attempt to
 256        start an expedited grace period found that someone else had
 257        completed an expedited grace period that satisfies the
 258        attempted request.  "Our work is done."
 259
 260o       "n" is number of times that contention was so great that
 261        the request was demoted from an expedited grace period to
 262        a normal grace period.
 263
 264o       "sc" is the number of times that the attempt to start a
 265        new expedited grace period succeeded.
 266
 267o       "dt" is the number of times that we attempted to update
 268        the "d" counter.
 269
 270o       "dl" is the number of times that we failed to update the "d"
 271        counter.
 272
 273o       "dx" is the number of times that we succeeded in updating
 274        the "d" counter.
 275
 276
 277The output of "cat rcu/rcu_preempt/rcugp" looks as follows:
 278
 279completed=31249  gpnum=31250  age=1  max=18
 280
 281These fields are taken from the rcu_state structure, and are as follows:
 282
 283o       "completed" is the number of grace periods that have completed.
 284        It is comparable to the "c" field from rcu/rcudata in that a
 285        CPU whose "c" field matches the value of "completed" is aware
 286        that the corresponding RCU grace period has completed.
 287
 288o       "gpnum" is the number of grace periods that have started.  It is
 289        similarly comparable to the "g" field from rcu/rcudata in that
 290        a CPU whose "g" field matches the value of "gpnum" is aware that
 291        the corresponding RCU grace period has started.
 292
 293        If these two fields are equal, then there is no grace period
 294        in progress, in other words, RCU is idle.  On the other hand,
 295        if the two fields differ (as they are above), then an RCU grace
 296        period is in progress.
 297
 298o       "age" is the number of jiffies that the current grace period
 299        has extended for, or zero if there is no grace period currently
 300        in effect.
 301
 302o       "max" is the age in jiffies of the longest-duration grace period
 303        thus far.
 304
 305The output of "cat rcu/rcu_preempt/rcuhier" looks as follows:
 306
 307c=14407 g=14408 s=0 jfq=2 j=c863 nfqs=12040/nfqsng=0(12040) fqlh=1051 oqlen=0/0
 3083/3 ..>. 0:7 ^0
 309e/e ..>. 0:3 ^0    d/d ..>. 4:7 ^1
 310
 311The fields are as follows:
 312
 313o       "c" is exactly the same as "completed" under rcu/rcu_preempt/rcugp.
 314
 315o       "g" is exactly the same as "gpnum" under rcu/rcu_preempt/rcugp.
 316
 317o       "s" is the current state of the force_quiescent_state()
 318        state machine.
 319
 320o       "jfq" is the number of jiffies remaining for this grace period
 321        before force_quiescent_state() is invoked to help push things
 322        along.  Note that CPUs in idle mode throughout the grace period
 323        will not report on their own, but rather must be check by some
 324        other CPU via force_quiescent_state().
 325
 326o       "j" is the low-order four hex digits of the jiffies counter.
 327        Yes, Paul did run into a number of problems that turned out to
 328        be due to the jiffies counter no longer counting.  Why do you ask?
 329
 330o       "nfqs" is the number of calls to force_quiescent_state() since
 331        boot.
 332
 333o       "nfqsng" is the number of useless calls to force_quiescent_state(),
 334        where there wasn't actually a grace period active.  This can
 335        no longer happen due to grace-period processing being pushed
 336        into a kthread.  The number in parentheses is the difference
 337        between "nfqs" and "nfqsng", or the number of times that
 338        force_quiescent_state() actually did some real work.
 339
 340o       "fqlh" is the number of calls to force_quiescent_state() that
 341        exited immediately (without even being counted in nfqs above)
 342        due to contention on ->fqslock.
 343
 344o       Each element of the form "3/3 ..>. 0:7 ^0" represents one rcu_node
 345        structure.  Each line represents one level of the hierarchy,
 346        from root to leaves.  It is best to think of the rcu_data
 347        structures as forming yet another level after the leaves.
 348        Note that there might be either one, two, three, or even four
 349        levels of rcu_node structures, depending on the relationship
 350        between CONFIG_RCU_FANOUT, CONFIG_RCU_FANOUT_LEAF (possibly
 351        adjusted using the rcu_fanout_leaf kernel boot parameter), and
 352        CONFIG_NR_CPUS (possibly adjusted using the nr_cpu_ids count of
 353        possible CPUs for the booting hardware).
 354
 355        o       The numbers separated by the "/" are the qsmask followed
 356                by the qsmaskinit.  The qsmask will have one bit
 357                set for each entity in the next lower level that has
 358                not yet checked in for the current grace period ("e"
 359                indicating CPUs 5, 6, and 7 in the example above).
 360                The qsmaskinit will have one bit for each entity that is
 361                currently expected to check in during each grace period.
 362                The value of qsmaskinit is assigned to that of qsmask
 363                at the beginning of each grace period.
 364
 365        o       The characters separated by the ">" indicate the state
 366                of the blocked-tasks lists.  A "G" preceding the ">"
 367                indicates that at least one task blocked in an RCU
 368                read-side critical section blocks the current grace
 369                period, while a "E" preceding the ">" indicates that
 370                at least one task blocked in an RCU read-side critical
 371                section blocks the current expedited grace period.
 372                A "T" character following the ">" indicates that at
 373                least one task is blocked within an RCU read-side
 374                critical section, regardless of whether any current
 375                grace period (expedited or normal) is inconvenienced.
 376                A "." character appears if the corresponding condition
 377                does not hold, so that "..>." indicates that no tasks
 378                are blocked.  In contrast, "GE>T" indicates maximal
 379                inconvenience from blocked tasks.  CONFIG_TREE_RCU
 380                builds of the kernel will always show "..>.".
 381
 382        o       The numbers separated by the ":" are the range of CPUs
 383                served by this struct rcu_node.  This can be helpful
 384                in working out how the hierarchy is wired together.
 385
 386                For example, the example rcu_node structure shown above
 387                has "0:7", indicating that it covers CPUs 0 through 7.
 388
 389        o       The number after the "^" indicates the bit in the
 390                next higher level rcu_node structure that this rcu_node
 391                structure corresponds to.  For example, the "d/d ..>. 4:7
 392                ^1" has a "1" in this position, indicating that it
 393                corresponds to the "1" bit in the "3" shown in the
 394                "3/3 ..>. 0:7 ^0" entry on the next level up.
 395
 396
 397The output of "cat rcu/rcu_sched/rcu_pending" looks as follows:
 398
 399  0!np=26111 qsp=29 rpq=5386 cbr=1 cng=570 gpc=3674 gps=577 nn=15903
 400  1!np=28913 qsp=35 rpq=6097 cbr=1 cng=448 gpc=3700 gps=554 nn=18113
 401  2!np=32740 qsp=37 rpq=6202 cbr=0 cng=476 gpc=4627 gps=546 nn=20889
 402  3 np=23679 qsp=22 rpq=5044 cbr=1 cng=415 gpc=3403 gps=347 nn=14469
 403  4!np=30714 qsp=4 rpq=5574 cbr=0 cng=528 gpc=3931 gps=639 nn=20042
 404  5 np=28910 qsp=2 rpq=5246 cbr=0 cng=428 gpc=4105 gps=709 nn=18422
 405  6!np=38648 qsp=5 rpq=7076 cbr=0 cng=840 gpc=4072 gps=961 nn=25699
 406  7 np=37275 qsp=2 rpq=6873 cbr=0 cng=868 gpc=3416 gps=971 nn=25147
 407
 408The fields are as follows:
 409
 410o       The leading number is the CPU number, with "!" indicating
 411        an offline CPU.
 412
 413o       "np" is the number of times that __rcu_pending() has been invoked
 414        for the corresponding flavor of RCU.
 415
 416o       "qsp" is the number of times that the RCU was waiting for a
 417        quiescent state from this CPU.
 418
 419o       "rpq" is the number of times that the CPU had passed through
 420        a quiescent state, but not yet reported it to RCU.
 421
 422o       "cbr" is the number of times that this CPU had RCU callbacks
 423        that had passed through a grace period, and were thus ready
 424        to be invoked.
 425
 426o       "cng" is the number of times that this CPU needed another
 427        grace period while RCU was idle.
 428
 429o       "gpc" is the number of times that an old grace period had
 430        completed, but this CPU was not yet aware of it.
 431
 432o       "gps" is the number of times that a new grace period had started,
 433        but this CPU was not yet aware of it.
 434
 435o       "nn" is the number of times that this CPU needed nothing.
 436
 437
 438The output of "cat rcu/rcuboost" looks as follows:
 439
 4400:3 tasks=.... kt=W ntb=0 neb=0 nnb=0 j=c864 bt=c894
 441    balk: nt=0 egt=4695 bt=0 nb=0 ny=56 nos=0
 4424:7 tasks=.... kt=W ntb=0 neb=0 nnb=0 j=c864 bt=c894
 443    balk: nt=0 egt=6541 bt=0 nb=0 ny=126 nos=0
 444
 445This information is output only for rcu_preempt.  Each two-line entry
 446corresponds to a leaf rcu_node strcuture.  The fields are as follows:
 447
 448o       "n:m" is the CPU-number range for the corresponding two-line
 449        entry.  In the sample output above, the first entry covers
 450        CPUs zero through three and the second entry covers CPUs four
 451        through seven.
 452
 453o       "tasks=TNEB" gives the state of the various segments of the
 454        rnp->blocked_tasks list:
 455
 456        "T"     This indicates that there are some tasks that blocked
 457                while running on one of the corresponding CPUs while
 458                in an RCU read-side critical section.
 459
 460        "N"     This indicates that some of the blocked tasks are preventing
 461                the current normal (non-expedited) grace period from
 462                completing.
 463
 464        "E"     This indicates that some of the blocked tasks are preventing
 465                the current expedited grace period from completing.
 466
 467        "B"     This indicates that some of the blocked tasks are in
 468                need of RCU priority boosting.
 469
 470        Each character is replaced with "." if the corresponding
 471        condition does not hold.
 472
 473o       "kt" is the state of the RCU priority-boosting kernel
 474        thread associated with the corresponding rcu_node structure.
 475        The state can be one of the following:
 476
 477        "S"     The kernel thread is stopped, in other words, all
 478                CPUs corresponding to this rcu_node structure are
 479                offline.
 480
 481        "R"     The kernel thread is running.
 482
 483        "W"     The kernel thread is waiting because there is no work
 484                for it to do.
 485
 486        "Y"     The kernel thread is yielding to avoid hogging CPU.
 487
 488        "?"     Unknown value, indicates a bug.
 489
 490o       "ntb" is the number of tasks boosted.
 491
 492o       "neb" is the number of tasks boosted in order to complete an
 493        expedited grace period.
 494
 495o       "nnb" is the number of tasks boosted in order to complete a
 496        normal (non-expedited) grace period.  When boosting a task
 497        that was blocking both an expedited and a normal grace period,
 498        it is counted against the expedited total above.
 499
 500o       "j" is the low-order 16 bits of the jiffies counter in
 501        hexadecimal.
 502
 503o       "bt" is the low-order 16 bits of the value that the jiffies
 504        counter will have when we next start boosting, assuming that
 505        the current grace period does not end beforehand.  This is
 506        also in hexadecimal.
 507
 508o       "balk: nt" counts the number of times we didn't boost (in
 509        other words, we balked) even though it was time to boost because
 510        there were no blocked tasks to boost.  This situation occurs
 511        when there is one blocked task on one rcu_node structure and
 512        none on some other rcu_node structure.
 513
 514o       "egt" counts the number of times we balked because although
 515        there were blocked tasks, none of them were blocking the
 516        current grace period, whether expedited or otherwise.
 517
 518o       "bt" counts the number of times we balked because boosting
 519        had already been initiated for the current grace period.
 520
 521o       "nb" counts the number of times we balked because there
 522        was at least one task blocking the current non-expedited grace
 523        period that never had blocked.  If it is already running, it
 524        just won't help to boost its priority!
 525
 526o       "ny" counts the number of times we balked because it was
 527        not yet time to start boosting.
 528
 529o       "nos" counts the number of times we balked for other
 530        reasons, e.g., the grace period ended first.
 531
 532
 533CONFIG_TINY_RCU and CONFIG_TINY_PREEMPT_RCU debugfs Files and Formats
 534
 535These implementations of RCU provides a single debugfs file under the
 536top-level directory RCU, namely rcu/rcudata, which displays fields in
 537rcu_bh_ctrlblk, rcu_sched_ctrlblk and, for CONFIG_TINY_PREEMPT_RCU,
 538rcu_preempt_ctrlblk.
 539
 540The output of "cat rcu/rcudata" is as follows:
 541
 542rcu_preempt: qlen=24 gp=1097669 g197/p197/c197 tasks=...
 543             ttb=. btg=no ntb=184 neb=0 nnb=183 j=01f7 bt=0274
 544             normal balk: nt=1097669 gt=0 bt=371 b=0 ny=25073378 nos=0
 545             exp balk: bt=0 nos=0
 546rcu_sched: qlen: 0
 547rcu_bh: qlen: 0
 548
 549This is split into rcu_preempt, rcu_sched, and rcu_bh sections, with the
 550rcu_preempt section appearing only in CONFIG_TINY_PREEMPT_RCU builds.
 551The last three lines of the rcu_preempt section appear only in
 552CONFIG_RCU_BOOST kernel builds.  The fields are as follows:
 553
 554o       "qlen" is the number of RCU callbacks currently waiting either
 555        for an RCU grace period or waiting to be invoked.  This is the
 556        only field present for rcu_sched and rcu_bh, due to the
 557        short-circuiting of grace period in those two cases.
 558
 559o       "gp" is the number of grace periods that have completed.
 560
 561o       "g197/p197/c197" displays the grace-period state, with the
 562        "g" number being the number of grace periods that have started
 563        (mod 256), the "p" number being the number of grace periods
 564        that the CPU has responded to (also mod 256), and the "c"
 565        number being the number of grace periods that have completed
 566        (once again mode 256).
 567
 568        Why have both "gp" and "g"?  Because the data flowing into
 569        "gp" is only present in a CONFIG_RCU_TRACE kernel.
 570
 571o       "tasks" is a set of bits.  The first bit is "T" if there are
 572        currently tasks that have recently blocked within an RCU
 573        read-side critical section, the second bit is "N" if any of the
 574        aforementioned tasks are blocking the current RCU grace period,
 575        and the third bit is "E" if any of the aforementioned tasks are
 576        blocking the current expedited grace period.  Each bit is "."
 577        if the corresponding condition does not hold.
 578
 579o       "ttb" is a single bit.  It is "B" if any of the blocked tasks
 580        need to be priority boosted and "." otherwise.
 581
 582o       "btg" indicates whether boosting has been carried out during
 583        the current grace period, with "exp" indicating that boosting
 584        is in progress for an expedited grace period, "no" indicating
 585        that boosting has not yet started for a normal grace period,
 586        "begun" indicating that boosting has bebug for a normal grace
 587        period, and "done" indicating that boosting has completed for
 588        a normal grace period.
 589
 590o       "ntb" is the total number of tasks subjected to RCU priority boosting
 591        periods since boot.
 592
 593o       "neb" is the number of expedited grace periods that have had
 594        to resort to RCU priority boosting since boot.
 595
 596o       "nnb" is the number of normal grace periods that have had
 597        to resort to RCU priority boosting since boot.
 598
 599o       "j" is the low-order 16 bits of the jiffies counter in hexadecimal.
 600
 601o       "bt" is the low-order 16 bits of the value that the jiffies counter
 602        will have at the next time that boosting is scheduled to begin.
 603
 604o       In the line beginning with "normal balk", the fields are as follows:
 605
 606        o       "nt" is the number of times that the system balked from
 607                boosting because there were no blocked tasks to boost.
 608                Note that the system will balk from boosting even if the
 609                grace period is overdue when the currently running task
 610                is looping within an RCU read-side critical section.
 611                There is no point in boosting in this case, because
 612                boosting a running task won't make it run any faster.
 613
 614        o       "gt" is the number of times that the system balked
 615                from boosting because, although there were blocked tasks,
 616                none of them were preventing the current grace period
 617                from completing.
 618
 619        o       "bt" is the number of times that the system balked
 620                from boosting because boosting was already in progress.
 621
 622        o       "b" is the number of times that the system balked from
 623                boosting because boosting had already completed for
 624                the grace period in question.
 625
 626        o       "ny" is the number of times that the system balked from
 627                boosting because it was not yet time to start boosting
 628                the grace period in question.
 629
 630        o       "nos" is the number of times that the system balked from
 631                boosting for inexplicable ("not otherwise specified")
 632                reasons.  This can actually happen due to races involving
 633                increments of the jiffies counter.
 634
 635o       In the line beginning with "exp balk", the fields are as follows:
 636
 637        o       "bt" is the number of times that the system balked from
 638                boosting because there were no blocked tasks to boost.
 639
 640        o       "nos" is the number of times that the system balked from
 641                 boosting for inexplicable ("not otherwise specified")
 642                 reasons.
 643
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