psi: pressure stall information for CPU, memory, and IO

When systems are overcommitted and resources become contended, it's hard
to tell exactly the impact this has on workload productivity, or how close
the system is to lockups and OOM kills.  In particular, when machines work
multiple jobs concurrently, the impact of overcommit in terms of latency
and throughput on the individual job can be enormous.

In order to maximize hardware utilization without sacrificing individual
job health or risk complete machine lockups, this patch implements a way
to quantify resource pressure in the system.

A kernel built with CONFIG_PSI=y creates files in /proc/pressure/ that
expose the percentage of time the system is stalled on CPU, memory, or IO,
respectively.  Stall states are aggregate versions of the per-task delay
accounting delays:

       cpu: some tasks are runnable but not executing on a CPU
       memory: tasks are reclaiming, or waiting for swapin or thrashing cache
       io: tasks are waiting for io completions

These percentages of walltime can be thought of as pressure percentages,
and they give a general sense of system health and productivity loss
incurred by resource overcommit.  They can also indicate when the system
is approaching lockup scenarios and OOMs.

To do this, psi keeps track of the task states associated with each CPU
and samples the time they spend in stall states.  Every 2 seconds, the
samples are averaged across CPUs - weighted by the CPUs' non-idle time to
eliminate artifacts from unused CPUs - and translated into percentages of
walltime.  A running average of those percentages is maintained over 10s,
1m, and 5m periods (similar to the loadaverage).

[hannes@cmpxchg.org: doc fixlet, per Randy]
  Link: http://lkml.kernel.org/r/20180828205625.GA14030@cmpxchg.org
[hannes@cmpxchg.org: code optimization]
  Link: http://lkml.kernel.org/r/20180907175015.GA8479@cmpxchg.org
[hannes@cmpxchg.org: rename psi_clock() to psi_update_work(), per Peter]
  Link: http://lkml.kernel.org/r/20180907145404.GB11088@cmpxchg.org
[hannes@cmpxchg.org: fix build]
  Link: http://lkml.kernel.org/r/20180913014222.GA2370@cmpxchg.org
Link: http://lkml.kernel.org/r/20180828172258.3185-9-hannes@cmpxchg.org
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Daniel Drake <drake@endlessm.com>
Tested-by: Suren Baghdasaryan <surenb@google.com>
Cc: Christopher Lameter <cl@linux.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Johannes Weiner <jweiner@fb.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Enderborg <peter.enderborg@sony.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Vinayak Menon <vinmenon@codeaurora.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

5 files changed, 756 insertions, 2 deletions

diff --git a/kernel/sched/Makefile b/kernel/sched/Makefile
index 7fe183404c38..21fb5a5662b5 100644
--- a/kernel/sched/Makefile
+++ b/kernel/sched/Makefile
@@ -29,3 +29,4 @@ obj-$(CONFIG_CPU_FREQ) += cpufreq.o
 obj-$(CONFIG_CPU_FREQ_GOV_SCHEDUTIL) += cpufreq_schedutil.o
 obj-$(CONFIG_MEMBARRIER) += membarrier.o
 obj-$(CONFIG_CPU_ISOLATION) += isolation.o
+obj-$(CONFIG_PSI) += psi.o
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index f3efef387797..fd2fce8a001b 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -722,8 +722,10 @@ static inline void enqueue_task(struct rq *rq, struct task_struct *p, int flags)
 	if (!(flags & ENQUEUE_NOCLOCK))
 		update_rq_clock(rq);
 
-	if (!(flags & ENQUEUE_RESTORE))
+	if (!(flags & ENQUEUE_RESTORE)) {
 		sched_info_queued(rq, p);
+		psi_enqueue(p, flags & ENQUEUE_WAKEUP);
+	}
 
 	p->sched_class->enqueue_task(rq, p, flags);
 }
@@ -733,8 +735,10 @@ static inline void dequeue_task(struct rq *rq, struct task_struct *p, int flags)
 	if (!(flags & DEQUEUE_NOCLOCK))
 		update_rq_clock(rq);
 
-	if (!(flags & DEQUEUE_SAVE))
+	if (!(flags & DEQUEUE_SAVE)) {
 		sched_info_dequeued(rq, p);
+		psi_dequeue(p, flags & DEQUEUE_SLEEP);
+	}
 
 	p->sched_class->dequeue_task(rq, p, flags);
 }
@@ -2037,6 +2041,7 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
 	cpu = select_task_rq(p, p->wake_cpu, SD_BALANCE_WAKE, wake_flags);
 	if (task_cpu(p) != cpu) {
 		wake_flags |= WF_MIGRATED;
+		psi_ttwu_dequeue(p);
 		set_task_cpu(p, cpu);
 	}
 
@@ -3051,6 +3056,7 @@ void scheduler_tick(void)
 	curr->sched_class->task_tick(rq, curr, 0);
 	cpu_load_update_active(rq);
 	calc_global_load_tick(rq);
+	psi_task_tick(rq);
 
 	rq_unlock(rq, &rf);
 
@@ -6067,6 +6073,8 @@ void __init sched_init(void)
 
 	init_schedstats();
 
+	psi_init();
+
 	scheduler_running = 1;
 }
 
diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
new file mode 100644
index 000000000000..595414599b98
--- /dev/null
+++ b/kernel/sched/psi.c
@@ -0,0 +1,657 @@
+/*
+ * Pressure stall information for CPU, memory and IO
+ *
+ * Copyright (c) 2018 Facebook, Inc.
+ * Author: Johannes Weiner <hannes@cmpxchg.org>
+ *
+ * When CPU, memory and IO are contended, tasks experience delays that
+ * reduce throughput and introduce latencies into the workload. Memory
+ * and IO contention, in addition, can cause a full loss of forward
+ * progress in which the CPU goes idle.
+ *
+ * This code aggregates individual task delays into resource pressure
+ * metrics that indicate problems with both workload health and
+ * resource utilization.
+ *
+ *			Model
+ *
+ * The time in which a task can execute on a CPU is our baseline for
+ * productivity. Pressure expresses the amount of time in which this
+ * potential cannot be realized due to resource contention.
+ *
+ * This concept of productivity has two components: the workload and
+ * the CPU. To measure the impact of pressure on both, we define two
+ * contention states for a resource: SOME and FULL.
+ *
+ * In the SOME state of a given resource, one or more tasks are
+ * delayed on that resource. This affects the workload's ability to
+ * perform work, but the CPU may still be executing other tasks.
+ *
+ * In the FULL state of a given resource, all non-idle tasks are
+ * delayed on that resource such that nobody is advancing and the CPU
+ * goes idle. This leaves both workload and CPU unproductive.
+ *
+ * (Naturally, the FULL state doesn't exist for the CPU resource.)
+ *
+ *	SOME = nr_delayed_tasks != 0
+ *	FULL = nr_delayed_tasks != 0 && nr_running_tasks == 0
+ *
+ * The percentage of wallclock time spent in those compound stall
+ * states gives pressure numbers between 0 and 100 for each resource,
+ * where the SOME percentage indicates workload slowdowns and the FULL
+ * percentage indicates reduced CPU utilization:
+ *
+ *	%SOME = time(SOME) / period
+ *	%FULL = time(FULL) / period
+ *
+ *			Multiple CPUs
+ *
+ * The more tasks and available CPUs there are, the more work can be
+ * performed concurrently. This means that the potential that can go
+ * unrealized due to resource contention *also* scales with non-idle
+ * tasks and CPUs.
+ *
+ * Consider a scenario where 257 number crunching tasks are trying to
+ * run concurrently on 256 CPUs. If we simply aggregated the task
+ * states, we would have to conclude a CPU SOME pressure number of
+ * 100%, since *somebody* is waiting on a runqueue at all
+ * times. However, that is clearly not the amount of contention the
+ * workload is experiencing: only one out of 256 possible exceution
+ * threads will be contended at any given time, or about 0.4%.
+ *
+ * Conversely, consider a scenario of 4 tasks and 4 CPUs where at any
+ * given time *one* of the tasks is delayed due to a lack of memory.
+ * Again, looking purely at the task state would yield a memory FULL
+ * pressure number of 0%, since *somebody* is always making forward
+ * progress. But again this wouldn't capture the amount of execution
+ * potential lost, which is 1 out of 4 CPUs, or 25%.
+ *
+ * To calculate wasted potential (pressure) with multiple processors,
+ * we have to base our calculation on the number of non-idle tasks in
+ * conjunction with the number of available CPUs, which is the number
+ * of potential execution threads. SOME becomes then the proportion of
+ * delayed tasks to possibe threads, and FULL is the share of possible
+ * threads that are unproductive due to delays:
+ *
+ *	threads = min(nr_nonidle_tasks, nr_cpus)
+ *	   SOME = min(nr_delayed_tasks / threads, 1)
+ *	   FULL = (threads - min(nr_running_tasks, threads)) / threads
+ *
+ * For the 257 number crunchers on 256 CPUs, this yields:
+ *
+ *	threads = min(257, 256)
+ *	   SOME = min(1 / 256, 1)             = 0.4%
+ *	   FULL = (256 - min(257, 256)) / 256 = 0%
+ *
+ * For the 1 out of 4 memory-delayed tasks, this yields:
+ *
+ *	threads = min(4, 4)
+ *	   SOME = min(1 / 4, 1)               = 25%
+ *	   FULL = (4 - min(3, 4)) / 4         = 25%
+ *
+ * [ Substitute nr_cpus with 1, and you can see that it's a natural
+ *   extension of the single-CPU model. ]
+ *
+ *			Implementation
+ *
+ * To assess the precise time spent in each such state, we would have
+ * to freeze the system on task changes and start/stop the state
+ * clocks accordingly. Obviously that doesn't scale in practice.
+ *
+ * Because the scheduler aims to distribute the compute load evenly
+ * among the available CPUs, we can track task state locally to each
+ * CPU and, at much lower frequency, extrapolate the global state for
+ * the cumulative stall times and the running averages.
+ *
+ * For each runqueue, we track:
+ *
+ *	   tSOME[cpu] = time(nr_delayed_tasks[cpu] != 0)
+ *	   tFULL[cpu] = time(nr_delayed_tasks[cpu] && !nr_running_tasks[cpu])
+ *	tNONIDLE[cpu] = time(nr_nonidle_tasks[cpu] != 0)
+ *
+ * and then periodically aggregate:
+ *
+ *	tNONIDLE = sum(tNONIDLE[i])
+ *
+ *	   tSOME = sum(tSOME[i] * tNONIDLE[i]) / tNONIDLE
+ *	   tFULL = sum(tFULL[i] * tNONIDLE[i]) / tNONIDLE
+ *
+ *	   %SOME = tSOME / period
+ *	   %FULL = tFULL / period
+ *
+ * This gives us an approximation of pressure that is practical
+ * cost-wise, yet way more sensitive and accurate than periodic
+ * sampling of the aggregate task states would be.
+ */
+
+#include <linux/sched/loadavg.h>
+#include <linux/seq_file.h>
+#include <linux/proc_fs.h>
+#include <linux/seqlock.h>
+#include <linux/cgroup.h>
+#include <linux/module.h>
+#include <linux/sched.h>
+#include <linux/psi.h>
+#include "sched.h"
+
+static int psi_bug __read_mostly;
+
+bool psi_disabled __read_mostly;
+core_param(psi_disabled, psi_disabled, bool, 0644);
+
+/* Running averages - we need to be higher-res than loadavg */
+#define PSI_FREQ	(2*HZ+1)	/* 2 sec intervals */
+#define EXP_10s		1677		/* 1/exp(2s/10s) as fixed-point */
+#define EXP_60s		1981		/* 1/exp(2s/60s) */
+#define EXP_300s	2034		/* 1/exp(2s/300s) */
+
+/* Sampling frequency in nanoseconds */
+static u64 psi_period __read_mostly;
+
+/* System-level pressure and stall tracking */
+static DEFINE_PER_CPU(struct psi_group_cpu, system_group_pcpu);
+static struct psi_group psi_system = {
+	.pcpu = &system_group_pcpu,
+};
+
+static void psi_update_work(struct work_struct *work);
+
+static void group_init(struct psi_group *group)
+{
+	int cpu;
+
+	for_each_possible_cpu(cpu)
+		seqcount_init(&per_cpu_ptr(group->pcpu, cpu)->seq);
+	group->next_update = sched_clock() + psi_period;
+	INIT_DELAYED_WORK(&group->clock_work, psi_update_work);
+	mutex_init(&group->stat_lock);
+}
+
+void __init psi_init(void)
+{
+	if (psi_disabled)
+		return;
+
+	psi_period = jiffies_to_nsecs(PSI_FREQ);
+	group_init(&psi_system);
+}
+
+static bool test_state(unsigned int *tasks, enum psi_states state)
+{
+	switch (state) {
+	case PSI_IO_SOME:
+		return tasks[NR_IOWAIT];
+	case PSI_IO_FULL:
+		return tasks[NR_IOWAIT] && !tasks[NR_RUNNING];
+	case PSI_MEM_SOME:
+		return tasks[NR_MEMSTALL];
+	case PSI_MEM_FULL:
+		return tasks[NR_MEMSTALL] && !tasks[NR_RUNNING];
+	case PSI_CPU_SOME:
+		return tasks[NR_RUNNING] > 1;
+	case PSI_NONIDLE:
+		return tasks[NR_IOWAIT] || tasks[NR_MEMSTALL] ||
+			tasks[NR_RUNNING];
+	default:
+		return false;
+	}
+}
+
+static void get_recent_times(struct psi_group *group, int cpu, u32 *times)
+{
+	struct psi_group_cpu *groupc = per_cpu_ptr(group->pcpu, cpu);
+	unsigned int tasks[NR_PSI_TASK_COUNTS];
+	u64 now, state_start;
+	unsigned int seq;
+	int s;
+
+	/* Snapshot a coherent view of the CPU state */
+	do {
+		seq = read_seqcount_begin(&groupc->seq);
+		now = cpu_clock(cpu);
+		memcpy(times, groupc->times, sizeof(groupc->times));
+		memcpy(tasks, groupc->tasks, sizeof(groupc->tasks));
+		state_start = groupc->state_start;
+	} while (read_seqcount_retry(&groupc->seq, seq));
+
+	/* Calculate state time deltas against the previous snapshot */
+	for (s = 0; s < NR_PSI_STATES; s++) {
+		u32 delta;
+		/*
+		 * In addition to already concluded states, we also
+		 * incorporate currently active states on the CPU,
+		 * since states may last for many sampling periods.
+		 *
+		 * This way we keep our delta sampling buckets small
+		 * (u32) and our reported pressure close to what's
+		 * actually happening.
+		 */
+		if (test_state(tasks, s))
+			times[s] += now - state_start;
+
+		delta = times[s] - groupc->times_prev[s];
+		groupc->times_prev[s] = times[s];
+
+		times[s] = delta;
+	}
+}
+
+static void calc_avgs(unsigned long avg[3], int missed_periods,
+		      u64 time, u64 period)
+{
+	unsigned long pct;
+
+	/* Fill in zeroes for periods of no activity */
+	if (missed_periods) {
+		avg[0] = calc_load_n(avg[0], EXP_10s, 0, missed_periods);
+		avg[1] = calc_load_n(avg[1], EXP_60s, 0, missed_periods);
+		avg[2] = calc_load_n(avg[2], EXP_300s, 0, missed_periods);
+	}
+
+	/* Sample the most recent active period */
+	pct = div_u64(time * 100, period);
+	pct *= FIXED_1;
+	avg[0] = calc_load(avg[0], EXP_10s, pct);
+	avg[1] = calc_load(avg[1], EXP_60s, pct);
+	avg[2] = calc_load(avg[2], EXP_300s, pct);
+}
+
+static bool update_stats(struct psi_group *group)
+{
+	u64 deltas[NR_PSI_STATES - 1] = { 0, };
+	unsigned long missed_periods = 0;
+	unsigned long nonidle_total = 0;
+	u64 now, expires, period;
+	int cpu;
+	int s;
+
+	mutex_lock(&group->stat_lock);
+
+	/*
+	 * Collect the per-cpu time buckets and average them into a
+	 * single time sample that is normalized to wallclock time.
+	 *
+	 * For averaging, each CPU is weighted by its non-idle time in
+	 * the sampling period. This eliminates artifacts from uneven
+	 * loading, or even entirely idle CPUs.
+	 */
+	for_each_possible_cpu(cpu) {
+		u32 times[NR_PSI_STATES];
+		u32 nonidle;
+
+		get_recent_times(group, cpu, times);
+
+		nonidle = nsecs_to_jiffies(times[PSI_NONIDLE]);
+		nonidle_total += nonidle;
+
+		for (s = 0; s < PSI_NONIDLE; s++)
+			deltas[s] += (u64)times[s] * nonidle;
+	}
+
+	/*
+	 * Integrate the sample into the running statistics that are
+	 * reported to userspace: the cumulative stall times and the
+	 * decaying averages.
+	 *
+	 * Pressure percentages are sampled at PSI_FREQ. We might be
+	 * called more often when the user polls more frequently than
+	 * that; we might be called less often when there is no task
+	 * activity, thus no data, and clock ticks are sporadic. The
+	 * below handles both.
+	 */
+
+	/* total= */
+	for (s = 0; s < NR_PSI_STATES - 1; s++)
+		group->total[s] += div_u64(deltas[s], max(nonidle_total, 1UL));
+
+	/* avgX= */
+	now = sched_clock();
+	expires = group->next_update;
+	if (now < expires)
+		goto out;
+	if (now - expires > psi_period)
+		missed_periods = div_u64(now - expires, psi_period);
+
+	/*
+	 * The periodic clock tick can get delayed for various
+	 * reasons, especially on loaded systems. To avoid clock
+	 * drift, we schedule the clock in fixed psi_period intervals.
+	 * But the deltas we sample out of the per-cpu buckets above
+	 * are based on the actual time elapsing between clock ticks.
+	 */
+	group->next_update = expires + ((1 + missed_periods) * psi_period);
+	period = now - (group->last_update + (missed_periods * psi_period));
+	group->last_update = now;
+
+	for (s = 0; s < NR_PSI_STATES - 1; s++) {
+		u32 sample;
+
+		sample = group->total[s] - group->total_prev[s];
+		/*
+		 * Due to the lockless sampling of the time buckets,
+		 * recorded time deltas can slip into the next period,
+		 * which under full pressure can result in samples in
+		 * excess of the period length.
+		 *
+		 * We don't want to report non-sensical pressures in
+		 * excess of 100%, nor do we want to drop such events
+		 * on the floor. Instead we punt any overage into the
+		 * future until pressure subsides. By doing this we
+		 * don't underreport the occurring pressure curve, we
+		 * just report it delayed by one period length.
+		 *
+		 * The error isn't cumulative. As soon as another
+		 * delta slips from a period P to P+1, by definition
+		 * it frees up its time T in P.
+		 */
+		if (sample > period)
+			sample = period;
+		group->total_prev[s] += sample;
+		calc_avgs(group->avg[s], missed_periods, sample, period);
+	}
+out:
+	mutex_unlock(&group->stat_lock);
+	return nonidle_total;
+}
+
+static void psi_update_work(struct work_struct *work)
+{
+	struct delayed_work *dwork;
+	struct psi_group *group;
+	bool nonidle;
+
+	dwork = to_delayed_work(work);
+	group = container_of(dwork, struct psi_group, clock_work);
+
+	/*
+	 * If there is task activity, periodically fold the per-cpu
+	 * times and feed samples into the running averages. If things
+	 * are idle and there is no data to process, stop the clock.
+	 * Once restarted, we'll catch up the running averages in one
+	 * go - see calc_avgs() and missed_periods.
+	 */
+
+	nonidle = update_stats(group);
+
+	if (nonidle) {
+		unsigned long delay = 0;
+		u64 now;
+
+		now = sched_clock();
+		if (group->next_update > now)
+			delay = nsecs_to_jiffies(group->next_update - now) + 1;
+		schedule_delayed_work(dwork, delay);
+	}
+}
+
+static void record_times(struct psi_group_cpu *groupc, int cpu,
+			 bool memstall_tick)
+{
+	u32 delta;
+	u64 now;
+
+	now = cpu_clock(cpu);
+	delta = now - groupc->state_start;
+	groupc->state_start = now;
+
+	if (test_state(groupc->tasks, PSI_IO_SOME)) {
+		groupc->times[PSI_IO_SOME] += delta;
+		if (test_state(groupc->tasks, PSI_IO_FULL))
+			groupc->times[PSI_IO_FULL] += delta;
+	}
+
+	if (test_state(groupc->tasks, PSI_MEM_SOME)) {
+		groupc->times[PSI_MEM_SOME] += delta;
+		if (test_state(groupc->tasks, PSI_MEM_FULL))
+			groupc->times[PSI_MEM_FULL] += delta;
+		else if (memstall_tick) {
+			u32 sample;
+			/*
+			 * Since we care about lost potential, a
+			 * memstall is FULL when there are no other
+			 * working tasks, but also when the CPU is
+			 * actively reclaiming and nothing productive
+			 * could run even if it were runnable.
+			 *
+			 * When the timer tick sees a reclaiming CPU,
+			 * regardless of runnable tasks, sample a FULL
+			 * tick (or less if it hasn't been a full tick
+			 * since the last state change).
+			 */
+			sample = min(delta, (u32)jiffies_to_nsecs(1));
+			groupc->times[PSI_MEM_FULL] += sample;
+		}
+	}
+
+	if (test_state(groupc->tasks, PSI_CPU_SOME))
+		groupc->times[PSI_CPU_SOME] += delta;
+
+	if (test_state(groupc->tasks, PSI_NONIDLE))
+		groupc->times[PSI_NONIDLE] += delta;
+}
+
+static void psi_group_change(struct psi_group *group, int cpu,
+			     unsigned int clear, unsigned int set)
+{
+	struct psi_group_cpu *groupc;
+	unsigned int t, m;
+
+	groupc = per_cpu_ptr(group->pcpu, cpu);
+
+	/*
+	 * First we assess the aggregate resource states this CPU's
+	 * tasks have been in since the last change, and account any
+	 * SOME and FULL time these may have resulted in.
+	 *
+	 * Then we update the task counts according to the state
+	 * change requested through the @clear and @set bits.
+	 */
+	write_seqcount_begin(&groupc->seq);
+
+	record_times(groupc, cpu, false);
+
+	for (t = 0, m = clear; m; m &= ~(1 << t), t++) {
+		if (!(m & (1 << t)))
+			continue;
+		if (groupc->tasks[t] == 0 && !psi_bug) {
+			printk_deferred(KERN_ERR "psi: task underflow! cpu=%d t=%d tasks=[%u %u %u] clear=%x set=%x\n",
+					cpu, t, groupc->tasks[0],
+					groupc->tasks[1], groupc->tasks[2],
+					clear, set);
+			psi_bug = 1;
+		}
+		groupc->tasks[t]--;
+	}
+
+	for (t = 0; set; set &= ~(1 << t), t++)
+		if (set & (1 << t))
+			groupc->tasks[t]++;
+
+	write_seqcount_end(&groupc->seq);
+
+	if (!delayed_work_pending(&group->clock_work))
+		schedule_delayed_work(&group->clock_work, PSI_FREQ);
+}
+
+void psi_task_change(struct task_struct *task, int clear, int set)
+{
+	int cpu = task_cpu(task);
+
+	if (!task->pid)
+		return;
+
+	if (((task->psi_flags & set) ||
+	     (task->psi_flags & clear) != clear) &&
+	    !psi_bug) {
+		printk_deferred(KERN_ERR "psi: inconsistent task state! task=%d:%s cpu=%d psi_flags=%x clear=%x set=%x\n",
+				task->pid, task->comm, cpu,
+				task->psi_flags, clear, set);
+		psi_bug = 1;
+	}
+
+	task->psi_flags &= ~clear;
+	task->psi_flags |= set;
+
+	psi_group_change(&psi_system, cpu, clear, set);
+}
+
+void psi_memstall_tick(struct task_struct *task, int cpu)
+{
+	struct psi_group_cpu *groupc;
+
+	groupc = per_cpu_ptr(psi_system.pcpu, cpu);
+	write_seqcount_begin(&groupc->seq);
+	record_times(groupc, cpu, true);
+	write_seqcount_end(&groupc->seq);
+}
+
+/**
+ * psi_memstall_enter - mark the beginning of a memory stall section
+ * @flags: flags to handle nested sections
+ *
+ * Marks the calling task as being stalled due to a lack of memory,
+ * such as waiting for a refault or performing reclaim.
+ */
+void psi_memstall_enter(unsigned long *flags)
+{
+	struct rq_flags rf;
+	struct rq *rq;
+
+	if (psi_disabled)
+		return;
+
+	*flags = current->flags & PF_MEMSTALL;
+	if (*flags)
+		return;
+	/*
+	 * PF_MEMSTALL setting & accounting needs to be atomic wrt
+	 * changes to the task's scheduling state, otherwise we can
+	 * race with CPU migration.
+	 */
+	rq = this_rq_lock_irq(&rf);
+
+	current->flags |= PF_MEMSTALL;
+	psi_task_change(current, 0, TSK_MEMSTALL);
+
+	rq_unlock_irq(rq, &rf);
+}
+
+/**
+ * psi_memstall_leave - mark the end of an memory stall section
+ * @flags: flags to handle nested memdelay sections
+ *
+ * Marks the calling task as no longer stalled due to lack of memory.
+ */
+void psi_memstall_leave(unsigned long *flags)
+{
+	struct rq_flags rf;
+	struct rq *rq;
+
+	if (psi_disabled)
+		return;
+
+	if (*flags)
+		return;
+	/*
+	 * PF_MEMSTALL clearing & accounting needs to be atomic wrt
+	 * changes to the task's scheduling state, otherwise we could
+	 * race with CPU migration.
+	 */
+	rq = this_rq_lock_irq(&rf);
+
+	current->flags &= ~PF_MEMSTALL;
+	psi_task_change(current, TSK_MEMSTALL, 0);
+
+	rq_unlock_irq(rq, &rf);
+}
+
+static int psi_show(struct seq_file *m, struct psi_group *group,
+		    enum psi_res res)
+{
+	int full;
+
+	if (psi_disabled)
+		return -EOPNOTSUPP;
+
+	update_stats(group);
+
+	for (full = 0; full < 2 - (res == PSI_CPU); full++) {
+		unsigned long avg[3];
+		u64 total;
+		int w;
+
+		for (w = 0; w < 3; w++)
+			avg[w] = group->avg[res * 2 + full][w];
+		total = div_u64(group->total[res * 2 + full], NSEC_PER_USEC);
+
+		seq_printf(m, "%s avg10=%lu.%02lu avg60=%lu.%02lu avg300=%lu.%02lu total=%llu\n",
+			   full ? "full" : "some",
+			   LOAD_INT(avg[0]), LOAD_FRAC(avg[0]),
+			   LOAD_INT(avg[1]), LOAD_FRAC(avg[1]),
+			   LOAD_INT(avg[2]), LOAD_FRAC(avg[2]),
+			   total);
+	}
+
+	return 0;
+}
+
+static int psi_io_show(struct seq_file *m, void *v)
+{
+	return psi_show(m, &psi_system, PSI_IO);
+}
+
+static int psi_memory_show(struct seq_file *m, void *v)
+{
+	return psi_show(m, &psi_system, PSI_MEM);
+}
+
+static int psi_cpu_show(struct seq_file *m, void *v)
+{
+	return psi_show(m, &psi_system, PSI_CPU);
+}
+
+static int psi_io_open(struct inode *inode, struct file *file)
+{
+	return single_open(file, psi_io_show, NULL);
+}
+
+static int psi_memory_open(struct inode *inode, struct file *file)
+{
+	return single_open(file, psi_memory_show, NULL);
+}
+
+static int psi_cpu_open(struct inode *inode, struct file *file)
+{
+	return single_open(file, psi_cpu_show, NULL);
+}
+
+static const struct file_operations psi_io_fops = {
+	.open           = psi_io_open,
+	.read           = seq_read,
+	.llseek         = seq_lseek,
+	.release        = single_release,
+};
+
+static const struct file_operations psi_memory_fops = {
+	.open           = psi_memory_open,
+	.read           = seq_read,
+	.llseek         = seq_lseek,
+	.release        = single_release,
+};
+
+static const struct file_operations psi_cpu_fops = {
+	.open           = psi_cpu_open,
+	.read           = seq_read,
+	.llseek         = seq_lseek,
+	.release        = single_release,
+};
+
+static int __init psi_proc_init(void)
+{
+	proc_mkdir("pressure", NULL);
+	proc_create("pressure/io", 0, NULL, &psi_io_fops);
+	proc_create("pressure/memory", 0, NULL, &psi_memory_fops);
+	proc_create("pressure/cpu", 0, NULL, &psi_cpu_fops);
+	return 0;
+}
+module_init(psi_proc_init);
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 1de189bb9209..618577fc9aa8 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -54,6 +54,7 @@
 #include <linux/proc_fs.h>
 #include <linux/prefetch.h>
 #include <linux/profile.h>
+#include <linux/psi.h>
 #include <linux/rcupdate_wait.h>
 #include <linux/security.h>
 #include <linux/stop_machine.h>
@@ -319,6 +320,7 @@ extern bool dl_cpu_busy(unsigned int cpu);
 #ifdef CONFIG_CGROUP_SCHED
 
 #include <linux/cgroup.h>
+#include <linux/psi.h>
 
 struct cfs_rq;
 struct rt_rq;
diff --git a/kernel/sched/stats.h b/kernel/sched/stats.h
index 8aea199a39b4..4904c4677000 100644
--- a/kernel/sched/stats.h
+++ b/kernel/sched/stats.h
@@ -55,6 +55,92 @@ static inline void rq_sched_info_depart  (struct rq *rq, unsigned long long delt
 # define   schedstat_val_or_zero(var)	0
 #endif /* CONFIG_SCHEDSTATS */
 
+#ifdef CONFIG_PSI
+/*
+ * PSI tracks state that persists across sleeps, such as iowaits and
+ * memory stalls. As a result, it has to distinguish between sleeps,
+ * where a task's runnable state changes, and requeues, where a task
+ * and its state are being moved between CPUs and runqueues.
+ */
+static inline void psi_enqueue(struct task_struct *p, bool wakeup)
+{
+	int clear = 0, set = TSK_RUNNING;
+
+	if (psi_disabled)
+		return;
+
+	if (!wakeup || p->sched_psi_wake_requeue) {
+		if (p->flags & PF_MEMSTALL)
+			set |= TSK_MEMSTALL;
+		if (p->sched_psi_wake_requeue)
+			p->sched_psi_wake_requeue = 0;
+	} else {
+		if (p->in_iowait)
+			clear |= TSK_IOWAIT;
+	}
+
+	psi_task_change(p, clear, set);
+}
+
+static inline void psi_dequeue(struct task_struct *p, bool sleep)
+{
+	int clear = TSK_RUNNING, set = 0;
+
+	if (psi_disabled)
+		return;
+
+	if (!sleep) {
+		if (p->flags & PF_MEMSTALL)
+			clear |= TSK_MEMSTALL;
+	} else {
+		if (p->in_iowait)
+			set |= TSK_IOWAIT;
+	}
+
+	psi_task_change(p, clear, set);
+}
+
+static inline void psi_ttwu_dequeue(struct task_struct *p)
+{
+	if (psi_disabled)
+		return;
+	/*
+	 * Is the task being migrated during a wakeup? Make sure to
+	 * deregister its sleep-persistent psi states from the old
+	 * queue, and let psi_enqueue() know it has to requeue.
+	 */
+	if (unlikely(p->in_iowait || (p->flags & PF_MEMSTALL))) {
+		struct rq_flags rf;
+		struct rq *rq;
+		int clear = 0;
+
+		if (p->in_iowait)
+			clear |= TSK_IOWAIT;
+		if (p->flags & PF_MEMSTALL)
+			clear |= TSK_MEMSTALL;
+
+		rq = __task_rq_lock(p, &rf);
+		psi_task_change(p, clear, 0);
+		p->sched_psi_wake_requeue = 1;
+		__task_rq_unlock(rq, &rf);
+	}
+}
+
+static inline void psi_task_tick(struct rq *rq)
+{
+	if (psi_disabled)
+		return;
+
+	if (unlikely(rq->curr->flags & PF_MEMSTALL))
+		psi_memstall_tick(rq->curr, cpu_of(rq));
+}
+#else /* CONFIG_PSI */
+static inline void psi_enqueue(struct task_struct *p, bool wakeup) {}
+static inline void psi_dequeue(struct task_struct *p, bool sleep) {}
+static inline void psi_ttwu_dequeue(struct task_struct *p) {}
+static inline void psi_task_tick(struct rq *rq) {}
+#endif /* CONFIG_PSI */
+
 #ifdef CONFIG_SCHED_INFO
 static inline void sched_info_reset_dequeued(struct task_struct *t)
 {