diff --git a/Documentation/accounting/psi.txt b/Documentation/accounting/psi.txt
new file mode 100644
index 000000000000..51e7ef14142e
--- /dev/null
+++ b/Documentation/accounting/psi.txt
@@ -0,0 +1,64 @@
+================================
+PSI - Pressure Stall Information
+================================
+
+:Date: April, 2018
+:Author: Johannes Weiner <hannes@cmpxchg.org>
+
+When CPU, memory or IO devices are contended, workloads experience
+latency spikes, throughput losses, and run the risk of OOM kills.
+
+Without an accurate measure of such contention, users are forced to
+either play it safe and under-utilize their hardware resources, or
+roll the dice and frequently suffer the disruptions resulting from
+excessive overcommit.
+
+The psi feature identifies and quantifies the disruptions caused by
+such resource crunches and the time impact it has on complex workloads
+or even entire systems.
+
+Having an accurate measure of productivity losses caused by resource
+scarcity aids users in sizing workloads to hardware--or provisioning
+hardware according to workload demand.
+
+As psi aggregates this information in realtime, systems can be managed
+dynamically using techniques such as load shedding, migrating jobs to
+other systems or data centers, or strategically pausing or killing low
+priority or restartable batch jobs.
+
+This allows maximizing hardware utilization without sacrificing
+workload health or risking major disruptions such as OOM kills.
+
+Pressure interface
+==================
+
+Pressure information for each resource is exported through the
+respective file in /proc/pressure/ -- cpu, memory, and io.
+
+In both cases, the format for CPU is as such:
+
+some avg10=0.00 avg60=0.00 avg300=0.00 total=0
+
+and for memory and IO:
+
+some avg10=0.00 avg60=0.00 avg300=0.00 total=0
+full avg10=0.00 avg60=0.00 avg300=0.00 total=0
+
+The "some" line indicates the share of time in which at least some
+tasks are stalled on a given resource.
+
+The "full" line indicates the share of time in which all non-idle
+tasks are stalled on a given resource simultaneously. In this state
+actual CPU cycles are going to waste, and a workload that spends
+extended time in this state is considered to be thrashing. This has
+severe impact on performance, and it's useful to distinguish this
+situation from a state where some tasks are stalled but the CPU is
+still doing productive work. As such, time spent in this subset of the
+stall state is tracked separately and exported in the "full" averages.
+
+The ratios are tracked as recent trends over ten, sixty, and three
+hundred second windows, which gives insight into short term events as
+well as medium and long term trends. The total absolute stall time is
+tracked and exported as well, to allow detection of latency spikes
+which wouldn't necessarily make a dent in the time averages, or to
+average trends over custom time frames.
diff --git a/include/linux/psi.h b/include/linux/psi.h
new file mode 100644
index 000000000000..371af1479699
--- /dev/null
+++ b/include/linux/psi.h
@@ -0,0 +1,27 @@
+#ifndef _LINUX_PSI_H
+#define _LINUX_PSI_H
+
+#include <linux/psi_types.h>
+#include <linux/sched.h>
+
+#ifdef CONFIG_PSI
+
+extern bool psi_disabled;
+
+void psi_init(void);
+
+void psi_task_change(struct task_struct *task, u64 now, int clear, int set);
+
+void psi_memstall_enter(unsigned long *flags);
+void psi_memstall_leave(unsigned long *flags);
+
+#else /* CONFIG_PSI */
+
+static inline void psi_init(void) {}
+
+static inline void psi_memstall_enter(unsigned long *flags) {}
+static inline void psi_memstall_leave(unsigned long *flags) {}
+
+#endif /* CONFIG_PSI */
+
+#endif /* _LINUX_PSI_H */
diff --git a/include/linux/psi_types.h b/include/linux/psi_types.h
new file mode 100644
index 000000000000..b6ff46362eb3
--- /dev/null
+++ b/include/linux/psi_types.h
@@ -0,0 +1,87 @@
+#ifndef _LINUX_PSI_TYPES_H
+#define _LINUX_PSI_TYPES_H
+
+#include <linux/types.h>
+
+#ifdef CONFIG_PSI
+
+/* Tracked task states */
+enum psi_task_count {
+	NR_IOWAIT,
+	NR_MEMSTALL,
+	NR_RUNNING,
+	NR_PSI_TASK_COUNTS,
+};
+
+/* Task state bitmasks */
+#define TSK_IOWAIT	(1 << NR_IOWAIT)
+#define TSK_MEMSTALL	(1 << NR_MEMSTALL)
+#define TSK_RUNNING	(1 << NR_RUNNING)
+
+/* Resources that workloads could be stalled on */
+enum psi_res {
+	PSI_IO,
+	PSI_MEM,
+	PSI_CPU,
+	NR_PSI_RESOURCES,
+};
+
+/*
+ * Pressure states for each resource:
+ *
+ * SOME: Stalled tasks & working tasks
+ * FULL: Stalled tasks & no working tasks
+ */
+enum psi_states {
+	PSI_IO_SOME,
+	PSI_IO_FULL,
+	PSI_MEM_SOME,
+	PSI_MEM_FULL,
+	PSI_CPU_SOME,
+	PSI_NONIDLE,
+	NR_PSI_STATES,
+};
+
+struct psi_group_cpu {
+	/* 1st cacheline updated by the scheduler */
+
+	/* States of the tasks belonging to this group */
+	unsigned int tasks[NR_PSI_TASK_COUNTS] ____cacheline_aligned_in_smp;
+
+	/* Period time sampling buckets for each state of interest (ns) */
+	u32 times[NR_PSI_STATES];
+
+	/* Time of last task change in this group (rq_clock) */
+	u64 state_start;
+
+	/* 2nd cacheline updated by the aggregator */
+
+	/* Delta detection against the sampling buckets */
+	u32 times_prev[NR_PSI_STATES] ____cacheline_aligned_in_smp;
+};
+
+struct psi_group {
+	/* Protects data updated during an aggregation */
+	struct mutex stat_lock;
+
+	/* Per-cpu task state & time tracking */
+	struct psi_group_cpu __percpu *pcpu;
+
+	/* Periodic aggregation state */
+	u64 total_prev[NR_PSI_STATES - 1];
+	u64 last_update;
+	u64 next_update;
+	struct delayed_work clock_work;
+
+	/* Total stall times and sampled pressure averages */
+	u64 total[NR_PSI_STATES - 1];
+	unsigned long avg[NR_PSI_STATES - 1][3];
+};
+
+#else /* CONFIG_PSI */
+
+struct psi_group { };
+
+#endif /* CONFIG_PSI */
+
+#endif /* _LINUX_PSI_TYPES_H */
diff --git a/include/linux/sched.h b/include/linux/sched.h
index ca3f3eae8980..d5e4ee234114 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -25,6 +25,7 @@
 #include <linux/latencytop.h>
 #include <linux/sched/prio.h>
 #include <linux/signal_types.h>
+#include <linux/psi_types.h>
 #include <linux/mm_types_task.h>
 #include <linux/task_io_accounting.h>
 
@@ -709,6 +710,10 @@ struct task_struct {
 	unsigned			sched_contributes_to_load:1;
 	unsigned			sched_migrated:1;
 	unsigned			sched_remote_wakeup:1;
+#ifdef CONFIG_PSI
+	unsigned			sched_psi_wake_requeue:1;
+#endif
+
 	/* Force alignment to the next boundary: */
 	unsigned			:0;
 
@@ -956,6 +961,10 @@ struct task_struct {
 	siginfo_t			*last_siginfo;
 
 	struct task_io_accounting	ioac;
+#ifdef CONFIG_PSI
+	/* Pressure stall state */
+	unsigned int			psi_flags;
+#endif
 #ifdef CONFIG_TASK_XACCT
 	/* Accumulated RSS usage: */
 	u64				acct_rss_mem1;
@@ -1385,6 +1394,7 @@ extern struct pid *cad_pid;
 #define PF_KTHREAD		0x00200000	/* I am a kernel thread */
 #define PF_RANDOMIZE		0x00400000	/* Randomize virtual address space */
 #define PF_SWAPWRITE		0x00800000	/* Allowed to write to swap */
+#define PF_MEMSTALL		0x01000000	/* Stalled due to lack of memory */
 #define PF_NO_SETAFFINITY	0x04000000	/* Userland is not allowed to meddle with cpus_allowed */
 #define PF_MCE_EARLY		0x08000000      /* Early kill for mce process policy */
 #define PF_MUTEX_TESTER		0x20000000	/* Thread belongs to the rt mutex tester */
diff --git a/init/Kconfig b/init/Kconfig
index 18b151f0ddc1..ad61ddb5d68e 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -457,6 +457,21 @@ config TASK_IO_ACCOUNTING
 
 	  Say N if unsure.
 
+config PSI
+	bool "Pressure stall information tracking"
+	help
+	  Collect metrics that indicate how overcommitted the CPU, memory,
+	  and IO capacity are in the system.
+
+	  If you say Y here, the kernel will create /proc/pressure/ with the
+	  pressure statistics files cpu, memory, and io. These will indicate
+	  the share of walltime in which some or all tasks in the system are
+	  delayed due to contention of the respective resource.
+
+	  For more details see Documentation/accounting/psi.txt.
+
+	  Say N if unsure.
+
 endmenu # "CPU/Task time and stats accounting"
 
 config CPU_ISOLATION
diff --git a/kernel/fork.c b/kernel/fork.c
index a5d21c42acfc..067aa5c28526 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1704,6 +1704,10 @@ static __latent_entropy struct task_struct *copy_process(
 
 	p->default_timer_slack_ns = current->timer_slack_ns;
 
+#ifdef CONFIG_PSI
+	p->psi_flags = 0;
+#endif
+
 	task_io_accounting_init(&p->ioac);
 	acct_clear_integrals(p);
 
diff --git a/kernel/sched/Makefile b/kernel/sched/Makefile
index d9a02b318108..b29bc18f2704 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 9586a8141f16..e53137df405b 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -743,8 +743,10 @@ 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(rq, p, flags & ENQUEUE_WAKEUP);
+	}
 
 	p->sched_class->enqueue_task(rq, p, flags);
 }
@@ -754,8 +756,10 @@ 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(rq, p, flags & DEQUEUE_SLEEP);
+	}
 
 	p->sched_class->dequeue_task(rq, p, flags);
 }
@@ -2058,6 +2062,7 @@ 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);
 	}
 
@@ -6124,6 +6129,8 @@ 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..57ec86592b5a
--- /dev/null
+++ b/kernel/sched/psi.c
@@ -0,0 +1,643 @@
+/*
+ * 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/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_clock(struct work_struct *work);
+
+static void psi_group_init(struct psi_group *group)
+{
+	group->next_update = sched_clock() + psi_period;
+	INIT_DELAYED_WORK(&group->clock_work, psi_clock);
+	mutex_init(&group->stat_lock);
+}
+
+void __init psi_init(void)
+{
+	if (psi_disabled)
+		return;
+
+	psi_period = jiffies_to_nsecs(PSI_FREQ);
+	psi_group_init(&psi_system);
+}
+
+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 test_state(unsigned int *tasks, int cpu, 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:
+		/*
+		 * Since we care about lost potential, things are
+		 * fully blocked on memory when there are no other
+		 * working tasks, but also when the CPU is actively
+		 * being used by a reclaimer and nothing productive
+		 * could run even if it were runnable.
+		 */
+		return tasks[NR_MEMSTALL] &&
+			(!tasks[NR_RUNNING] ||
+			 cpu_curr(cpu)->flags & PF_MEMSTALL);
+	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 bool psi_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.
+	 *
+	 * We don't need to synchronize against CPU hotplugging. If we
+	 * see a CPU that's online and has samples, we incorporate it.
+	 */
+	for_each_online_cpu(cpu) {
+		struct psi_group_cpu *groupc = per_cpu_ptr(group->pcpu, cpu);
+		u32 uninitialized_var(nonidle);
+
+		BUILD_BUG_ON(PSI_NONIDLE != NR_PSI_STATES - 1);
+
+		for (s = PSI_NONIDLE; s >= 0; s--) {
+			u32 time, delta;
+
+			time = READ_ONCE(groupc->times[s]);
+			/*
+			 * 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(groupc->tasks, cpu, s)) {
+				/*
+				 * We can race with a state change and
+				 * need to make sure the state_start
+				 * update is ordered against the
+				 * updates to the live state and the
+				 * time buckets (groupc->times).
+				 *
+				 * 1. If we observe task state that
+				 * needs to be recorded, make sure we
+				 * see state_start from when that
+				 * state went into effect or we'll
+				 * count time from the previous state.
+				 *
+				 * 2. If the time delta has already
+				 * been added to the bucket, make sure
+				 * we don't see it in state_start or
+				 * we'll count it twice.
+				 *
+				 * If the time delta is out of
+				 * state_start but not in the time
+				 * bucket yet, we'll miss it entirely
+				 * and handle it in the next period.
+				 */
+				smp_rmb();
+				time += cpu_clock(cpu) - groupc->state_start;
+			}
+			delta = time - groupc->times_prev[s];
+			groupc->times_prev[s] = time;
+
+			if (s == PSI_NONIDLE) {
+				nonidle = nsecs_to_jiffies(delta);
+				nonidle_total += nonidle;
+			} else {
+				deltas[s] += (u64)delta * 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_clock(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 = psi_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 psi_group_change(struct psi_group *group, int cpu, u64 now,
+			     unsigned int clear, unsigned int set)
+{
+	struct psi_group_cpu *groupc;
+	unsigned int t, m;
+	u32 delta;
+
+	groupc = per_cpu_ptr(group->pcpu, cpu);
+
+	/*
+	 * First we assess the aggregate resource states these CPU's
+	 * tasks have been in since the last change, and account any
+	 * SOME and FULL time that may have resulted in.
+	 *
+	 * Then we update the task counts according to the state
+	 * change requested through the @clear and @set bits.
+	 */
+
+	delta = now - groupc->state_start;
+	groupc->state_start = now;
+
+	/*
+	 * Update state_start before recording time in the sampling
+	 * buckets and changing task counts, to prevent a racing
+	 * aggregation from counting the delta twice or attributing it
+	 * to an old state.
+	 */
+	smp_wmb();
+
+	if (test_state(groupc->tasks, cpu, PSI_IO_SOME)) {
+		groupc->times[PSI_IO_SOME] += delta;
+		if (test_state(groupc->tasks, cpu, PSI_IO_FULL))
+			groupc->times[PSI_IO_FULL] += delta;
+	}
+	if (test_state(groupc->tasks, cpu, PSI_MEM_SOME)) {
+		groupc->times[PSI_MEM_SOME] += delta;
+		if (test_state(groupc->tasks, cpu, PSI_MEM_FULL))
+			groupc->times[PSI_MEM_FULL] += delta;
+	}
+	if (test_state(groupc->tasks, cpu, PSI_CPU_SOME))
+		groupc->times[PSI_CPU_SOME] += delta;
+	if (test_state(groupc->tasks, cpu, PSI_NONIDLE))
+		groupc->times[PSI_NONIDLE] += delta;
+
+	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]++;
+
+	if (!delayed_work_pending(&group->clock_work))
+		schedule_delayed_work(&group->clock_work, PSI_FREQ);
+}
+
+void psi_task_change(struct task_struct *task, u64 now, int clear, int set)
+{
+	int cpu = task_cpu(task);
+
+	if (psi_disabled)
+		return;
+
+	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, now, clear, set);
+}
+
+/**
+ * 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);
+
+	update_rq_clock(rq);
+
+	current->flags |= PF_MEMSTALL;
+	psi_task_change(current, rq_clock(rq), 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);
+
+	update_rq_clock(rq);
+
+	current->flags &= ~PF_MEMSTALL;
+	psi_task_change(current, rq_clock(rq), 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;
+
+	psi_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 bc798c7cb4d4..e798491ff329 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/stackprotector.h>
@@ -320,6 +321,7 @@ 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..f3e0267eb47d 100644
--- a/kernel/sched/stats.h
+++ b/kernel/sched/stats.h
@@ -55,6 +55,86 @@ 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 rq *rq, 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, rq_clock(rq), clear, set);
+}
+
+static inline void psi_dequeue(struct rq *rq, 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, rq_clock(rq), 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);
+		update_rq_clock(rq);
+		psi_task_change(p, rq_clock(rq), clear, 0);
+		p->sched_psi_wake_requeue = 1;
+		__task_rq_unlock(rq, &rf);
+	}
+}
+#else /* CONFIG_PSI */
+static inline void psi_enqueue(struct rq *rq, struct task_struct *p,
+			       bool wakeup) {}
+static inline void psi_dequeue(struct rq *rq, struct task_struct *p,
+			       bool sleep) {}
+static inline void psi_ttwu_dequeue(struct task_struct *p) {}
+#endif /* CONFIG_PSI */
+
 #ifdef CONFIG_SCHED_INFO
 static inline void sched_info_reset_dequeued(struct task_struct *t)
 {
diff --git a/mm/compaction.c b/mm/compaction.c
index 29bd1df18b98..8f9566745902 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -22,6 +22,7 @@
 #include <linux/kthread.h>
 #include <linux/freezer.h>
 #include <linux/page_owner.h>
+#include <linux/psi.h>
 #include "internal.h"
 
 #ifdef CONFIG_COMPACTION
@@ -2068,11 +2069,15 @@ static int kcompactd(void *p)
 	pgdat->kcompactd_classzone_idx = pgdat->nr_zones - 1;
 
 	while (!kthread_should_stop()) {
+		unsigned long pflags;
+
 		trace_mm_compaction_kcompactd_sleep(pgdat->node_id);
 		wait_event_freezable(pgdat->kcompactd_wait,
 				kcompactd_work_requested(pgdat));
 
+		psi_memstall_enter(&pflags);
 		kcompactd_do_work(pgdat);
+		psi_memstall_leave(&pflags);
 	}
 
 	return 0;
diff --git a/mm/filemap.c b/mm/filemap.c
index e49961e13dd9..eee06145b997 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -37,6 +37,7 @@
 #include <linux/shmem_fs.h>
 #include <linux/rmap.h>
 #include <linux/delayacct.h>
+#include <linux/psi.h>
 #include "internal.h"
 
 #define CREATE_TRACE_POINTS
@@ -1075,11 +1076,14 @@ static inline int wait_on_page_bit_common(wait_queue_head_t *q,
 	struct wait_page_queue wait_page;
 	wait_queue_entry_t *wait = &wait_page.wait;
 	bool thrashing = false;
+	unsigned long pflags;
 	int ret = 0;
 
-	if (bit_nr == PG_locked && !PageSwapBacked(page) &&
+	if (bit_nr == PG_locked &&
 	    !PageUptodate(page) && PageWorkingset(page)) {
-		delayacct_thrashing_start();
+		if (!PageSwapBacked(page))
+			delayacct_thrashing_start();
+		psi_memstall_enter(&pflags);
 		thrashing = true;
 	}
 
@@ -1121,8 +1125,11 @@ static inline int wait_on_page_bit_common(wait_queue_head_t *q,
 
 	finish_wait(q, wait);
 
-	if (thrashing)
-		delayacct_thrashing_end();
+	if (thrashing) {
+		if (!PageSwapBacked(page))
+			delayacct_thrashing_end();
+		psi_memstall_leave(&pflags);
+	}
 
 	/*
 	 * A signal could leave PageWaiters set. Clearing it here if
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 22320ea27489..8469f34e6731 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -67,6 +67,7 @@
 #include <linux/ftrace.h>
 #include <linux/lockdep.h>
 #include <linux/nmi.h>
+#include <linux/psi.h>
 
 #include <asm/sections.h>
 #include <asm/tlbflush.h>
@@ -3552,15 +3553,20 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
 		enum compact_priority prio, enum compact_result *compact_result)
 {
 	struct page *page;
+	unsigned long pflags;
 	unsigned int noreclaim_flag;
 
 	if (!order)
 		return NULL;
 
+	psi_memstall_enter(&pflags);
 	noreclaim_flag = memalloc_noreclaim_save();
+
 	*compact_result = try_to_compact_pages(gfp_mask, order, alloc_flags, ac,
 									prio);
+
 	memalloc_noreclaim_restore(noreclaim_flag);
+	psi_memstall_leave(&pflags);
 
 	if (*compact_result <= COMPACT_INACTIVE)
 		return NULL;
@@ -3749,11 +3755,14 @@ __perform_reclaim(gfp_t gfp_mask, unsigned int order,
 	struct reclaim_state reclaim_state;
 	int progress;
 	unsigned int noreclaim_flag;
+	unsigned long pflags;
 
 	cond_resched();
 
 	/* We now go into synchronous reclaim */
 	cpuset_memory_pressure_bump();
+
+	psi_memstall_enter(&pflags);
 	noreclaim_flag = memalloc_noreclaim_save();
 	fs_reclaim_acquire(gfp_mask);
 	reclaim_state.reclaimed_slab = 0;
@@ -3765,6 +3774,7 @@ __perform_reclaim(gfp_t gfp_mask, unsigned int order,
 	current->reclaim_state = NULL;
 	fs_reclaim_release(gfp_mask);
 	memalloc_noreclaim_restore(noreclaim_flag);
+	psi_memstall_leave(&pflags);
 
 	cond_resched();
 
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 8d1ad48ffbcd..ee91e8cbeb5a 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -49,6 +49,7 @@
 #include <linux/prefetch.h>
 #include <linux/printk.h>
 #include <linux/dax.h>
+#include <linux/psi.h>
 
 #include <asm/tlbflush.h>
 #include <asm/div64.h>
@@ -3115,6 +3116,7 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
 {
 	struct zonelist *zonelist;
 	unsigned long nr_reclaimed;
+	unsigned long pflags;
 	int nid;
 	unsigned int noreclaim_flag;
 	struct scan_control sc = {
@@ -3143,9 +3145,13 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
 					    sc.gfp_mask,
 					    sc.reclaim_idx);
 
+	psi_memstall_enter(&pflags);
 	noreclaim_flag = memalloc_noreclaim_save();
+
 	nr_reclaimed = do_try_to_free_pages(zonelist, &sc);
+
 	memalloc_noreclaim_restore(noreclaim_flag);
+	psi_memstall_leave(&pflags);
 
 	trace_mm_vmscan_memcg_reclaim_end(nr_reclaimed);
 
@@ -3565,6 +3571,7 @@ static int kswapd(void *p)
 	pgdat->kswapd_order = 0;
 	pgdat->kswapd_classzone_idx = MAX_NR_ZONES;
 	for ( ; ; ) {
+		unsigned long pflags;
 		bool ret;
 
 		alloc_order = reclaim_order = pgdat->kswapd_order;
@@ -3601,9 +3608,15 @@ static int kswapd(void *p)
 		 */
 		trace_mm_vmscan_kswapd_wake(pgdat->node_id, classzone_idx,
 						alloc_order);
+
+		psi_memstall_enter(&pflags);
 		fs_reclaim_acquire(GFP_KERNEL);
+
 		reclaim_order = balance_pgdat(pgdat, alloc_order, classzone_idx);
+
 		fs_reclaim_release(GFP_KERNEL);
+		psi_memstall_leave(&pflags);
+
 		if (reclaim_order < alloc_order)
 			goto kswapd_try_sleep;
 	}
