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Date: Wed, 10 Jan 2024 14:49:39 +0000
From: Mark Rutland <mark.rutland@....com>
To: Namhyung Kim <namhyung@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>,
Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
Arnaldo Carvalho de Melo <acme@...nel.org>,
LKML <linux-kernel@...r.kernel.org>,
Mingwei Zhang <mizhang@...gle.com>, Ian Rogers <irogers@...gle.com>,
Kan Liang <kan.liang@...ux.intel.com>
Subject: Re: [PATCH RESEND 1/2] perf/core: Update
perf_adjust_freq_unthr_context()
On Tue, Jan 09, 2024 at 01:36:22PM -0800, Namhyung Kim wrote:
> It was unnecessarily disabling and enabling PMUs for each event. It
> should be done at PMU level. Add pmu_ctx->nr_freq counter to check it
> at each PMU. As pmu context has separate active lists for pinned group
> and flexible group, factor out a new function to do the job.
>
> Another minor optimization is that it can skip PMUs w/ CAP_NO_INTERRUPT
> even if it needs to unthrottle sampling events.
>
> Reviewed-by: Ian Rogers <irogers@...gle.com>
> Reviewed-by: Kan Liang <kan.liang@...ux.intel.com>
> Tested-by: Mingwei Zhang <mizhang@...gle.com>
> Signed-off-by: Namhyung Kim <namhyung@...nel.org>
Hi,
I've taken a quick look and I don't think this is quite right for
hybrid/big.LITTLE, but I think that should be relatively simple to fix (more on
that below).
This seems to be a bunch of optimizations; was that based on inspection alone,
or have you found a workload where this has a measureable impact?
> ---
> include/linux/perf_event.h | 1 +
> kernel/events/core.c | 68 +++++++++++++++++++++++---------------
> 2 files changed, 43 insertions(+), 26 deletions(-)
>
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index d2a15c0c6f8a..b2ff60fa487e 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -883,6 +883,7 @@ struct perf_event_pmu_context {
>
> unsigned int nr_events;
> unsigned int nr_cgroups;
> + unsigned int nr_freq;
>
> atomic_t refcount; /* event <-> epc */
> struct rcu_head rcu_head;
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 59b332cce9e7..ce9db9dbfd4c 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -2277,8 +2277,10 @@ event_sched_out(struct perf_event *event, struct perf_event_context *ctx)
>
> if (!is_software_event(event))
> cpc->active_oncpu--;
> - if (event->attr.freq && event->attr.sample_freq)
> + if (event->attr.freq && event->attr.sample_freq) {
> ctx->nr_freq--;
> + epc->nr_freq--;
> + }
> if (event->attr.exclusive || !cpc->active_oncpu)
> cpc->exclusive = 0;
>
> @@ -2533,9 +2535,10 @@ event_sched_in(struct perf_event *event, struct perf_event_context *ctx)
>
> if (!is_software_event(event))
> cpc->active_oncpu++;
> - if (event->attr.freq && event->attr.sample_freq)
> + if (event->attr.freq && event->attr.sample_freq) {
> ctx->nr_freq++;
> -
> + epc->nr_freq++;
> + }
> if (event->attr.exclusive)
> cpc->exclusive = 1;
>
> @@ -4098,30 +4101,14 @@ static void perf_adjust_period(struct perf_event *event, u64 nsec, u64 count, bo
> }
> }
>
> -/*
> - * 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, bool unthrottle)
> +static void perf_adjust_freq_unthr_events(struct list_head *event_list)
> {
> struct perf_event *event;
> struct hw_perf_event *hwc;
> u64 now, period = TICK_NSEC;
> s64 delta;
>
> - /*
> - * 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 || unthrottle))
> - return;
> -
> - raw_spin_lock(&ctx->lock);
> -
> - list_for_each_entry_rcu(event, &ctx->event_list, event_entry) {
> + list_for_each_entry(event, event_list, active_list) {
> if (event->state != PERF_EVENT_STATE_ACTIVE)
> continue;
>
> @@ -4129,8 +4116,6 @@ perf_adjust_freq_unthr_context(struct perf_event_context *ctx, bool unthrottle)
> if (!event_filter_match(event))
> continue;
>
> - perf_pmu_disable(event->pmu);
> -
> hwc = &event->hw;
>
> if (hwc->interrupts == MAX_INTERRUPTS) {
> @@ -4140,7 +4125,7 @@ perf_adjust_freq_unthr_context(struct perf_event_context *ctx, bool unthrottle)
> }
>
> if (!event->attr.freq || !event->attr.sample_freq)
> - goto next;
> + continue;
>
> /*
> * stop the event and update event->count
> @@ -4162,8 +4147,39 @@ perf_adjust_freq_unthr_context(struct perf_event_context *ctx, bool unthrottle)
> perf_adjust_period(event, period, delta, false);
>
> event->pmu->start(event, delta > 0 ? PERF_EF_RELOAD : 0);
> - next:
> - perf_pmu_enable(event->pmu);
> + }
> +}
> +
> +/*
> + * 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, bool unthrottle)
> +{
> + struct perf_event_pmu_context *pmu_ctx;
> +
> + /*
> + * 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 || unthrottle))
> + return;
> +
> + raw_spin_lock(&ctx->lock);
> +
> + list_for_each_entry(pmu_ctx, &ctx->pmu_ctx_list, pmu_ctx_entry) {
> + if (!(pmu_ctx->nr_freq || unthrottle))
> + continue;
> + if (pmu_ctx->pmu->capabilities & PERF_PMU_CAP_NO_INTERRUPT)
> + continue;
> +
> + perf_pmu_disable(pmu_ctx->pmu);
> + perf_adjust_freq_unthr_events(&pmu_ctx->pinned_active);
> + perf_adjust_freq_unthr_events(&pmu_ctx->flexible_active);
> + perf_pmu_enable(pmu_ctx->pmu);
> }
I don't think this is correct for big.LITTLE/hybrid systems.
Imagine a system where CPUs 0-1 have pmu_a, CPUs 2-3 have pmu_b, and a task has
events for both pmu_a and pmu_b. The perf_event_context for that task will have
a perf_event_pmu_context for each PMU in its pmu_ctx_list.
Say that task is run on CPU0, and perf_event_task_tick() is called. That will
call perf_adjust_freq_unthr_context(), and it will iterate over the
pmu_ctx_list. Note that regardless of pmu_ctx->nr_freq, if 'unthottle' is true,
we'll go ahead and call the following for all of the pmu contexts in the
pmu_ctx_list:
perf_pmu_disable(pmu_ctx->pmu);
perf_adjust_freq_unthr_events(&pmu_ctx->pinned_active);
perf_adjust_freq_unthr_events(&pmu_ctx->flexible_active);
perf_pmu_enable(pmu_ctx->pmu);
.. and that means we might call that for pmu_b, even though it's not
associated with CPU0. That could be fatal depending on what those callbacks do.
The old logic avoided that possibility implicitly, since the events for pmu_b
couldn't be active, and so the check at the start of the look would skip all of
pmu_b's events:
if (event->state != PERF_EVENT_STATE_ACTIVE)
continue;
We could do similar by keeping track of how many active events each
perf_event_pmu_context has, which'd allow us to do something like:
if (pmu_ctx->nr_active == 0)
continue;
How does that sound to you?
Mark.
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