linux/kernel/latencytop.c
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   1/*
   2 * latencytop.c: Latency display infrastructure
   3 *
   4 * (C) Copyright 2008 Intel Corporation
   5 * Author: Arjan van de Ven <arjan@linux.intel.com>
   6 *
   7 * This program is free software; you can redistribute it and/or
   8 * modify it under the terms of the GNU General Public License
   9 * as published by the Free Software Foundation; version 2
  10 * of the License.
  11 */
  12
  13/*
  14 * CONFIG_LATENCYTOP enables a kernel latency tracking infrastructure that is
  15 * used by the "latencytop" userspace tool. The latency that is tracked is not
  16 * the 'traditional' interrupt latency (which is primarily caused by something
  17 * else consuming CPU), but instead, it is the latency an application encounters
  18 * because the kernel sleeps on its behalf for various reasons.
  19 *
  20 * This code tracks 2 levels of statistics:
  21 * 1) System level latency
  22 * 2) Per process latency
  23 *
  24 * The latency is stored in fixed sized data structures in an accumulated form;
  25 * if the "same" latency cause is hit twice, this will be tracked as one entry
  26 * in the data structure. Both the count, total accumulated latency and maximum
  27 * latency are tracked in this data structure. When the fixed size structure is
  28 * full, no new causes are tracked until the buffer is flushed by writing to
  29 * the /proc file; the userspace tool does this on a regular basis.
  30 *
  31 * A latency cause is identified by a stringified backtrace at the point that
  32 * the scheduler gets invoked. The userland tool will use this string to
  33 * identify the cause of the latency in human readable form.
  34 *
  35 * The information is exported via /proc/latency_stats and /proc/<pid>/latency.
  36 * These files look like this:
  37 *
  38 * Latency Top version : v0.1
  39 * 70 59433 4897 i915_irq_wait drm_ioctl vfs_ioctl do_vfs_ioctl sys_ioctl
  40 * |    |    |    |
  41 * |    |    |    +----> the stringified backtrace
  42 * |    |    +---------> The maximum latency for this entry in microseconds
  43 * |    +--------------> The accumulated latency for this entry (microseconds)
  44 * +-------------------> The number of times this entry is hit
  45 *
  46 * (note: the average latency is the accumulated latency divided by the number
  47 * of times)
  48 */
  49
  50#include <linux/latencytop.h>
  51#include <linux/kallsyms.h>
  52#include <linux/seq_file.h>
  53#include <linux/notifier.h>
  54#include <linux/spinlock.h>
  55#include <linux/proc_fs.h>
  56#include <linux/module.h>
  57#include <linux/sched.h>
  58#include <linux/list.h>
  59#include <linux/slab.h>
  60#include <linux/stacktrace.h>
  61
  62static DEFINE_SPINLOCK(latency_lock);
  63
  64#define MAXLR 128
  65static struct latency_record latency_record[MAXLR];
  66
  67int latencytop_enabled;
  68
  69void clear_all_latency_tracing(struct task_struct *p)
  70{
  71        unsigned long flags;
  72
  73        if (!latencytop_enabled)
  74                return;
  75
  76        spin_lock_irqsave(&latency_lock, flags);
  77        memset(&p->latency_record, 0, sizeof(p->latency_record));
  78        p->latency_record_count = 0;
  79        spin_unlock_irqrestore(&latency_lock, flags);
  80}
  81
  82static void clear_global_latency_tracing(void)
  83{
  84        unsigned long flags;
  85
  86        spin_lock_irqsave(&latency_lock, flags);
  87        memset(&latency_record, 0, sizeof(latency_record));
  88        spin_unlock_irqrestore(&latency_lock, flags);
  89}
  90
  91static void __sched
  92account_global_scheduler_latency(struct task_struct *tsk, struct latency_record *lat)
  93{
  94        int firstnonnull = MAXLR + 1;
  95        int i;
  96
  97        if (!latencytop_enabled)
  98                return;
  99
 100        /* skip kernel threads for now */
 101        if (!tsk->mm)
 102                return;
 103
 104        for (i = 0; i < MAXLR; i++) {
 105                int q, same = 1;
 106
 107                /* Nothing stored: */
 108                if (!latency_record[i].backtrace[0]) {
 109                        if (firstnonnull > i)
 110                                firstnonnull = i;
 111                        continue;
 112                }
 113                for (q = 0; q < LT_BACKTRACEDEPTH; q++) {
 114                        unsigned long record = lat->backtrace[q];
 115
 116                        if (latency_record[i].backtrace[q] != record) {
 117                                same = 0;
 118                                break;
 119                        }
 120
 121                        /* 0 and ULONG_MAX entries mean end of backtrace: */
 122                        if (record == 0 || record == ULONG_MAX)
 123                                break;
 124                }
 125                if (same) {
 126                        latency_record[i].count++;
 127                        latency_record[i].time += lat->time;
 128                        if (lat->time > latency_record[i].max)
 129                                latency_record[i].max = lat->time;
 130                        return;
 131                }
 132        }
 133
 134        i = firstnonnull;
 135        if (i >= MAXLR - 1)
 136                return;
 137
 138        /* Allocted a new one: */
 139        memcpy(&latency_record[i], lat, sizeof(struct latency_record));
 140}
 141
 142/*
 143 * Iterator to store a backtrace into a latency record entry
 144 */
 145static inline void store_stacktrace(struct task_struct *tsk,
 146                                        struct latency_record *lat)
 147{
 148        struct stack_trace trace;
 149
 150        memset(&trace, 0, sizeof(trace));
 151        trace.max_entries = LT_BACKTRACEDEPTH;
 152        trace.entries = &lat->backtrace[0];
 153        save_stack_trace_tsk(tsk, &trace);
 154}
 155
 156/**
 157 * __account_scheduler_latency - record an occured latency
 158 * @tsk - the task struct of the task hitting the latency
 159 * @usecs - the duration of the latency in microseconds
 160 * @inter - 1 if the sleep was interruptible, 0 if uninterruptible
 161 *
 162 * This function is the main entry point for recording latency entries
 163 * as called by the scheduler.
 164 *
 165 * This function has a few special cases to deal with normal 'non-latency'
 166 * sleeps: specifically, interruptible sleep longer than 5 msec is skipped
 167 * since this usually is caused by waiting for events via select() and co.
 168 *
 169 * Negative latencies (caused by time going backwards) are also explicitly
 170 * skipped.
 171 */
 172void __sched
 173__account_scheduler_latency(struct task_struct *tsk, int usecs, int inter)
 174{
 175        unsigned long flags;
 176        int i, q;
 177        struct latency_record lat;
 178
 179        /* Long interruptible waits are generally user requested... */
 180        if (inter && usecs > 5000)
 181                return;
 182
 183        /* Negative sleeps are time going backwards */
 184        /* Zero-time sleeps are non-interesting */
 185        if (usecs <= 0)
 186                return;
 187
 188        memset(&lat, 0, sizeof(lat));
 189        lat.count = 1;
 190        lat.time = usecs;
 191        lat.max = usecs;
 192        store_stacktrace(tsk, &lat);
 193
 194        spin_lock_irqsave(&latency_lock, flags);
 195
 196        account_global_scheduler_latency(tsk, &lat);
 197
 198        /*
 199         * short term hack; if we're > 32 we stop; future we recycle:
 200         */
 201        tsk->latency_record_count++;
 202        if (tsk->latency_record_count >= LT_SAVECOUNT)
 203                goto out_unlock;
 204
 205        for (i = 0; i < LT_SAVECOUNT; i++) {
 206                struct latency_record *mylat;
 207                int same = 1;
 208
 209                mylat = &tsk->latency_record[i];
 210                for (q = 0; q < LT_BACKTRACEDEPTH; q++) {
 211                        unsigned long record = lat.backtrace[q];
 212
 213                        if (mylat->backtrace[q] != record) {
 214                                same = 0;
 215                                break;
 216                        }
 217
 218                        /* 0 and ULONG_MAX entries mean end of backtrace: */
 219                        if (record == 0 || record == ULONG_MAX)
 220                                break;
 221                }
 222                if (same) {
 223                        mylat->count++;
 224                        mylat->time += lat.time;
 225                        if (lat.time > mylat->max)
 226                                mylat->max = lat.time;
 227                        goto out_unlock;
 228                }
 229        }
 230
 231        /* Allocated a new one: */
 232        i = tsk->latency_record_count;
 233        memcpy(&tsk->latency_record[i], &lat, sizeof(struct latency_record));
 234
 235out_unlock:
 236        spin_unlock_irqrestore(&latency_lock, flags);
 237}
 238
 239static int lstats_show(struct seq_file *m, void *v)
 240{
 241        int i;
 242
 243        seq_puts(m, "Latency Top version : v0.1\n");
 244
 245        for (i = 0; i < MAXLR; i++) {
 246                if (latency_record[i].backtrace[0]) {
 247                        int q;
 248                        seq_printf(m, "%i %lu %lu ",
 249                                latency_record[i].count,
 250                                latency_record[i].time,
 251                                latency_record[i].max);
 252                        for (q = 0; q < LT_BACKTRACEDEPTH; q++) {
 253                                char sym[KSYM_SYMBOL_LEN];
 254                                char *c;
 255                                if (!latency_record[i].backtrace[q])
 256                                        break;
 257                                if (latency_record[i].backtrace[q] == ULONG_MAX)
 258                                        break;
 259                                sprint_symbol(sym, latency_record[i].backtrace[q]);
 260                                c = strchr(sym, '+');
 261                                if (c)
 262                                        *c = 0;
 263                                seq_printf(m, "%s ", sym);
 264                        }
 265                        seq_printf(m, "\n");
 266                }
 267        }
 268        return 0;
 269}
 270
 271static ssize_t
 272lstats_write(struct file *file, const char __user *buf, size_t count,
 273             loff_t *offs)
 274{
 275        clear_global_latency_tracing();
 276
 277        return count;
 278}
 279
 280static int lstats_open(struct inode *inode, struct file *filp)
 281{
 282        return single_open(filp, lstats_show, NULL);
 283}
 284
 285static const struct file_operations lstats_fops = {
 286        .open           = lstats_open,
 287        .read           = seq_read,
 288        .write          = lstats_write,
 289        .llseek         = seq_lseek,
 290        .release        = single_release,
 291};
 292
 293static int __init init_lstats_procfs(void)
 294{
 295        proc_create("latency_stats", 0644, NULL, &lstats_fops);
 296        return 0;
 297}
 298device_initcall(init_lstats_procfs);
 299