[PATCH 8/9] psi: pressure stall information for CPU, memory, and IO

Johannes Weiner
2018-08-01T15:19:57Z

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).

v2:
- stable clock tick, as per Peter
- data structure layout optimization, as per Peter
- fix u64 divisions on 32 bit, as per Peter
- outermost psi_disabled checks, as per Peter
- coding style fixes, as per Peter
- just-in-time stats aggregation, as per Suren
- fix task state corruption with CONFIG_PREEMPT, as per Suren
- CONFIG_PSI=n build error
- avoid writing p->sched_psi_wake_requeue unnecessarily
- documentation & comment updates

v3:
- pack scheduler hotpath data into one cacheline, as per Peter and Linus
- drop unnecessary SCHED_INFO dependency, as per Peter
- lockless live-state aggregation, as per Peter
- do_div -> div64_ul and some other cleanups, as per Peter
- realtime sampling period and slipped sample handling, as per Tejun

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
Documentation/accounting/psi.txt | 64 +++
include/linux/psi.h | 27 ++
include/linux/psi_types.h | 87 +++++
include/linux/sched.h | 10 +
init/Kconfig | 15 +
kernel/fork.c | 4 +
kernel/sched/Makefile | 1 +
kernel/sched/core.c | 11 +-
kernel/sched/psi.c | 643 +++++++++++++++++++++++++++++++
kernel/sched/sched.h | 2 +
kernel/sched/stats.h | 80 ++++
mm/compaction.c | 5 +
mm/filemap.c | 15 +-
mm/page_alloc.c | 10 +
mm/vmscan.c | 13 +
15 files changed, 981 insertions(+), 6 deletions(-)
create mode 100644 Documentation/accounting/psi.txt
create mode 100644 include/linux/psi.h
create mode 100644 include/linux/psi_types.h
create mode 100644 kernel/sched/psi.c

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;
}
--
2.18.0