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Linux/kernel/latencytop.c

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  1 // SPDX-License-Identifier: GPL-2.0-only
  2 /*
  3  * latencytop.c: Latency display infrastructure
  4  *
  5  * (C) Copyright 2008 Intel Corporation
  6  * Author: Arjan van de Ven <arjan@linux.intel.com>
  7  */
  8 
  9 /*
 10  * CONFIG_LATENCYTOP enables a kernel latency tracking infrastructure that is
 11  * used by the "latencytop" userspace tool. The latency that is tracked is not
 12  * the 'traditional' interrupt latency (which is primarily caused by something
 13  * else consuming CPU), but instead, it is the latency an application encounters
 14  * because the kernel sleeps on its behalf for various reasons.
 15  *
 16  * This code tracks 2 levels of statistics:
 17  * 1) System level latency
 18  * 2) Per process latency
 19  *
 20  * The latency is stored in fixed sized data structures in an accumulated form;
 21  * if the "same" latency cause is hit twice, this will be tracked as one entry
 22  * in the data structure. Both the count, total accumulated latency and maximum
 23  * latency are tracked in this data structure. When the fixed size structure is
 24  * full, no new causes are tracked until the buffer is flushed by writing to
 25  * the /proc file; the userspace tool does this on a regular basis.
 26  *
 27  * A latency cause is identified by a stringified backtrace at the point that
 28  * the scheduler gets invoked. The userland tool will use this string to
 29  * identify the cause of the latency in human readable form.
 30  *
 31  * The information is exported via /proc/latency_stats and /proc/<pid>/latency.
 32  * These files look like this:
 33  *
 34  * Latency Top version : v0.1
 35  * 70 59433 4897 i915_irq_wait drm_ioctl vfs_ioctl do_vfs_ioctl sys_ioctl
 36  * |    |    |    |
 37  * |    |    |    +----> the stringified backtrace
 38  * |    |    +---------> The maximum latency for this entry in microseconds
 39  * |    +--------------> The accumulated latency for this entry (microseconds)
 40  * +-------------------> The number of times this entry is hit
 41  *
 42  * (note: the average latency is the accumulated latency divided by the number
 43  * of times)
 44  */
 45 
 46 #include <linux/kallsyms.h>
 47 #include <linux/seq_file.h>
 48 #include <linux/notifier.h>
 49 #include <linux/spinlock.h>
 50 #include <linux/proc_fs.h>
 51 #include <linux/latencytop.h>
 52 #include <linux/export.h>
 53 #include <linux/sched.h>
 54 #include <linux/sched/debug.h>
 55 #include <linux/sched/stat.h>
 56 #include <linux/list.h>
 57 #include <linux/stacktrace.h>
 58 
 59 static DEFINE_RAW_SPINLOCK(latency_lock);
 60 
 61 #define MAXLR 128
 62 static struct latency_record latency_record[MAXLR];
 63 
 64 int latencytop_enabled;
 65 
 66 void clear_tsk_latency_tracing(struct task_struct *p)
 67 {
 68         unsigned long flags;
 69 
 70         raw_spin_lock_irqsave(&latency_lock, flags);
 71         memset(&p->latency_record, 0, sizeof(p->latency_record));
 72         p->latency_record_count = 0;
 73         raw_spin_unlock_irqrestore(&latency_lock, flags);
 74 }
 75 
 76 static void clear_global_latency_tracing(void)
 77 {
 78         unsigned long flags;
 79 
 80         raw_spin_lock_irqsave(&latency_lock, flags);
 81         memset(&latency_record, 0, sizeof(latency_record));
 82         raw_spin_unlock_irqrestore(&latency_lock, flags);
 83 }
 84 
 85 static void __sched
 86 account_global_scheduler_latency(struct task_struct *tsk,
 87                                  struct latency_record *lat)
 88 {
 89         int firstnonnull = MAXLR + 1;
 90         int i;
 91 
 92         /* skip kernel threads for now */
 93         if (!tsk->mm)
 94                 return;
 95 
 96         for (i = 0; i < MAXLR; i++) {
 97                 int q, same = 1;
 98 
 99                 /* Nothing stored: */
100                 if (!latency_record[i].backtrace[0]) {
101                         if (firstnonnull > i)
102                                 firstnonnull = i;
103                         continue;
104                 }
105                 for (q = 0; q < LT_BACKTRACEDEPTH; q++) {
106                         unsigned long record = lat->backtrace[q];
107 
108                         if (latency_record[i].backtrace[q] != record) {
109                                 same = 0;
110                                 break;
111                         }
112 
113                         /* 0 entry marks end of backtrace: */
114                         if (!record)
115                                 break;
116                 }
117                 if (same) {
118                         latency_record[i].count++;
119                         latency_record[i].time += lat->time;
120                         if (lat->time > latency_record[i].max)
121                                 latency_record[i].max = lat->time;
122                         return;
123                 }
124         }
125 
126         i = firstnonnull;
127         if (i >= MAXLR - 1)
128                 return;
129 
130         /* Allocted a new one: */
131         memcpy(&latency_record[i], lat, sizeof(struct latency_record));
132 }
133 
134 /**
135  * __account_scheduler_latency - record an occurred latency
136  * @tsk - the task struct of the task hitting the latency
137  * @usecs - the duration of the latency in microseconds
138  * @inter - 1 if the sleep was interruptible, 0 if uninterruptible
139  *
140  * This function is the main entry point for recording latency entries
141  * as called by the scheduler.
142  *
143  * This function has a few special cases to deal with normal 'non-latency'
144  * sleeps: specifically, interruptible sleep longer than 5 msec is skipped
145  * since this usually is caused by waiting for events via select() and co.
146  *
147  * Negative latencies (caused by time going backwards) are also explicitly
148  * skipped.
149  */
150 void __sched
151 __account_scheduler_latency(struct task_struct *tsk, int usecs, int inter)
152 {
153         unsigned long flags;
154         int i, q;
155         struct latency_record lat;
156 
157         /* Long interruptible waits are generally user requested... */
158         if (inter && usecs > 5000)
159                 return;
160 
161         /* Negative sleeps are time going backwards */
162         /* Zero-time sleeps are non-interesting */
163         if (usecs <= 0)
164                 return;
165 
166         memset(&lat, 0, sizeof(lat));
167         lat.count = 1;
168         lat.time = usecs;
169         lat.max = usecs;
170 
171         stack_trace_save_tsk(tsk, lat.backtrace, LT_BACKTRACEDEPTH, 0);
172 
173         raw_spin_lock_irqsave(&latency_lock, flags);
174 
175         account_global_scheduler_latency(tsk, &lat);
176 
177         for (i = 0; i < tsk->latency_record_count; i++) {
178                 struct latency_record *mylat;
179                 int same = 1;
180 
181                 mylat = &tsk->latency_record[i];
182                 for (q = 0; q < LT_BACKTRACEDEPTH; q++) {
183                         unsigned long record = lat.backtrace[q];
184 
185                         if (mylat->backtrace[q] != record) {
186                                 same = 0;
187                                 break;
188                         }
189 
190                         /* 0 entry is end of backtrace */
191                         if (!record)
192                                 break;
193                 }
194                 if (same) {
195                         mylat->count++;
196                         mylat->time += lat.time;
197                         if (lat.time > mylat->max)
198                                 mylat->max = lat.time;
199                         goto out_unlock;
200                 }
201         }
202 
203         /*
204          * short term hack; if we're > 32 we stop; future we recycle:
205          */
206         if (tsk->latency_record_count >= LT_SAVECOUNT)
207                 goto out_unlock;
208 
209         /* Allocated a new one: */
210         i = tsk->latency_record_count++;
211         memcpy(&tsk->latency_record[i], &lat, sizeof(struct latency_record));
212 
213 out_unlock:
214         raw_spin_unlock_irqrestore(&latency_lock, flags);
215 }
216 
217 static int lstats_show(struct seq_file *m, void *v)
218 {
219         int i;
220 
221         seq_puts(m, "Latency Top version : v0.1\n");
222 
223         for (i = 0; i < MAXLR; i++) {
224                 struct latency_record *lr = &latency_record[i];
225 
226                 if (lr->backtrace[0]) {
227                         int q;
228                         seq_printf(m, "%i %lu %lu",
229                                    lr->count, lr->time, lr->max);
230                         for (q = 0; q < LT_BACKTRACEDEPTH; q++) {
231                                 unsigned long bt = lr->backtrace[q];
232 
233                                 if (!bt)
234                                         break;
235 
236                                 seq_printf(m, " %ps", (void *)bt);
237                         }
238                         seq_puts(m, "\n");
239                 }
240         }
241         return 0;
242 }
243 
244 static ssize_t
245 lstats_write(struct file *file, const char __user *buf, size_t count,
246              loff_t *offs)
247 {
248         clear_global_latency_tracing();
249 
250         return count;
251 }
252 
253 static int lstats_open(struct inode *inode, struct file *filp)
254 {
255         return single_open(filp, lstats_show, NULL);
256 }
257 
258 static const struct proc_ops lstats_proc_ops = {
259         .proc_open      = lstats_open,
260         .proc_read      = seq_read,
261         .proc_write     = lstats_write,
262         .proc_lseek     = seq_lseek,
263         .proc_release   = single_release,
264 };
265 
266 static int __init init_lstats_procfs(void)
267 {
268         proc_create("latency_stats", 0644, NULL, &lstats_proc_ops);
269         return 0;
270 }
271 
272 int sysctl_latencytop(struct ctl_table *table, int write, void *buffer,
273                 size_t *lenp, loff_t *ppos)
274 {
275         int err;
276 
277         err = proc_dointvec(table, write, buffer, lenp, ppos);
278         if (latencytop_enabled)
279                 force_schedstat_enabled();
280 
281         return err;
282 }
283 device_initcall(init_lstats_procfs);
284 

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