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Message-ID: <CAP-5=fXgLJR2EQoK3SVrGWOKVWuZ4+ZVWFHLU8B_w2ibWtFzwg@mail.gmail.com>
Date: Wed, 7 May 2025 09:52:24 -0700
From: Ian Rogers <irogers@...gle.com>
To: kan.liang@...ux.intel.com
Cc: peterz@...radead.org, mingo@...hat.com, namhyung@...nel.org,
mark.rutland@....com, linux-kernel@...r.kernel.org,
linux-perf-users@...r.kernel.org, eranian@...gle.com, ctshao@...gle.com,
tmricht@...ux.ibm.com
Subject: Re: [RFC PATCH 01/15] perf: Fix the throttle logic for a group
On Tue, May 6, 2025 at 9:48 AM <kan.liang@...ux.intel.com> wrote:
>
> From: Kan Liang <kan.liang@...ux.intel.com>
>
> The current throttle logic doesn't work well with a group, e.g., the
> following sampling-read case.
>
> $ perf record -e "{cycles,cycles}:S" ...
>
> $ perf report -D | grep THROTTLE | tail -2
> THROTTLE events: 426 ( 9.0%)
> UNTHROTTLE events: 425 ( 9.0%)
>
> $ perf report -D | grep PERF_RECORD_SAMPLE -a4 | tail -n 5
> 0 1020120874009167 0x74970 [0x68]: PERF_RECORD_SAMPLE(IP, 0x1):
> ... sample_read:
> .... group nr 2
> ..... id 0000000000000327, value 000000000cbb993a, lost 0
> ..... id 0000000000000328, value 00000002211c26df, lost 0
>
> The second cycles event has a much larger value than the first cycles
> event in the same group.
>
> The current throttle logic in the generic code only logs the THROTTLE
> event. It relies on the specific driver implementation to disable
> events. However, for all ARCHs, the implementation is similar. It only
> disable the event, rather than the group.
>
> The logic to disable the group should be generic for all ARCHs. Add the
> logic in the generic code. The following patch will remove the buggy
> driver-specific implementation.
>
> The throttle only happens when an event is overflowed. Stop the entire
> group when any event in the group triggers the throttle. Set the
> MAX_INTERRUPTS to the leader event to indicate the group is throttled.
>
> The unthrottled could happen in 3 places.
> - event/group sched. All events in the group are scheduled one by one.
> All of them will be unthrottled eventually. Nothing needs to be
> changed.
> - The perf_adjust_freq_unthr_events for each tick. Needs to restart the
> group altogether.
> - The __perf_event_period(). The whole group needs to be restarted
> altogether as well.
>
> With the fix,
> $ sudo perf report -D | grep PERF_RECORD_SAMPLE -a4 | tail -n 5
> 0 3573470770332 0x12f5f8 [0x70]: PERF_RECORD_SAMPLE(IP, 0x2):
> ... sample_read:
> .... group nr 2
> ..... id 0000000000000a28, value 00000004fd3dfd8f, lost 0
> ..... id 0000000000000a29, value 00000004fd3dfd8f, lost 0
Thanks Kan! The patches look good to me. As I understand it patches 2
to 15 are just removing the logic where an event is unnecessarily
stopped twice, so is it possible to test just this patch in isolation?
Given the logic is generic it is applied to software events, so you
should be able to repeat the problem with `perf record -e
"{cpu-clock,cpu-clock}:S" ...` possibly by reducing the period or
increasing the frequency. This would be nice to show that it fixes the
problem more generically than just the Intel PMU.
Ian
> Signed-off-by: Kan Liang <kan.liang@...ux.intel.com>
> ---
> kernel/events/core.c | 55 +++++++++++++++++++++++++++++++++-----------
> 1 file changed, 41 insertions(+), 14 deletions(-)
>
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index a84abc2b7f20..eb0dc871f4f1 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -2734,6 +2734,38 @@ void perf_event_disable_inatomic(struct perf_event *event)
> static void perf_log_throttle(struct perf_event *event, int enable);
> static void perf_log_itrace_start(struct perf_event *event);
>
> +static void perf_event_group_unthrottle(struct perf_event *event, bool start_event)
> +{
> + struct perf_event *leader = event->group_leader;
> + struct perf_event *sibling;
> +
> + if (leader != event || start_event)
> + leader->pmu->start(leader, 0);
> + leader->hw.interrupts = 0;
> +
> + for_each_sibling_event(sibling, leader) {
> + if (sibling != event || start_event)
> + sibling->pmu->start(sibling, 0);
> + sibling->hw.interrupts = 0;
> + }
> +
> + perf_log_throttle(leader, 1);
> +}
> +
> +static void perf_event_group_throttle(struct perf_event *event)
> +{
> + struct perf_event *leader = event->group_leader;
> + struct perf_event *sibling;
> +
> + leader->hw.interrupts = MAX_INTERRUPTS;
> + leader->pmu->stop(leader, 0);
> +
> + for_each_sibling_event(sibling, leader)
> + sibling->pmu->stop(sibling, 0);
> +
> + perf_log_throttle(leader, 0);
> +}
> +
> static int
> event_sched_in(struct perf_event *event, struct perf_event_context *ctx)
> {
> @@ -4389,10 +4421,8 @@ static void perf_adjust_freq_unthr_events(struct list_head *event_list)
> hwc = &event->hw;
>
> if (hwc->interrupts == MAX_INTERRUPTS) {
> - hwc->interrupts = 0;
> - perf_log_throttle(event, 1);
> - if (!event->attr.freq || !event->attr.sample_freq)
> - event->pmu->start(event, 0);
> + perf_event_group_unthrottle(event,
> + !event->attr.freq || !event->attr.sample_freq);
> }
>
> if (!event->attr.freq || !event->attr.sample_freq)
> @@ -6421,14 +6451,6 @@ static void __perf_event_period(struct perf_event *event,
> active = (event->state == PERF_EVENT_STATE_ACTIVE);
> if (active) {
> perf_pmu_disable(event->pmu);
> - /*
> - * We could be throttled; unthrottle now to avoid the tick
> - * trying to unthrottle while we already re-started the event.
> - */
> - if (event->hw.interrupts == MAX_INTERRUPTS) {
> - event->hw.interrupts = 0;
> - perf_log_throttle(event, 1);
> - }
> event->pmu->stop(event, PERF_EF_UPDATE);
> }
>
> @@ -6436,6 +6458,12 @@ static void __perf_event_period(struct perf_event *event,
>
> if (active) {
> event->pmu->start(event, PERF_EF_RELOAD);
> + /*
> + * We could be throttled; unthrottle now to avoid the tick
> + * trying to unthrottle while we already re-started the event.
> + */
> + if (event->group_leader->hw.interrupts == MAX_INTERRUPTS)
> + perf_event_group_unthrottle(event, false);
> perf_pmu_enable(event->pmu);
> }
> }
> @@ -10326,8 +10354,7 @@ __perf_event_account_interrupt(struct perf_event *event, int throttle)
> if (unlikely(throttle && hwc->interrupts >= max_samples_per_tick)) {
> __this_cpu_inc(perf_throttled_count);
> tick_dep_set_cpu(smp_processor_id(), TICK_DEP_BIT_PERF_EVENTS);
> - hwc->interrupts = MAX_INTERRUPTS;
> - perf_log_throttle(event, 0);
> + perf_event_group_throttle(event);
> ret = 1;
> }
>
> --
> 2.38.1
>
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