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Linux/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/kallsyms.h>
 51 #include <linux/seq_file.h>
 52 #include <linux/notifier.h>
 53 #include <linux/spinlock.h>
 54 #include <linux/proc_fs.h>
 55 #include <linux/latencytop.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_RAW_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         raw_spin_lock_irqsave(&latency_lock, flags);
 76         memset(&p->latency_record, 0, sizeof(p->latency_record));
 77         p->latency_record_count = 0;
 78         raw_spin_unlock_irqrestore(&latency_lock, flags);
 79 }
 80 
 81 static void clear_global_latency_tracing(void)
 82 {
 83         unsigned long flags;
 84 
 85         raw_spin_lock_irqsave(&latency_lock, flags);
 86         memset(&latency_record, 0, sizeof(latency_record));
 87         raw_spin_unlock_irqrestore(&latency_lock, flags);
 88 }
 89 
 90 static void __sched
 91 account_global_scheduler_latency(struct task_struct *tsk,
 92                                  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  */
145 static 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 occurred 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  */
172 void __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         raw_spin_lock_irqsave(&latency_lock, flags);
195 
196         account_global_scheduler_latency(tsk, &lat);
197 
198         for (i = 0; i < tsk->latency_record_count; i++) {
199                 struct latency_record *mylat;
200                 int same = 1;
201 
202                 mylat = &tsk->latency_record[i];
203                 for (q = 0; q < LT_BACKTRACEDEPTH; q++) {
204                         unsigned long record = lat.backtrace[q];
205 
206                         if (mylat->backtrace[q] != record) {
207                                 same = 0;
208                                 break;
209                         }
210 
211                         /* 0 and ULONG_MAX entries mean end of backtrace: */
212                         if (record == 0 || record == ULONG_MAX)
213                                 break;
214                 }
215                 if (same) {
216                         mylat->count++;
217                         mylat->time += lat.time;
218                         if (lat.time > mylat->max)
219                                 mylat->max = lat.time;
220                         goto out_unlock;
221                 }
222         }
223 
224         /*
225          * short term hack; if we're > 32 we stop; future we recycle:
226          */
227         if (tsk->latency_record_count >= LT_SAVECOUNT)
228                 goto out_unlock;
229 
230         /* Allocated a new one: */
231         i = tsk->latency_record_count++;
232         memcpy(&tsk->latency_record[i], &lat, sizeof(struct latency_record));
233 
234 out_unlock:
235         raw_spin_unlock_irqrestore(&latency_lock, flags);
236 }
237 
238 static int lstats_show(struct seq_file *m, void *v)
239 {
240         int i;
241 
242         seq_puts(m, "Latency Top version : v0.1\n");
243 
244         for (i = 0; i < MAXLR; i++) {
245                 struct latency_record *lr = &latency_record[i];
246 
247                 if (lr->backtrace[0]) {
248                         int q;
249                         seq_printf(m, "%i %lu %lu",
250                                    lr->count, lr->time, lr->max);
251                         for (q = 0; q < LT_BACKTRACEDEPTH; q++) {
252                                 unsigned long bt = lr->backtrace[q];
253                                 if (!bt)
254                                         break;
255                                 if (bt == ULONG_MAX)
256                                         break;
257                                 seq_printf(m, " %ps", (void *)bt);
258                         }
259                         seq_puts(m, "\n");
260                 }
261         }
262         return 0;
263 }
264 
265 static ssize_t
266 lstats_write(struct file *file, const char __user *buf, size_t count,
267              loff_t *offs)
268 {
269         clear_global_latency_tracing();
270 
271         return count;
272 }
273 
274 static int lstats_open(struct inode *inode, struct file *filp)
275 {
276         return single_open(filp, lstats_show, NULL);
277 }
278 
279 static const struct file_operations lstats_fops = {
280         .open           = lstats_open,
281         .read           = seq_read,
282         .write          = lstats_write,
283         .llseek         = seq_lseek,
284         .release        = single_release,
285 };
286 
287 static int __init init_lstats_procfs(void)
288 {
289         proc_create("latency_stats", 0644, NULL, &lstats_fops);
290         return 0;
291 }
292 
293 int sysctl_latencytop(struct ctl_table *table, int write,
294                         void __user *buffer, size_t *lenp, loff_t *ppos)
295 {
296         int err;
297 
298         err = proc_dointvec(table, write, buffer, lenp, ppos);
299         if (latencytop_enabled)
300                 force_schedstat_enabled();
301 
302         return err;
303 }
304 device_initcall(init_lstats_procfs);
305 

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