GNU Linux-libre 5.17.9-gnu

[releases.git] / kernel / sched / psi.c

// SPDX-License-Identifier: GPL-2.0
/*
 * Pressure stall information for CPU, memory and IO
 *
 * Copyright (c) 2018 Facebook, Inc.
 * Author: Johannes Weiner <hannes@cmpxchg.org>
 *
 * Polling support by Suren Baghdasaryan <surenb@google.com>
 * Copyright (c) 2018 Google, Inc.
 *
 * 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.
 *
 *      SOME = nr_delayed_tasks != 0
 *      FULL = nr_delayed_tasks != 0 && nr_productive_tasks == 0
 *
 * What it means for a task to be productive is defined differently
 * for each resource. For IO, productive means a running task. For
 * memory, productive means a running task that isn't a reclaimer. For
 * CPU, productive means an oncpu task.
 *
 * Naturally, the FULL state doesn't exist for the CPU resource at the
 * system level, but exist at the cgroup level. At the cgroup level,
 * FULL means all non-idle tasks in the cgroup are delayed on the CPU
 * resource which is being used by others outside of the cgroup or
 * throttled by the cgroup cpu.max configuration.
 *
 * 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 execution
 * 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 possible 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_productive_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(256, 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_productive_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 "../workqueue_internal.h"
#include <linux/sched/loadavg.h>
#include <linux/seq_file.h>
#include <linux/proc_fs.h>
#include <linux/seqlock.h>
#include <linux/uaccess.h>
#include <linux/cgroup.h>
#include <linux/module.h>
#include <linux/sched.h>
#include <linux/ctype.h>
#include <linux/file.h>
#include <linux/poll.h>
#include <linux/psi.h>
#include "sched.h"

static int psi_bug __read_mostly;

DEFINE_STATIC_KEY_FALSE(psi_disabled);
DEFINE_STATIC_KEY_TRUE(psi_cgroups_enabled);

#ifdef CONFIG_PSI_DEFAULT_DISABLED
static bool psi_enable;
#else
static bool psi_enable = true;
#endif
static int __init setup_psi(char *str)
{
        return kstrtobool(str, &psi_enable) == 0;
}
__setup("psi=", setup_psi);

/* 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) */

/* PSI trigger definitions */
#define WINDOW_MIN_US 500000    /* Min window size is 500ms */
#define WINDOW_MAX_US 10000000  /* Max window size is 10s */
#define UPDATES_PER_WINDOW 10   /* 10 updates per window */

/* 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);
struct psi_group psi_system = {
        .pcpu = &system_group_pcpu,
};

static void psi_avgs_work(struct work_struct *work);

static void poll_timer_fn(struct timer_list *t);

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->avg_last_update = sched_clock();
        group->avg_next_update = group->avg_last_update + psi_period;
        INIT_DELAYED_WORK(&group->avgs_work, psi_avgs_work);
        mutex_init(&group->avgs_lock);
        /* Init trigger-related members */
        mutex_init(&group->trigger_lock);
        INIT_LIST_HEAD(&group->triggers);
        memset(group->nr_triggers, 0, sizeof(group->nr_triggers));
        group->poll_states = 0;
        group->poll_min_period = U32_MAX;
        memset(group->polling_total, 0, sizeof(group->polling_total));
        group->polling_next_update = ULLONG_MAX;
        group->polling_until = 0;
        init_waitqueue_head(&group->poll_wait);
        timer_setup(&group->poll_timer, poll_timer_fn, 0);
        rcu_assign_pointer(group->poll_task, NULL);
}

void __init psi_init(void)
{
        if (!psi_enable) {
                static_branch_enable(&psi_disabled);
                return;
        }

        if (!cgroup_psi_enabled())
                static_branch_disable(&psi_cgroups_enabled);

        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 unlikely(tasks[NR_IOWAIT]);
        case PSI_IO_FULL:
                return unlikely(tasks[NR_IOWAIT] && !tasks[NR_RUNNING]);
        case PSI_MEM_SOME:
                return unlikely(tasks[NR_MEMSTALL]);
        case PSI_MEM_FULL:
                return unlikely(tasks[NR_MEMSTALL] &&
                        tasks[NR_RUNNING] == tasks[NR_MEMSTALL_RUNNING]);
        case PSI_CPU_SOME:
                return unlikely(tasks[NR_RUNNING] > tasks[NR_ONCPU]);
        case PSI_CPU_FULL:
                return unlikely(tasks[NR_RUNNING] && !tasks[NR_ONCPU]);
        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,
                             enum psi_aggregators aggregator, u32 *times,
                             u32 *pchanged_states)
{
        struct psi_group_cpu *groupc = per_cpu_ptr(group->pcpu, cpu);
        u64 now, state_start;
        enum psi_states s;
        unsigned int seq;
        u32 state_mask;

        *pchanged_states = 0;

        /* 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));
                state_mask = groupc->state_mask;
                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 (state_mask & (1 << s))
                        times[s] += now - state_start;

                delta = times[s] - groupc->times_prev[aggregator][s];
                groupc->times_prev[aggregator][s] = times[s];

                times[s] = delta;
                if (delta)
                        *pchanged_states |= (1 << s);
        }
}

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 void collect_percpu_times(struct psi_group *group,
                                 enum psi_aggregators aggregator,
                                 u32 *pchanged_states)
{
        u64 deltas[NR_PSI_STATES - 1] = { 0, };
        unsigned long nonidle_total = 0;
        u32 changed_states = 0;
        int cpu;
        int s;

        /*
         * 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;
                u32 cpu_changed_states;

                get_recent_times(group, cpu, aggregator, times,
                                &cpu_changed_states);
                changed_states |= cpu_changed_states;

                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[aggregator][s] +=
                                div_u64(deltas[s], max(nonidle_total, 1UL));

        if (pchanged_states)
                *pchanged_states = changed_states;
}

static u64 update_averages(struct psi_group *group, u64 now)
{
        unsigned long missed_periods = 0;
        u64 expires, period;
        u64 avg_next_update;
        int s;

        /* avgX= */
        expires = group->avg_next_update;
        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.
         */
        avg_next_update = expires + ((1 + missed_periods) * psi_period);
        period = now - (group->avg_last_update + (missed_periods * psi_period));
        group->avg_last_update = now;

        for (s = 0; s < NR_PSI_STATES - 1; s++) {
                u32 sample;

                sample = group->total[PSI_AVGS][s] - group->avg_total[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->avg_total[s] += sample;
                calc_avgs(group->avg[s], missed_periods, sample, period);
        }

        return avg_next_update;
}

static void psi_avgs_work(struct work_struct *work)
{
        struct delayed_work *dwork;
        struct psi_group *group;
        u32 changed_states;
        bool nonidle;
        u64 now;

        dwork = to_delayed_work(work);
        group = container_of(dwork, struct psi_group, avgs_work);

        mutex_lock(&group->avgs_lock);

        now = sched_clock();

        collect_percpu_times(group, PSI_AVGS, &changed_states);
        nonidle = changed_states & (1 << PSI_NONIDLE);
        /*
         * 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.
         */
        if (now >= group->avg_next_update)
                group->avg_next_update = update_averages(group, now);

        if (nonidle) {
                schedule_delayed_work(dwork, nsecs_to_jiffies(
                                group->avg_next_update - now) + 1);
        }

        mutex_unlock(&group->avgs_lock);
}

/* Trigger tracking window manipulations */
static void window_reset(struct psi_window *win, u64 now, u64 value,
                         u64 prev_growth)
{
        win->start_time = now;
        win->start_value = value;
        win->prev_growth = prev_growth;
}

/*
 * PSI growth tracking window update and growth calculation routine.
 *
 * This approximates a sliding tracking window by interpolating
 * partially elapsed windows using historical growth data from the
 * previous intervals. This minimizes memory requirements (by not storing
 * all the intermediate values in the previous window) and simplifies
 * the calculations. It works well because PSI signal changes only in
 * positive direction and over relatively small window sizes the growth
 * is close to linear.
 */
static u64 window_update(struct psi_window *win, u64 now, u64 value)
{
        u64 elapsed;
        u64 growth;

        elapsed = now - win->start_time;
        growth = value - win->start_value;
        /*
         * After each tracking window passes win->start_value and
         * win->start_time get reset and win->prev_growth stores
         * the average per-window growth of the previous window.
         * win->prev_growth is then used to interpolate additional
         * growth from the previous window assuming it was linear.
         */
        if (elapsed > win->size)
                window_reset(win, now, value, growth);
        else {
                u32 remaining;

                remaining = win->size - elapsed;
                growth += div64_u64(win->prev_growth * remaining, win->size);
        }

        return growth;
}

static void init_triggers(struct psi_group *group, u64 now)
{
        struct psi_trigger *t;

        list_for_each_entry(t, &group->triggers, node)
                window_reset(&t->win, now,
                                group->total[PSI_POLL][t->state], 0);
        memcpy(group->polling_total, group->total[PSI_POLL],
                   sizeof(group->polling_total));
        group->polling_next_update = now + group->poll_min_period;
}

static u64 update_triggers(struct psi_group *group, u64 now)
{
        struct psi_trigger *t;
        bool new_stall = false;
        u64 *total = group->total[PSI_POLL];

        /*
         * On subsequent updates, calculate growth deltas and let
         * watchers know when their specified thresholds are exceeded.
         */
        list_for_each_entry(t, &group->triggers, node) {
                u64 growth;

                /* Check for stall activity */
                if (group->polling_total[t->state] == total[t->state])
                        continue;

                /*
                 * Multiple triggers might be looking at the same state,
                 * remember to update group->polling_total[] once we've
                 * been through all of them. Also remember to extend the
                 * polling time if we see new stall activity.
                 */
                new_stall = true;

                /* Calculate growth since last update */
                growth = window_update(&t->win, now, total[t->state]);
                if (growth < t->threshold)
                        continue;

                /* Limit event signaling to once per window */
                if (now < t->last_event_time + t->win.size)
                        continue;

                /* Generate an event */
                if (cmpxchg(&t->event, 0, 1) == 0)
                        wake_up_interruptible(&t->event_wait);
                t->last_event_time = now;
        }

        if (new_stall)
                memcpy(group->polling_total, total,
                                sizeof(group->polling_total));

        return now + group->poll_min_period;
}

/* Schedule polling if it's not already scheduled. */
static void psi_schedule_poll_work(struct psi_group *group, unsigned long delay)
{
        struct task_struct *task;

        /*
         * Do not reschedule if already scheduled.
         * Possible race with a timer scheduled after this check but before
         * mod_timer below can be tolerated because group->polling_next_update
         * will keep updates on schedule.
         */
        if (timer_pending(&group->poll_timer))
                return;

        rcu_read_lock();

        task = rcu_dereference(group->poll_task);
        /*
         * kworker might be NULL in case psi_trigger_destroy races with
         * psi_task_change (hotpath) which can't use locks
         */
        if (likely(task))
                mod_timer(&group->poll_timer, jiffies + delay);

        rcu_read_unlock();
}

static void psi_poll_work(struct psi_group *group)
{
        u32 changed_states;
        u64 now;

        mutex_lock(&group->trigger_lock);

        now = sched_clock();

        collect_percpu_times(group, PSI_POLL, &changed_states);

        if (changed_states & group->poll_states) {
                /* Initialize trigger windows when entering polling mode */
                if (now > group->polling_until)
                        init_triggers(group, now);

                /*
                 * Keep the monitor active for at least the duration of the
                 * minimum tracking window as long as monitor states are
                 * changing.
                 */
                group->polling_until = now +
                        group->poll_min_period * UPDATES_PER_WINDOW;
        }

        if (now > group->polling_until) {
                group->polling_next_update = ULLONG_MAX;
                goto out;
        }

        if (now >= group->polling_next_update)
                group->polling_next_update = update_triggers(group, now);

        psi_schedule_poll_work(group,
                nsecs_to_jiffies(group->polling_next_update - now) + 1);

out:
        mutex_unlock(&group->trigger_lock);
}

static int psi_poll_worker(void *data)
{
        struct psi_group *group = (struct psi_group *)data;

        sched_set_fifo_low(current);

        while (true) {
                wait_event_interruptible(group->poll_wait,
                                atomic_cmpxchg(&group->poll_wakeup, 1, 0) ||
                                kthread_should_stop());
                if (kthread_should_stop())
                        break;

                psi_poll_work(group);
        }
        return 0;
}

static void poll_timer_fn(struct timer_list *t)
{
        struct psi_group *group = from_timer(group, t, poll_timer);

        atomic_set(&group->poll_wakeup, 1);
        wake_up_interruptible(&group->poll_wait);
}

static void record_times(struct psi_group_cpu *groupc, u64 now)
{
        u32 delta;

        delta = now - groupc->state_start;
        groupc->state_start = now;

        if (groupc->state_mask & (1 << PSI_IO_SOME)) {
                groupc->times[PSI_IO_SOME] += delta;
                if (groupc->state_mask & (1 << PSI_IO_FULL))
                        groupc->times[PSI_IO_FULL] += delta;
        }

        if (groupc->state_mask & (1 << PSI_MEM_SOME)) {
                groupc->times[PSI_MEM_SOME] += delta;
                if (groupc->state_mask & (1 << PSI_MEM_FULL))
                        groupc->times[PSI_MEM_FULL] += delta;
        }

        if (groupc->state_mask & (1 << PSI_CPU_SOME)) {
                groupc->times[PSI_CPU_SOME] += delta;
                if (groupc->state_mask & (1 << PSI_CPU_FULL))
                        groupc->times[PSI_CPU_FULL] += delta;
        }

        if (groupc->state_mask & (1 << PSI_NONIDLE))
                groupc->times[PSI_NONIDLE] += delta;
}

static void psi_group_change(struct psi_group *group, int cpu,
                             unsigned int clear, unsigned int set, u64 now,
                             bool wake_clock)
{
        struct psi_group_cpu *groupc;
        u32 state_mask = 0;
        unsigned int t, m;
        enum psi_states s;

        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, now);

        for (t = 0, m = clear; m; m &= ~(1 << t), t++) {
                if (!(m & (1 << t)))
                        continue;
                if (groupc->tasks[t]) {
                        groupc->tasks[t]--;
                } else if (!psi_bug) {
                        printk_deferred(KERN_ERR "psi: task underflow! cpu=%d t=%d tasks=[%u %u %u %u %u] clear=%x set=%x\n",
                                        cpu, t, groupc->tasks[0],
                                        groupc->tasks[1], groupc->tasks[2],
                                        groupc->tasks[3], groupc->tasks[4],
                                        clear, set);
                        psi_bug = 1;
                }
        }

        for (t = 0; set; set &= ~(1 << t), t++)
                if (set & (1 << t))
                        groupc->tasks[t]++;

        /* Calculate state mask representing active states */
        for (s = 0; s < NR_PSI_STATES; s++) {
                if (test_state(groupc->tasks, s))
                        state_mask |= (1 << s);
        }

        /*
         * 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. So when the current
         * task in a cgroup is in_memstall, the corresponding groupc
         * on that cpu is in PSI_MEM_FULL state.
         */
        if (unlikely(groupc->tasks[NR_ONCPU] && cpu_curr(cpu)->in_memstall))
                state_mask |= (1 << PSI_MEM_FULL);

        groupc->state_mask = state_mask;

        write_seqcount_end(&groupc->seq);

        if (state_mask & group->poll_states)
                psi_schedule_poll_work(group, 1);

        if (wake_clock && !delayed_work_pending(&group->avgs_work))
                schedule_delayed_work(&group->avgs_work, PSI_FREQ);
}

static struct psi_group *iterate_groups(struct task_struct *task, void **iter)
{
        if (*iter == &psi_system)
                return NULL;

#ifdef CONFIG_CGROUPS
        if (static_branch_likely(&psi_cgroups_enabled)) {
                struct cgroup *cgroup = NULL;

                if (!*iter)
                        cgroup = task->cgroups->dfl_cgrp;
                else
                        cgroup = cgroup_parent(*iter);

                if (cgroup && cgroup_parent(cgroup)) {
                        *iter = cgroup;
                        return cgroup_psi(cgroup);
                }
        }
#endif
        *iter = &psi_system;
        return &psi_system;
}

static void psi_flags_change(struct task_struct *task, int clear, int set)
{
        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, task_cpu(task),
                                task->psi_flags, clear, set);
                psi_bug = 1;
        }

        task->psi_flags &= ~clear;
        task->psi_flags |= set;
}

void psi_task_change(struct task_struct *task, int clear, int set)
{
        int cpu = task_cpu(task);
        struct psi_group *group;
        bool wake_clock = true;
        void *iter = NULL;
        u64 now;

        if (!task->pid)
                return;

        psi_flags_change(task, clear, set);

        now = cpu_clock(cpu);
        /*
         * Periodic aggregation shuts off if there is a period of no
         * task changes, so we wake it back up if necessary. However,
         * don't do this if the task change is the aggregation worker
         * itself going to sleep, or we'll ping-pong forever.
         */
        if (unlikely((clear & TSK_RUNNING) &&
                     (task->flags & PF_WQ_WORKER) &&
                     wq_worker_last_func(task) == psi_avgs_work))
                wake_clock = false;

        while ((group = iterate_groups(task, &iter)))
                psi_group_change(group, cpu, clear, set, now, wake_clock);
}

void psi_task_switch(struct task_struct *prev, struct task_struct *next,
                     bool sleep)
{
        struct psi_group *group, *common = NULL;
        int cpu = task_cpu(prev);
        void *iter;
        u64 now = cpu_clock(cpu);

        if (next->pid) {
                bool identical_state;

                psi_flags_change(next, 0, TSK_ONCPU);
                /*
                 * When switching between tasks that have an identical
                 * runtime state, the cgroup that contains both tasks
                 * we reach the first common ancestor. Iterate @next's
                 * ancestors only until we encounter @prev's ONCPU.
                 */
                identical_state = prev->psi_flags == next->psi_flags;
                iter = NULL;
                while ((group = iterate_groups(next, &iter))) {
                        if (identical_state &&
                            per_cpu_ptr(group->pcpu, cpu)->tasks[NR_ONCPU]) {
                                common = group;
                                break;
                        }

                        psi_group_change(group, cpu, 0, TSK_ONCPU, now, true);
                }
        }

        if (prev->pid) {
                int clear = TSK_ONCPU, set = 0;

                /*
                 * When we're going to sleep, psi_dequeue() lets us
                 * handle TSK_RUNNING, TSK_MEMSTALL_RUNNING and
                 * TSK_IOWAIT here, where we can combine it with
                 * TSK_ONCPU and save walking common ancestors twice.
                 */
                if (sleep) {
                        clear |= TSK_RUNNING;
                        if (prev->in_memstall)
                                clear |= TSK_MEMSTALL_RUNNING;
                        if (prev->in_iowait)
                                set |= TSK_IOWAIT;
                }

                psi_flags_change(prev, clear, set);

                iter = NULL;
                while ((group = iterate_groups(prev, &iter)) && group != common)
                        psi_group_change(group, cpu, clear, set, now, true);

                /*
                 * TSK_ONCPU is handled up to the common ancestor. If we're tasked
                 * with dequeuing too, finish that for the rest of the hierarchy.
                 */
                if (sleep) {
                        clear &= ~TSK_ONCPU;
                        for (; group; group = iterate_groups(prev, &iter))
                                psi_group_change(group, cpu, clear, set, now, true);
                }
        }
}

/**
 * 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 (static_branch_likely(&psi_disabled))
                return;

        *flags = current->in_memstall;
        if (*flags)
                return;
        /*
         * in_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->in_memstall = 1;
        psi_task_change(current, 0, TSK_MEMSTALL | TSK_MEMSTALL_RUNNING);

        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 (static_branch_likely(&psi_disabled))
                return;

        if (*flags)
                return;
        /*
         * in_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->in_memstall = 0;
        psi_task_change(current, TSK_MEMSTALL | TSK_MEMSTALL_RUNNING, 0);

        rq_unlock_irq(rq, &rf);
}

#ifdef CONFIG_CGROUPS
int psi_cgroup_alloc(struct cgroup *cgroup)
{
        if (static_branch_likely(&psi_disabled))
                return 0;

        cgroup->psi.pcpu = alloc_percpu(struct psi_group_cpu);
        if (!cgroup->psi.pcpu)
                return -ENOMEM;
        group_init(&cgroup->psi);
        return 0;
}

void psi_cgroup_free(struct cgroup *cgroup)
{
        if (static_branch_likely(&psi_disabled))
                return;

        cancel_delayed_work_sync(&cgroup->psi.avgs_work);
        free_percpu(cgroup->psi.pcpu);
        /* All triggers must be removed by now */
        WARN_ONCE(cgroup->psi.poll_states, "psi: trigger leak\n");
}

/**
 * cgroup_move_task - move task to a different cgroup
 * @task: the task
 * @to: the target css_set
 *
 * Move task to a new cgroup and safely migrate its associated stall
 * state between the different groups.
 *
 * This function acquires the task's rq lock to lock out concurrent
 * changes to the task's scheduling state and - in case the task is
 * running - concurrent changes to its stall state.
 */
void cgroup_move_task(struct task_struct *task, struct css_set *to)
{
        unsigned int task_flags;
        struct rq_flags rf;
        struct rq *rq;

        if (static_branch_likely(&psi_disabled)) {
                /*
                 * Lame to do this here, but the scheduler cannot be locked
                 * from the outside, so we move cgroups from inside sched/.
                 */
                rcu_assign_pointer(task->cgroups, to);
                return;
        }

        rq = task_rq_lock(task, &rf);

        /*
         * We may race with schedule() dropping the rq lock between
         * deactivating prev and switching to next. Because the psi
         * updates from the deactivation are deferred to the switch
         * callback to save cgroup tree updates, the task's scheduling
         * state here is not coherent with its psi state:
         *
         * schedule()                   cgroup_move_task()
         *   rq_lock()
         *   deactivate_task()
         *     p->on_rq = 0
         *     psi_dequeue() // defers TSK_RUNNING & TSK_IOWAIT updates
         *   pick_next_task()
         *     rq_unlock()
         *                                rq_lock()
         *                                psi_task_change() // old cgroup
         *                                task->cgroups = to
         *                                psi_task_change() // new cgroup
         *                                rq_unlock()
         *     rq_lock()
         *   psi_sched_switch() // does deferred updates in new cgroup
         *
         * Don't rely on the scheduling state. Use psi_flags instead.
         */
        task_flags = task->psi_flags;

        if (task_flags)
                psi_task_change(task, task_flags, 0);

        /* See comment above */
        rcu_assign_pointer(task->cgroups, to);

        if (task_flags)
                psi_task_change(task, 0, task_flags);

        task_rq_unlock(rq, task, &rf);
}
#endif /* CONFIG_CGROUPS */

int psi_show(struct seq_file *m, struct psi_group *group, enum psi_res res)
{
        int full;
        u64 now;

        if (static_branch_likely(&psi_disabled))
                return -EOPNOTSUPP;

        /* Update averages before reporting them */
        mutex_lock(&group->avgs_lock);
        now = sched_clock();
        collect_percpu_times(group, PSI_AVGS, NULL);
        if (now >= group->avg_next_update)
                group->avg_next_update = update_averages(group, now);
        mutex_unlock(&group->avgs_lock);

        for (full = 0; full < 2; 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[PSI_AVGS][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;
}

struct psi_trigger *psi_trigger_create(struct psi_group *group,
                        char *buf, size_t nbytes, enum psi_res res)
{
        struct psi_trigger *t;
        enum psi_states state;
        u32 threshold_us;
        u32 window_us;

        if (static_branch_likely(&psi_disabled))
                return ERR_PTR(-EOPNOTSUPP);

        if (sscanf(buf, "some %u %u", &threshold_us, &window_us) == 2)
                state = PSI_IO_SOME + res * 2;
        else if (sscanf(buf, "full %u %u", &threshold_us, &window_us) == 2)
                state = PSI_IO_FULL + res * 2;
        else
                return ERR_PTR(-EINVAL);

        if (state >= PSI_NONIDLE)
                return ERR_PTR(-EINVAL);

        if (window_us < WINDOW_MIN_US ||
                window_us > WINDOW_MAX_US)
                return ERR_PTR(-EINVAL);

        /* Check threshold */
        if (threshold_us == 0 || threshold_us > window_us)
                return ERR_PTR(-EINVAL);

        t = kmalloc(sizeof(*t), GFP_KERNEL);
        if (!t)
                return ERR_PTR(-ENOMEM);

        t->group = group;
        t->state = state;
        t->threshold = threshold_us * NSEC_PER_USEC;
        t->win.size = window_us * NSEC_PER_USEC;
        window_reset(&t->win, 0, 0, 0);

        t->event = 0;
        t->last_event_time = 0;
        init_waitqueue_head(&t->event_wait);

        mutex_lock(&group->trigger_lock);

        if (!rcu_access_pointer(group->poll_task)) {
                struct task_struct *task;

                task = kthread_create(psi_poll_worker, group, "psimon");
                if (IS_ERR(task)) {
                        kfree(t);
                        mutex_unlock(&group->trigger_lock);
                        return ERR_CAST(task);
                }
                atomic_set(&group->poll_wakeup, 0);
                wake_up_process(task);
                rcu_assign_pointer(group->poll_task, task);
        }

        list_add(&t->node, &group->triggers);
        group->poll_min_period = min(group->poll_min_period,
                div_u64(t->win.size, UPDATES_PER_WINDOW));
        group->nr_triggers[t->state]++;
        group->poll_states |= (1 << t->state);

        mutex_unlock(&group->trigger_lock);

        return t;
}

void psi_trigger_destroy(struct psi_trigger *t)
{
        struct psi_group *group;
        struct task_struct *task_to_destroy = NULL;

        /*
         * We do not check psi_disabled since it might have been disabled after
         * the trigger got created.
         */
        if (!t)
                return;

        group = t->group;
        /*
         * Wakeup waiters to stop polling. Can happen if cgroup is deleted
         * from under a polling process.
         */
        wake_up_interruptible(&t->event_wait);

        mutex_lock(&group->trigger_lock);

        if (!list_empty(&t->node)) {
                struct psi_trigger *tmp;
                u64 period = ULLONG_MAX;

                list_del(&t->node);
                group->nr_triggers[t->state]--;
                if (!group->nr_triggers[t->state])
                        group->poll_states &= ~(1 << t->state);
                /* reset min update period for the remaining triggers */
                list_for_each_entry(tmp, &group->triggers, node)
                        period = min(period, div_u64(tmp->win.size,
                                        UPDATES_PER_WINDOW));
                group->poll_min_period = period;
                /* Destroy poll_task when the last trigger is destroyed */
                if (group->poll_states == 0) {
                        group->polling_until = 0;
                        task_to_destroy = rcu_dereference_protected(
                                        group->poll_task,
                                        lockdep_is_held(&group->trigger_lock));
                        rcu_assign_pointer(group->poll_task, NULL);
                        del_timer(&group->poll_timer);
                }
        }

        mutex_unlock(&group->trigger_lock);

        /*
         * Wait for psi_schedule_poll_work RCU to complete its read-side
         * critical section before destroying the trigger and optionally the
         * poll_task.
         */
        synchronize_rcu();
        /*
         * Stop kthread 'psimon' after releasing trigger_lock to prevent a
         * deadlock while waiting for psi_poll_work to acquire trigger_lock
         */
        if (task_to_destroy) {
                /*
                 * After the RCU grace period has expired, the worker
                 * can no longer be found through group->poll_task.
                 */
                kthread_stop(task_to_destroy);
        }
        kfree(t);
}

__poll_t psi_trigger_poll(void **trigger_ptr,
                                struct file *file, poll_table *wait)
{
        __poll_t ret = DEFAULT_POLLMASK;
        struct psi_trigger *t;

        if (static_branch_likely(&psi_disabled))
                return DEFAULT_POLLMASK | EPOLLERR | EPOLLPRI;

        t = smp_load_acquire(trigger_ptr);
        if (!t)
                return DEFAULT_POLLMASK | EPOLLERR | EPOLLPRI;

        poll_wait(file, &t->event_wait, wait);

        if (cmpxchg(&t->event, 1, 0) == 1)
                ret |= EPOLLPRI;

        return ret;
}

#ifdef CONFIG_PROC_FS
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_open(struct file *file, int (*psi_show)(struct seq_file *, void *))
{
        if (file->f_mode & FMODE_WRITE && !capable(CAP_SYS_RESOURCE))
                return -EPERM;

        return single_open(file, psi_show, NULL);
}

static int psi_io_open(struct inode *inode, struct file *file)
{
        return psi_open(file, psi_io_show);
}

static int psi_memory_open(struct inode *inode, struct file *file)
{
        return psi_open(file, psi_memory_show);
}

static int psi_cpu_open(struct inode *inode, struct file *file)
{
        return psi_open(file, psi_cpu_show);
}

static ssize_t psi_write(struct file *file, const char __user *user_buf,
                         size_t nbytes, enum psi_res res)
{
        char buf[32];
        size_t buf_size;
        struct seq_file *seq;
        struct psi_trigger *new;

        if (static_branch_likely(&psi_disabled))
                return -EOPNOTSUPP;

        if (!nbytes)
                return -EINVAL;

        buf_size = min(nbytes, sizeof(buf));
        if (copy_from_user(buf, user_buf, buf_size))
                return -EFAULT;

        buf[buf_size - 1] = '\0';

        seq = file->private_data;

        /* Take seq->lock to protect seq->private from concurrent writes */
        mutex_lock(&seq->lock);

        /* Allow only one trigger per file descriptor */
        if (seq->private) {
                mutex_unlock(&seq->lock);
                return -EBUSY;
        }

        new = psi_trigger_create(&psi_system, buf, nbytes, res);
        if (IS_ERR(new)) {
                mutex_unlock(&seq->lock);
                return PTR_ERR(new);
        }

        smp_store_release(&seq->private, new);
        mutex_unlock(&seq->lock);

        return nbytes;
}

static ssize_t psi_io_write(struct file *file, const char __user *user_buf,
                            size_t nbytes, loff_t *ppos)
{
        return psi_write(file, user_buf, nbytes, PSI_IO);
}

static ssize_t psi_memory_write(struct file *file, const char __user *user_buf,
                                size_t nbytes, loff_t *ppos)
{
        return psi_write(file, user_buf, nbytes, PSI_MEM);
}

static ssize_t psi_cpu_write(struct file *file, const char __user *user_buf,
                             size_t nbytes, loff_t *ppos)
{
        return psi_write(file, user_buf, nbytes, PSI_CPU);
}

static __poll_t psi_fop_poll(struct file *file, poll_table *wait)
{
        struct seq_file *seq = file->private_data;

        return psi_trigger_poll(&seq->private, file, wait);
}

static int psi_fop_release(struct inode *inode, struct file *file)
{
        struct seq_file *seq = file->private_data;

        psi_trigger_destroy(seq->private);
        return single_release(inode, file);
}

static const struct proc_ops psi_io_proc_ops = {
        .proc_open      = psi_io_open,
        .proc_read      = seq_read,
        .proc_lseek     = seq_lseek,
        .proc_write     = psi_io_write,
        .proc_poll      = psi_fop_poll,
        .proc_release   = psi_fop_release,
};

static const struct proc_ops psi_memory_proc_ops = {
        .proc_open      = psi_memory_open,
        .proc_read      = seq_read,
        .proc_lseek     = seq_lseek,
        .proc_write     = psi_memory_write,
        .proc_poll      = psi_fop_poll,
        .proc_release   = psi_fop_release,
};

static const struct proc_ops psi_cpu_proc_ops = {
        .proc_open      = psi_cpu_open,
        .proc_read      = seq_read,
        .proc_lseek     = seq_lseek,
        .proc_write     = psi_cpu_write,
        .proc_poll      = psi_fop_poll,
        .proc_release   = psi_fop_release,
};

static int __init psi_proc_init(void)
{
        if (psi_enable) {
                proc_mkdir("pressure", NULL);
                proc_create("pressure/io", 0666, NULL, &psi_io_proc_ops);
                proc_create("pressure/memory", 0666, NULL, &psi_memory_proc_ops);
                proc_create("pressure/cpu", 0666, NULL, &psi_cpu_proc_ops);
        }
        return 0;
}
module_init(psi_proc_init);

#endif /* CONFIG_PROC_FS */