2 * Pressure stall information for CPU, memory and IO
4 * Copyright (c) 2018 Facebook, Inc.
5 * Author: Johannes Weiner <hannes@cmpxchg.org>
7 * When CPU, memory and IO are contended, tasks experience delays that
8 * reduce throughput and introduce latencies into the workload. Memory
9 * and IO contention, in addition, can cause a full loss of forward
10 * progress in which the CPU goes idle.
12 * This code aggregates individual task delays into resource pressure
13 * metrics that indicate problems with both workload health and
14 * resource utilization.
18 * The time in which a task can execute on a CPU is our baseline for
19 * productivity. Pressure expresses the amount of time in which this
20 * potential cannot be realized due to resource contention.
22 * This concept of productivity has two components: the workload and
23 * the CPU. To measure the impact of pressure on both, we define two
24 * contention states for a resource: SOME and FULL.
26 * In the SOME state of a given resource, one or more tasks are
27 * delayed on that resource. This affects the workload's ability to
28 * perform work, but the CPU may still be executing other tasks.
30 * In the FULL state of a given resource, all non-idle tasks are
31 * delayed on that resource such that nobody is advancing and the CPU
32 * goes idle. This leaves both workload and CPU unproductive.
34 * (Naturally, the FULL state doesn't exist for the CPU resource.)
36 * SOME = nr_delayed_tasks != 0
37 * FULL = nr_delayed_tasks != 0 && nr_running_tasks == 0
39 * The percentage of wallclock time spent in those compound stall
40 * states gives pressure numbers between 0 and 100 for each resource,
41 * where the SOME percentage indicates workload slowdowns and the FULL
42 * percentage indicates reduced CPU utilization:
44 * %SOME = time(SOME) / period
45 * %FULL = time(FULL) / period
49 * The more tasks and available CPUs there are, the more work can be
50 * performed concurrently. This means that the potential that can go
51 * unrealized due to resource contention *also* scales with non-idle
54 * Consider a scenario where 257 number crunching tasks are trying to
55 * run concurrently on 256 CPUs. If we simply aggregated the task
56 * states, we would have to conclude a CPU SOME pressure number of
57 * 100%, since *somebody* is waiting on a runqueue at all
58 * times. However, that is clearly not the amount of contention the
59 * workload is experiencing: only one out of 256 possible exceution
60 * threads will be contended at any given time, or about 0.4%.
62 * Conversely, consider a scenario of 4 tasks and 4 CPUs where at any
63 * given time *one* of the tasks is delayed due to a lack of memory.
64 * Again, looking purely at the task state would yield a memory FULL
65 * pressure number of 0%, since *somebody* is always making forward
66 * progress. But again this wouldn't capture the amount of execution
67 * potential lost, which is 1 out of 4 CPUs, or 25%.
69 * To calculate wasted potential (pressure) with multiple processors,
70 * we have to base our calculation on the number of non-idle tasks in
71 * conjunction with the number of available CPUs, which is the number
72 * of potential execution threads. SOME becomes then the proportion of
73 * delayed tasks to possibe threads, and FULL is the share of possible
74 * threads that are unproductive due to delays:
76 * threads = min(nr_nonidle_tasks, nr_cpus)
77 * SOME = min(nr_delayed_tasks / threads, 1)
78 * FULL = (threads - min(nr_running_tasks, threads)) / threads
80 * For the 257 number crunchers on 256 CPUs, this yields:
82 * threads = min(257, 256)
83 * SOME = min(1 / 256, 1) = 0.4%
84 * FULL = (256 - min(257, 256)) / 256 = 0%
86 * For the 1 out of 4 memory-delayed tasks, this yields:
89 * SOME = min(1 / 4, 1) = 25%
90 * FULL = (4 - min(3, 4)) / 4 = 25%
92 * [ Substitute nr_cpus with 1, and you can see that it's a natural
93 * extension of the single-CPU model. ]
97 * To assess the precise time spent in each such state, we would have
98 * to freeze the system on task changes and start/stop the state
99 * clocks accordingly. Obviously that doesn't scale in practice.
101 * Because the scheduler aims to distribute the compute load evenly
102 * among the available CPUs, we can track task state locally to each
103 * CPU and, at much lower frequency, extrapolate the global state for
104 * the cumulative stall times and the running averages.
106 * For each runqueue, we track:
108 * tSOME[cpu] = time(nr_delayed_tasks[cpu] != 0)
109 * tFULL[cpu] = time(nr_delayed_tasks[cpu] && !nr_running_tasks[cpu])
110 * tNONIDLE[cpu] = time(nr_nonidle_tasks[cpu] != 0)
112 * and then periodically aggregate:
114 * tNONIDLE = sum(tNONIDLE[i])
116 * tSOME = sum(tSOME[i] * tNONIDLE[i]) / tNONIDLE
117 * tFULL = sum(tFULL[i] * tNONIDLE[i]) / tNONIDLE
119 * %SOME = tSOME / period
120 * %FULL = tFULL / period
122 * This gives us an approximation of pressure that is practical
123 * cost-wise, yet way more sensitive and accurate than periodic
124 * sampling of the aggregate task states would be.
127 #include "../workqueue_internal.h"
128 #include <linux/sched/loadavg.h>
129 #include <linux/seq_file.h>
130 #include <linux/proc_fs.h>
131 #include <linux/seqlock.h>
132 #include <linux/cgroup.h>
133 #include <linux/module.h>
134 #include <linux/sched.h>
135 #include <linux/psi.h>
138 static int psi_bug __read_mostly
;
140 DEFINE_STATIC_KEY_FALSE(psi_disabled
);
142 #ifdef CONFIG_PSI_DEFAULT_DISABLED
145 bool psi_enable
= true;
147 static int __init
setup_psi(char *str
)
149 return kstrtobool(str
, &psi_enable
) == 0;
151 __setup("psi=", setup_psi
);
153 /* Running averages - we need to be higher-res than loadavg */
154 #define PSI_FREQ (2*HZ+1) /* 2 sec intervals */
155 #define EXP_10s 1677 /* 1/exp(2s/10s) as fixed-point */
156 #define EXP_60s 1981 /* 1/exp(2s/60s) */
157 #define EXP_300s 2034 /* 1/exp(2s/300s) */
159 /* Sampling frequency in nanoseconds */
160 static u64 psi_period __read_mostly
;
162 /* System-level pressure and stall tracking */
163 static DEFINE_PER_CPU(struct psi_group_cpu
, system_group_pcpu
);
164 static struct psi_group psi_system
= {
165 .pcpu
= &system_group_pcpu
,
168 static void psi_update_work(struct work_struct
*work
);
170 static void group_init(struct psi_group
*group
)
174 for_each_possible_cpu(cpu
)
175 seqcount_init(&per_cpu_ptr(group
->pcpu
, cpu
)->seq
);
176 group
->next_update
= sched_clock() + psi_period
;
177 INIT_DELAYED_WORK(&group
->clock_work
, psi_update_work
);
178 mutex_init(&group
->stat_lock
);
181 void __init
psi_init(void)
184 static_branch_enable(&psi_disabled
);
188 psi_period
= jiffies_to_nsecs(PSI_FREQ
);
189 group_init(&psi_system
);
192 static bool test_state(unsigned int *tasks
, enum psi_states state
)
196 return tasks
[NR_IOWAIT
];
198 return tasks
[NR_IOWAIT
] && !tasks
[NR_RUNNING
];
200 return tasks
[NR_MEMSTALL
];
202 return tasks
[NR_MEMSTALL
] && !tasks
[NR_RUNNING
];
204 return tasks
[NR_RUNNING
] > 1;
206 return tasks
[NR_IOWAIT
] || tasks
[NR_MEMSTALL
] ||
213 static void get_recent_times(struct psi_group
*group
, int cpu
, u32
*times
)
215 struct psi_group_cpu
*groupc
= per_cpu_ptr(group
->pcpu
, cpu
);
216 unsigned int tasks
[NR_PSI_TASK_COUNTS
];
217 u64 now
, state_start
;
221 /* Snapshot a coherent view of the CPU state */
223 seq
= read_seqcount_begin(&groupc
->seq
);
224 now
= cpu_clock(cpu
);
225 memcpy(times
, groupc
->times
, sizeof(groupc
->times
));
226 memcpy(tasks
, groupc
->tasks
, sizeof(groupc
->tasks
));
227 state_start
= groupc
->state_start
;
228 } while (read_seqcount_retry(&groupc
->seq
, seq
));
230 /* Calculate state time deltas against the previous snapshot */
231 for (s
= 0; s
< NR_PSI_STATES
; s
++) {
234 * In addition to already concluded states, we also
235 * incorporate currently active states on the CPU,
236 * since states may last for many sampling periods.
238 * This way we keep our delta sampling buckets small
239 * (u32) and our reported pressure close to what's
240 * actually happening.
242 if (test_state(tasks
, s
))
243 times
[s
] += now
- state_start
;
245 delta
= times
[s
] - groupc
->times_prev
[s
];
246 groupc
->times_prev
[s
] = times
[s
];
252 static void calc_avgs(unsigned long avg
[3], int missed_periods
,
253 u64 time
, u64 period
)
257 /* Fill in zeroes for periods of no activity */
258 if (missed_periods
) {
259 avg
[0] = calc_load_n(avg
[0], EXP_10s
, 0, missed_periods
);
260 avg
[1] = calc_load_n(avg
[1], EXP_60s
, 0, missed_periods
);
261 avg
[2] = calc_load_n(avg
[2], EXP_300s
, 0, missed_periods
);
264 /* Sample the most recent active period */
265 pct
= div_u64(time
* 100, period
);
267 avg
[0] = calc_load(avg
[0], EXP_10s
, pct
);
268 avg
[1] = calc_load(avg
[1], EXP_60s
, pct
);
269 avg
[2] = calc_load(avg
[2], EXP_300s
, pct
);
272 static bool update_stats(struct psi_group
*group
)
274 u64 deltas
[NR_PSI_STATES
- 1] = { 0, };
275 unsigned long missed_periods
= 0;
276 unsigned long nonidle_total
= 0;
277 u64 now
, expires
, period
;
281 mutex_lock(&group
->stat_lock
);
284 * Collect the per-cpu time buckets and average them into a
285 * single time sample that is normalized to wallclock time.
287 * For averaging, each CPU is weighted by its non-idle time in
288 * the sampling period. This eliminates artifacts from uneven
289 * loading, or even entirely idle CPUs.
291 for_each_possible_cpu(cpu
) {
292 u32 times
[NR_PSI_STATES
];
295 get_recent_times(group
, cpu
, times
);
297 nonidle
= nsecs_to_jiffies(times
[PSI_NONIDLE
]);
298 nonidle_total
+= nonidle
;
300 for (s
= 0; s
< PSI_NONIDLE
; s
++)
301 deltas
[s
] += (u64
)times
[s
] * nonidle
;
305 * Integrate the sample into the running statistics that are
306 * reported to userspace: the cumulative stall times and the
309 * Pressure percentages are sampled at PSI_FREQ. We might be
310 * called more often when the user polls more frequently than
311 * that; we might be called less often when there is no task
312 * activity, thus no data, and clock ticks are sporadic. The
313 * below handles both.
317 for (s
= 0; s
< NR_PSI_STATES
- 1; s
++)
318 group
->total
[s
] += div_u64(deltas
[s
], max(nonidle_total
, 1UL));
322 expires
= group
->next_update
;
325 if (now
- expires
>= psi_period
)
326 missed_periods
= div_u64(now
- expires
, psi_period
);
329 * The periodic clock tick can get delayed for various
330 * reasons, especially on loaded systems. To avoid clock
331 * drift, we schedule the clock in fixed psi_period intervals.
332 * But the deltas we sample out of the per-cpu buckets above
333 * are based on the actual time elapsing between clock ticks.
335 group
->next_update
= expires
+ ((1 + missed_periods
) * psi_period
);
336 period
= now
- (group
->last_update
+ (missed_periods
* psi_period
));
337 group
->last_update
= now
;
339 for (s
= 0; s
< NR_PSI_STATES
- 1; s
++) {
342 sample
= group
->total
[s
] - group
->total_prev
[s
];
344 * Due to the lockless sampling of the time buckets,
345 * recorded time deltas can slip into the next period,
346 * which under full pressure can result in samples in
347 * excess of the period length.
349 * We don't want to report non-sensical pressures in
350 * excess of 100%, nor do we want to drop such events
351 * on the floor. Instead we punt any overage into the
352 * future until pressure subsides. By doing this we
353 * don't underreport the occurring pressure curve, we
354 * just report it delayed by one period length.
356 * The error isn't cumulative. As soon as another
357 * delta slips from a period P to P+1, by definition
358 * it frees up its time T in P.
362 group
->total_prev
[s
] += sample
;
363 calc_avgs(group
->avg
[s
], missed_periods
, sample
, period
);
366 mutex_unlock(&group
->stat_lock
);
367 return nonidle_total
;
370 static void psi_update_work(struct work_struct
*work
)
372 struct delayed_work
*dwork
;
373 struct psi_group
*group
;
376 dwork
= to_delayed_work(work
);
377 group
= container_of(dwork
, struct psi_group
, clock_work
);
380 * If there is task activity, periodically fold the per-cpu
381 * times and feed samples into the running averages. If things
382 * are idle and there is no data to process, stop the clock.
383 * Once restarted, we'll catch up the running averages in one
384 * go - see calc_avgs() and missed_periods.
387 nonidle
= update_stats(group
);
390 unsigned long delay
= 0;
394 if (group
->next_update
> now
)
395 delay
= nsecs_to_jiffies(group
->next_update
- now
) + 1;
396 schedule_delayed_work(dwork
, delay
);
400 static void record_times(struct psi_group_cpu
*groupc
, int cpu
,
406 now
= cpu_clock(cpu
);
407 delta
= now
- groupc
->state_start
;
408 groupc
->state_start
= now
;
410 if (test_state(groupc
->tasks
, PSI_IO_SOME
)) {
411 groupc
->times
[PSI_IO_SOME
] += delta
;
412 if (test_state(groupc
->tasks
, PSI_IO_FULL
))
413 groupc
->times
[PSI_IO_FULL
] += delta
;
416 if (test_state(groupc
->tasks
, PSI_MEM_SOME
)) {
417 groupc
->times
[PSI_MEM_SOME
] += delta
;
418 if (test_state(groupc
->tasks
, PSI_MEM_FULL
))
419 groupc
->times
[PSI_MEM_FULL
] += delta
;
420 else if (memstall_tick
) {
423 * Since we care about lost potential, a
424 * memstall is FULL when there are no other
425 * working tasks, but also when the CPU is
426 * actively reclaiming and nothing productive
427 * could run even if it were runnable.
429 * When the timer tick sees a reclaiming CPU,
430 * regardless of runnable tasks, sample a FULL
431 * tick (or less if it hasn't been a full tick
432 * since the last state change).
434 sample
= min(delta
, (u32
)jiffies_to_nsecs(1));
435 groupc
->times
[PSI_MEM_FULL
] += sample
;
439 if (test_state(groupc
->tasks
, PSI_CPU_SOME
))
440 groupc
->times
[PSI_CPU_SOME
] += delta
;
442 if (test_state(groupc
->tasks
, PSI_NONIDLE
))
443 groupc
->times
[PSI_NONIDLE
] += delta
;
446 static void psi_group_change(struct psi_group
*group
, int cpu
,
447 unsigned int clear
, unsigned int set
)
449 struct psi_group_cpu
*groupc
;
452 groupc
= per_cpu_ptr(group
->pcpu
, cpu
);
455 * First we assess the aggregate resource states this CPU's
456 * tasks have been in since the last change, and account any
457 * SOME and FULL time these may have resulted in.
459 * Then we update the task counts according to the state
460 * change requested through the @clear and @set bits.
462 write_seqcount_begin(&groupc
->seq
);
464 record_times(groupc
, cpu
, false);
466 for (t
= 0, m
= clear
; m
; m
&= ~(1 << t
), t
++) {
469 if (groupc
->tasks
[t
] == 0 && !psi_bug
) {
470 printk_deferred(KERN_ERR
"psi: task underflow! cpu=%d t=%d tasks=[%u %u %u] clear=%x set=%x\n",
471 cpu
, t
, groupc
->tasks
[0],
472 groupc
->tasks
[1], groupc
->tasks
[2],
479 for (t
= 0; set
; set
&= ~(1 << t
), t
++)
483 write_seqcount_end(&groupc
->seq
);
486 static struct psi_group
*iterate_groups(struct task_struct
*task
, void **iter
)
488 #ifdef CONFIG_CGROUPS
489 struct cgroup
*cgroup
= NULL
;
492 cgroup
= task
->cgroups
->dfl_cgrp
;
493 else if (*iter
== &psi_system
)
496 cgroup
= cgroup_parent(*iter
);
498 if (cgroup
&& cgroup_parent(cgroup
)) {
500 return cgroup_psi(cgroup
);
510 void psi_task_change(struct task_struct
*task
, int clear
, int set
)
512 int cpu
= task_cpu(task
);
513 struct psi_group
*group
;
514 bool wake_clock
= true;
520 if (((task
->psi_flags
& set
) ||
521 (task
->psi_flags
& clear
) != clear
) &&
523 printk_deferred(KERN_ERR
"psi: inconsistent task state! task=%d:%s cpu=%d psi_flags=%x clear=%x set=%x\n",
524 task
->pid
, task
->comm
, cpu
,
525 task
->psi_flags
, clear
, set
);
529 task
->psi_flags
&= ~clear
;
530 task
->psi_flags
|= set
;
533 * Periodic aggregation shuts off if there is a period of no
534 * task changes, so we wake it back up if necessary. However,
535 * don't do this if the task change is the aggregation worker
536 * itself going to sleep, or we'll ping-pong forever.
538 if (unlikely((clear
& TSK_RUNNING
) &&
539 (task
->flags
& PF_WQ_WORKER
) &&
540 wq_worker_last_func(task
) == psi_update_work
))
543 while ((group
= iterate_groups(task
, &iter
))) {
544 psi_group_change(group
, cpu
, clear
, set
);
545 if (wake_clock
&& !delayed_work_pending(&group
->clock_work
))
546 schedule_delayed_work(&group
->clock_work
, PSI_FREQ
);
550 void psi_memstall_tick(struct task_struct
*task
, int cpu
)
552 struct psi_group
*group
;
555 while ((group
= iterate_groups(task
, &iter
))) {
556 struct psi_group_cpu
*groupc
;
558 groupc
= per_cpu_ptr(group
->pcpu
, cpu
);
559 write_seqcount_begin(&groupc
->seq
);
560 record_times(groupc
, cpu
, true);
561 write_seqcount_end(&groupc
->seq
);
566 * psi_memstall_enter - mark the beginning of a memory stall section
567 * @flags: flags to handle nested sections
569 * Marks the calling task as being stalled due to a lack of memory,
570 * such as waiting for a refault or performing reclaim.
572 void psi_memstall_enter(unsigned long *flags
)
577 if (static_branch_likely(&psi_disabled
))
580 *flags
= current
->flags
& PF_MEMSTALL
;
584 * PF_MEMSTALL setting & accounting needs to be atomic wrt
585 * changes to the task's scheduling state, otherwise we can
586 * race with CPU migration.
588 rq
= this_rq_lock_irq(&rf
);
590 current
->flags
|= PF_MEMSTALL
;
591 psi_task_change(current
, 0, TSK_MEMSTALL
);
593 rq_unlock_irq(rq
, &rf
);
597 * psi_memstall_leave - mark the end of an memory stall section
598 * @flags: flags to handle nested memdelay sections
600 * Marks the calling task as no longer stalled due to lack of memory.
602 void psi_memstall_leave(unsigned long *flags
)
607 if (static_branch_likely(&psi_disabled
))
613 * PF_MEMSTALL clearing & accounting needs to be atomic wrt
614 * changes to the task's scheduling state, otherwise we could
615 * race with CPU migration.
617 rq
= this_rq_lock_irq(&rf
);
619 current
->flags
&= ~PF_MEMSTALL
;
620 psi_task_change(current
, TSK_MEMSTALL
, 0);
622 rq_unlock_irq(rq
, &rf
);
625 #ifdef CONFIG_CGROUPS
626 int psi_cgroup_alloc(struct cgroup
*cgroup
)
628 if (static_branch_likely(&psi_disabled
))
631 cgroup
->psi
.pcpu
= alloc_percpu(struct psi_group_cpu
);
632 if (!cgroup
->psi
.pcpu
)
634 group_init(&cgroup
->psi
);
638 void psi_cgroup_free(struct cgroup
*cgroup
)
640 if (static_branch_likely(&psi_disabled
))
643 cancel_delayed_work_sync(&cgroup
->psi
.clock_work
);
644 free_percpu(cgroup
->psi
.pcpu
);
648 * cgroup_move_task - move task to a different cgroup
650 * @to: the target css_set
652 * Move task to a new cgroup and safely migrate its associated stall
653 * state between the different groups.
655 * This function acquires the task's rq lock to lock out concurrent
656 * changes to the task's scheduling state and - in case the task is
657 * running - concurrent changes to its stall state.
659 void cgroup_move_task(struct task_struct
*task
, struct css_set
*to
)
661 unsigned int task_flags
= 0;
665 if (static_branch_likely(&psi_disabled
)) {
667 * Lame to do this here, but the scheduler cannot be locked
668 * from the outside, so we move cgroups from inside sched/.
670 rcu_assign_pointer(task
->cgroups
, to
);
674 rq
= task_rq_lock(task
, &rf
);
676 if (task_on_rq_queued(task
))
677 task_flags
= TSK_RUNNING
;
678 else if (task
->in_iowait
)
679 task_flags
= TSK_IOWAIT
;
681 if (task
->flags
& PF_MEMSTALL
)
682 task_flags
|= TSK_MEMSTALL
;
685 psi_task_change(task
, task_flags
, 0);
687 /* See comment above */
688 rcu_assign_pointer(task
->cgroups
, to
);
691 psi_task_change(task
, 0, task_flags
);
693 task_rq_unlock(rq
, task
, &rf
);
695 #endif /* CONFIG_CGROUPS */
697 int psi_show(struct seq_file
*m
, struct psi_group
*group
, enum psi_res res
)
701 if (static_branch_likely(&psi_disabled
))
706 for (full
= 0; full
< 2 - (res
== PSI_CPU
); full
++) {
707 unsigned long avg
[3];
711 for (w
= 0; w
< 3; w
++)
712 avg
[w
] = group
->avg
[res
* 2 + full
][w
];
713 total
= div_u64(group
->total
[res
* 2 + full
], NSEC_PER_USEC
);
715 seq_printf(m
, "%s avg10=%lu.%02lu avg60=%lu.%02lu avg300=%lu.%02lu total=%llu\n",
716 full
? "full" : "some",
717 LOAD_INT(avg
[0]), LOAD_FRAC(avg
[0]),
718 LOAD_INT(avg
[1]), LOAD_FRAC(avg
[1]),
719 LOAD_INT(avg
[2]), LOAD_FRAC(avg
[2]),
726 static int psi_io_show(struct seq_file
*m
, void *v
)
728 return psi_show(m
, &psi_system
, PSI_IO
);
731 static int psi_memory_show(struct seq_file
*m
, void *v
)
733 return psi_show(m
, &psi_system
, PSI_MEM
);
736 static int psi_cpu_show(struct seq_file
*m
, void *v
)
738 return psi_show(m
, &psi_system
, PSI_CPU
);
741 static int psi_io_open(struct inode
*inode
, struct file
*file
)
743 return single_open(file
, psi_io_show
, NULL
);
746 static int psi_memory_open(struct inode
*inode
, struct file
*file
)
748 return single_open(file
, psi_memory_show
, NULL
);
751 static int psi_cpu_open(struct inode
*inode
, struct file
*file
)
753 return single_open(file
, psi_cpu_show
, NULL
);
756 static const struct file_operations psi_io_fops
= {
760 .release
= single_release
,
763 static const struct file_operations psi_memory_fops
= {
764 .open
= psi_memory_open
,
767 .release
= single_release
,
770 static const struct file_operations psi_cpu_fops
= {
771 .open
= psi_cpu_open
,
774 .release
= single_release
,
777 static int __init
psi_proc_init(void)
779 proc_mkdir("pressure", NULL
);
780 proc_create("pressure/io", 0, NULL
, &psi_io_fops
);
781 proc_create("pressure/memory", 0, NULL
, &psi_memory_fops
);
782 proc_create("pressure/cpu", 0, NULL
, &psi_cpu_fops
);
785 module_init(psi_proc_init
);