kernel/latencytop.c

   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/export.h>
  57 #include <linux/sched.h>
  58 #include <linux/list.h>
  59 #include <linux/stacktrace.h>
  60
  61 static DEFINE_SPINLOCK(latency_lock);
  62
  63 #define MAXLR 128
  64 static struct latency_record latency_record[MAXLR];
  65
  66 int latencytop_enabled;
  67
  68 void clear_all_latency_tracing(struct task_struct *p)
  69 {
  70         unsigned long flags;
  71
  72         if (!latencytop_enabled)
  73                 return;
  74
  75         spin_lock_irqsave(&latency_lock, flags);
  76         memset(&p->latency_record, 0, sizeof(p->latency_record));
  77         p->latency_record_count = 0;
  78         spin_unlock_irqrestore(&latency_lock, flags);
  79 }
  80
  81 static void clear_global_latency_tracing(void)
  82 {
  83         unsigned long flags;
  84
  85         spin_lock_irqsave(&latency_lock, flags);
  86         memset(&latency_record, 0, sizeof(latency_record));
  87         spin_unlock_irqrestore(&latency_lock, flags);
  88 }
  89
  90 static void __sched
  91 account_global_scheduler_latency(struct task_struct *tsk, struct latency_record *lat)
  92 {
  93         int firstnonnull = MAXLR + 1;
  94         int i;
  95
  96         if (!latencytop_enabled)
  97                 return;
  98
  99         /* skip kernel threads for now */
 100         if (!tsk->mm)
 101                 return;
 102
 103         for (i = 0; i < MAXLR; i++) {
 104                 int q, same = 1;
 105
 106                 /* Nothing stored: */
 107                 if (!latency_record[i].backtrace[0]) {
 108                         if (firstnonnull > i)
 109                                 firstnonnull = i;
 110                         continue;
 111                 }
 112                 for (q = 0; q < LT_BACKTRACEDEPTH; q++) {
 113                         unsigned long record = lat->backtrace[q];
 114
 115                         if (latency_record[i].backtrace[q] != record) {
 116                                 same = 0;
 117                                 break;
 118                         }
 119
 120                         /* 0 and ULONG_MAX entries mean end of backtrace: */
 121                         if (record == 0 || record == ULONG_MAX)
 122                                 break;
 123                 }
 124                 if (same) {
 125                         latency_record[i].count++;
 126                         latency_record[i].time += lat->time;
 127                         if (lat->time > latency_record[i].max)
 128                                 latency_record[i].max = lat->time;
 129                         return;
 130                 }
 131         }
 132
 133         i = firstnonnull;
 134         if (i >= MAXLR - 1)
 135                 return;
 136
 137         /* Allocted a new one: */
 138         memcpy(&latency_record[i], lat, sizeof(struct latency_record));
 139 }
 140
 141 /*
 142  * Iterator to store a backtrace into a latency record entry
 143  */
 144 static inline void store_stacktrace(struct task_struct *tsk,
 145                                         struct latency_record *lat)
 146 {
 147         struct stack_trace trace;
 148
 149         memset(&trace, 0, sizeof(trace));
 150         trace.max_entries = LT_BACKTRACEDEPTH;
 151         trace.entries = &lat->backtrace[0];
 152         save_stack_trace_tsk(tsk, &trace);
 153 }
 154
 155 /**
 156  * __account_scheduler_latency - record an occurred latency
 157  * @tsk - the task struct of the task hitting the latency
 158  * @usecs - the duration of the latency in microseconds
 159  * @inter - 1 if the sleep was interruptible, 0 if uninterruptible
 160  *
 161  * This function is the main entry point for recording latency entries
 162  * as called by the scheduler.
 163  *
 164  * This function has a few special cases to deal with normal 'non-latency'
 165  * sleeps: specifically, interruptible sleep longer than 5 msec is skipped
 166  * since this usually is caused by waiting for events via select() and co.
 167  *
 168  * Negative latencies (caused by time going backwards) are also explicitly
 169  * skipped.
 170  */
 171 void __sched
 172 __account_scheduler_latency(struct task_struct *tsk, int usecs, int inter)
 173 {
 174         unsigned long flags;
 175         int i, q;
 176         struct latency_record lat;
 177
 178         /* Long interruptible waits are generally user requested... */
 179         if (inter && usecs > 5000)
 180                 return;
 181
 182         /* Negative sleeps are time going backwards */
 183         /* Zero-time sleeps are non-interesting */
 184         if (usecs <= 0)
 185                 return;
 186
 187         memset(&lat, 0, sizeof(lat));
 188         lat.count = 1;
 189         lat.time = usecs;
 190         lat.max = usecs;
 191         store_stacktrace(tsk, &lat);
 192
 193         spin_lock_irqsave(&latency_lock, flags);
 194
 195         account_global_scheduler_latency(tsk, &lat);
 196
 197         for (i = 0; i < tsk->latency_record_count; i++) {
 198                 struct latency_record *mylat;
 199                 int same = 1;
 200
 201                 mylat = &tsk->latency_record[i];
 202                 for (q = 0; q < LT_BACKTRACEDEPTH; q++) {
 203                         unsigned long record = lat.backtrace[q];
 204
 205                         if (mylat->backtrace[q] != record) {
 206                                 same = 0;
 207                                 break;
 208                         }
 209
 210                         /* 0 and ULONG_MAX entries mean end of backtrace: */
 211                         if (record == 0 || record == ULONG_MAX)
 212                                 break;
 213                 }
 214                 if (same) {
 215                         mylat->count++;
 216                         mylat->time += lat.time;
 217                         if (lat.time > mylat->max)
 218                                 mylat->max = lat.time;
 219                         goto out_unlock;
 220                 }
 221         }
 222
 223         /*
 224          * short term hack; if we're > 32 we stop; future we recycle:
 225          */
 226         if (tsk->latency_record_count >= LT_SAVECOUNT)
 227                 goto out_unlock;
 228
 229         /* Allocated a new one: */
 230         i = tsk->latency_record_count++;
 231         memcpy(&tsk->latency_record[i], &lat, sizeof(struct latency_record));
 232
 233 out_unlock:
 234         spin_unlock_irqrestore(&latency_lock, flags);
 235 }
 236
 237 static int lstats_show(struct seq_file *m, void *v)
 238 {
 239         int i;
 240
 241         seq_puts(m, "Latency Top version : v0.1\n");
 242
 243         for (i = 0; i < MAXLR; i++) {
 244                 struct latency_record *lr = &latency_record[i];
 245
 246                 if (lr->backtrace[0]) {
 247                         int q;
 248                         seq_printf(m, "%i %lu %lu",
 249                                    lr->count, lr->time, lr->max);
 250                         for (q = 0; q < LT_BACKTRACEDEPTH; q++) {
 251                                 unsigned long bt = lr->backtrace[q];
 252                                 if (!bt)
 253                                         break;
 254                                 if (bt == ULONG_MAX)
 255                                         break;
 256                                 seq_printf(m, " %ps", (void *)bt);
 257                         }
 258                         seq_printf(m, "\n");
 259                 }
 260         }
 261         return 0;
 262 }
 263
 264 static ssize_t
 265 lstats_write(struct file *file, const char __user *buf, size_t count,
 266              loff_t *offs)
 267 {
 268         clear_global_latency_tracing();
 269
 270         return count;
 271 }
 272
 273 static int lstats_open(struct inode *inode, struct file *filp)
 274 {
 275         return single_open(filp, lstats_show, NULL);
 276 }
 277
 278 static const struct file_operations lstats_fops = {
 279         .open           = lstats_open,
 280         .read           = seq_read,
 281         .write          = lstats_write,
 282         .llseek         = seq_lseek,
 283         .release        = single_release,
 284 };
 285
 286 static int __init init_lstats_procfs(void)
 287 {
 288         proc_create("latency_stats", 0644, NULL, &lstats_fops);
 289         return 0;
 290 }
 291 device_initcall(init_lstats_procfs);