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Date: Thu, 09 Feb 2012 10:53:36 +0530
From: Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To: Stephane Eranian <eranian@...gle.com>
CC: linux-kernel@...r.kernel.org, peterz@...radead.org, mingo@...e.hu,
gleb@...hat.com, wcohen@...hat.com, vince@...ter.net,
asharma@...com, andi@...stfloor.org
Subject: Re: [PATCH] perf_events: fix broken intr throttling (v3)
On Thursday 26 January 2012 09:33 PM, Stephane Eranian wrote:
>
> This patch fixes the throttling mechanism. It was broken
> in 3.2.0. Events were not being unthrottled. The unthrottling
> mechanism required that events be checked at each timer tick.
>
> This patch solves this problem and also separates:
> - unthrottling
> - multiplexing
> - frequency-mode period adjustments
>
> Not all of them need to be executed at each timer tick.
>
> This third version of the patch is based on my original patch +
> PeterZ proposal (https://lkml.org/lkml/2012/1/7/87).
>
> At each timer tick, for each context:
> - if the current CPU has throttled events, we unthrottle events
>
> - if context has frequency-based events, we adjust sampling periods
>
> - if we have reached the jiffies interval, we multiplex (rotate)
>
> We decoupled rotation (multiplexing) from frequency-mode sampling
> period adjustments. They should not necessarily happen at the same
> rate. Multiplexing is subject to jiffies_interval (currently at 1
> but could be higher once the tunable is exposed via sysfs).
I am wondering how much higher value would jiffies_interval can be assigned
to ? Once its exposed to the user through sysfs it can have any value. Are
we planning to impose some sort of MAX limit on this ?
>
> We have grouped frequency-mode adjustment and unthrottling into the
> same routine to minimize code duplication. When throttled while in
> frequency mode, we scan the events only once.
>
> We have fixed the threshold enforcement code in __perf_event_overflow().
> There was a bug whereby it would allow more than the authorized rate
> because an increment of hwc->interrupts was not executed at the right
> place.
>
> The patch was tested with low sampling limit (2000) and fixed periods,
> frequency mode, overcommitted PMU.
>
> On a 2.1GHz AMD CPU:
> $ cat /proc/sys/kernel/perf_event_max_sample_rate
> 2000
>
> We set a rate of 3000 samples/sec (2.1GHz/3000 = 700000):
>
> $ perf record -e cycles,cycles -c 700000 noploop 10
> $ perf report -D | tail -21
> Aggregated stats:
> TOTAL events: 80086
> MMAP events: 88
> COMM events: 2
> EXIT events: 4
> THROTTLE events: 19996
> UNTHROTTLE events: 19996
> SAMPLE events: 40000
> cycles stats:
> TOTAL events: 40006
> MMAP events: 5
> COMM events: 1
> EXIT events: 4
> THROTTLE events: 9998
> UNTHROTTLE events: 9998
> SAMPLE events: 20000
> cycles stats:
> TOTAL events: 39996
> THROTTLE events: 9998
> UNTHROTTLE events: 9998
> SAMPLE events: 20000
>
> For 10s, the cap is 2x2000x10 = 40000 samples.
> We get exactly that: 20000 samples/event.
>
> Signed-off-by: Stephane Eranian <eranian@...gle.com>
> ---
>
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index 0b91db2..412b790 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -589,6 +589,7 @@ struct hw_perf_event {
> u64 sample_period;
> u64 last_period;
> local64_t period_left;
> + u64 interrupts_seq;
> u64 interrupts;
>
> u64 freq_time_stamp;
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index de859fb..7c3b9de 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -2300,6 +2300,9 @@ do { \
> return div64_u64(dividend, divisor);
> }
>
> +static DEFINE_PER_CPU(int, perf_throttled_count);
> +static DEFINE_PER_CPU(u64, perf_throttled_seq);
> +
> static void perf_adjust_period(struct perf_event *event, u64 nsec, u64 count)
> {
> struct hw_perf_event *hwc = &event->hw;
> @@ -2325,16 +2328,29 @@ static void perf_adjust_period(struct perf_event *event, u64 nsec, u64 count)
> }
> }
>
> -static void perf_ctx_adjust_freq(struct perf_event_context *ctx, u64 period)
> +/*
> + * combine freq adjustment with unthrottling to avoid two passes over the
> + * events. At the same time, make sure, having freq events does not change
> + * the rate of unthrottling as that would introduce bias.
> + */
> +static void perf_adjust_freq_unthr_context(struct perf_event_context *ctx,
> + int needs_unthr)
> {
> struct perf_event *event;
> struct hw_perf_event *hwc;
> - u64 interrupts, now;
> + u64 now, period = TICK_NSEC;
> s64 delta;
>
> - if (!ctx->nr_freq)
> + /*
> + * only need to iterate over all events iff:
> + * - context have events in frequency mode (needs freq adjust)
> + * - there are events to unthrottle on this cpu
> + */
> + if (!(ctx->nr_freq || needs_unthr))
> return;
>
> + raw_spin_lock(&ctx->lock);
> +
> list_for_each_entry_rcu(event, &ctx->event_list, event_entry) {
> if (event->state != PERF_EVENT_STATE_ACTIVE)
> continue;
> @@ -2344,13 +2360,8 @@ static void perf_ctx_adjust_freq(struct perf_event_context *ctx, u64 period)
>
> hwc = &event->hw;
>
> - interrupts = hwc->interrupts;
> - hwc->interrupts = 0;
> -
> - /*
> - * unthrottle events on the tick
> - */
> - if (interrupts == MAX_INTERRUPTS) {
> + if (needs_unthr && hwc->interrupts == MAX_INTERRUPTS) {
> + hwc->interrupts = 0;
> perf_log_throttle(event, 1);
> event->pmu->start(event, 0);
> }
> @@ -2358,14 +2369,26 @@ static void perf_ctx_adjust_freq(struct perf_event_context *ctx, u64 period)
> if (!event->attr.freq || !event->attr.sample_freq)
> continue;
>
> - event->pmu->read(event);
> + /*
> + * stop the event and update event->count
> + */
> + event->pmu->stop(event, PERF_EF_UPDATE);
> +
> now = local64_read(&event->count);
> delta = now - hwc->freq_count_stamp;
> hwc->freq_count_stamp = now;
>
> + /*
> + * restart the event
> + * reload only if value has changed
> + */
> if (delta > 0)
> perf_adjust_period(event, period, delta);
> +
> + event->pmu->start(event, delta > 0 ? PERF_EF_RELOAD : 0);
> }
> +
> + raw_spin_unlock(&ctx->lock);
> }
>
> /*
> @@ -2388,16 +2411,13 @@ static void rotate_ctx(struct perf_event_context *ctx)
> */
> static void perf_rotate_context(struct perf_cpu_context *cpuctx)
> {
> - u64 interval = (u64)cpuctx->jiffies_interval * TICK_NSEC;
> struct perf_event_context *ctx = NULL;
> - int rotate = 0, remove = 1, freq = 0;
> + int rotate = 0, remove = 1;
>
> if (cpuctx->ctx.nr_events) {
> remove = 0;
> if (cpuctx->ctx.nr_events != cpuctx->ctx.nr_active)
> rotate = 1;
> - if (cpuctx->ctx.nr_freq)
> - freq = 1;
> }
>
> ctx = cpuctx->task_ctx;
> @@ -2405,37 +2425,26 @@ static void perf_rotate_context(struct perf_cpu_context *cpuctx)
> remove = 0;
> if (ctx->nr_events != ctx->nr_active)
> rotate = 1;
> - if (ctx->nr_freq)
> - freq = 1;
> }
>
> - if (!rotate && !freq)
> + if (!rotate)
> goto done;
>
> perf_ctx_lock(cpuctx, cpuctx->task_ctx);
> perf_pmu_disable(cpuctx->ctx.pmu);
>
> - if (freq) {
> - perf_ctx_adjust_freq(&cpuctx->ctx, interval);
> - if (ctx)
> - perf_ctx_adjust_freq(ctx, interval);
> - }
> -
> - if (rotate) {
> - cpu_ctx_sched_out(cpuctx, EVENT_FLEXIBLE);
> - if (ctx)
> - ctx_sched_out(ctx, cpuctx, EVENT_FLEXIBLE);
> + cpu_ctx_sched_out(cpuctx, EVENT_FLEXIBLE);
> + if (ctx)
> + ctx_sched_out(ctx, cpuctx, EVENT_FLEXIBLE);
>
> - rotate_ctx(&cpuctx->ctx);
> - if (ctx)
> - rotate_ctx(ctx);
> + rotate_ctx(&cpuctx->ctx);
> + if (ctx)
> + rotate_ctx(ctx);
>
> - perf_event_sched_in(cpuctx, ctx, current);
> - }
> + perf_event_sched_in(cpuctx, ctx, current);
>
> perf_pmu_enable(cpuctx->ctx.pmu);
> perf_ctx_unlock(cpuctx, cpuctx->task_ctx);
> -
> done:
> if (remove)
> list_del_init(&cpuctx->rotation_list);
> @@ -2445,10 +2454,22 @@ void perf_event_task_tick(void)
> {
> struct list_head *head = &__get_cpu_var(rotation_list);
> struct perf_cpu_context *cpuctx, *tmp;
> + struct perf_event_context *ctx;
> + int throttled;
>
> WARN_ON(!irqs_disabled());
>
> + __this_cpu_inc(perf_throttled_seq);
> + throttled = __this_cpu_xchg(perf_throttled_count, 0);
> +
> list_for_each_entry_safe(cpuctx, tmp, head, rotation_list) {
> + ctx = &cpuctx->ctx;
> + perf_adjust_freq_unthr_context(ctx, throttled);
> +
> + ctx = cpuctx->task_ctx;
> + if (ctx)
> + perf_adjust_freq_unthr_context(ctx, throttled);
> +
> if (cpuctx->jiffies_interval == 1 ||
> !(jiffies % cpuctx->jiffies_interval))
> perf_rotate_context(cpuctx);
> @@ -4514,6 +4535,7 @@ static int __perf_event_overflow(struct perf_event *event,
> {
> int events = atomic_read(&event->event_limit);
> struct hw_perf_event *hwc = &event->hw;
> + u64 seq;
> int ret = 0;
>
> /*
> @@ -4523,14 +4545,20 @@ static int __perf_event_overflow(struct perf_event *event,
> if (unlikely(!is_sampling_event(event)))
> return 0;
>
> - if (unlikely(hwc->interrupts >= max_samples_per_tick)) {
> - if (throttle) {
> + seq = __this_cpu_read(perf_throttled_seq);
> + if (seq != hwc->interrupts_seq) {
> + hwc->interrupts_seq = seq;
> + hwc->interrupts = 1;
> + } else {
> + hwc->interrupts++;
> + if (unlikely(throttle
> + && hwc->interrupts >= max_samples_per_tick)) {
> + __this_cpu_inc(perf_throttled_count);
> hwc->interrupts = MAX_INTERRUPTS;
> perf_log_throttle(event, 0);
> ret = 1;
> }
> - } else
> - hwc->interrupts++;
> + }
>
> if (event->attr.freq) {
> u64 now = perf_clock();
> --
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