Re: [PATCH V4] perf: Fix the throttle error of some clock events

[Leo Yan]

Hi Kan,

On Fri, Jun 06, 2025 at 12:25:46PM -0700, kan.liang@xxxxxxxxxxxxxxx wrote:

[...]

> There may be two ways to fix it.
> - Introduce a PMU flag to track the case. Avoid the event_stop in
>   perf_event_throttle() if the flag is detected.
>   It has been implemented in the
>   https://lore.kernel.org/lkml/20250528175832.2999139-1-kan.liang@xxxxxxxxxxxxxxx/
>   The new flag was thought to be an overkill for the issue.
> - Add a check in the event_stop. Return immediately if the throttle is
>   invoked in the hrtimer handler. Rely on the existing HRTIMER_NORESTART
>   method to stop the timer.
> 
> The latter is implemented here.
> 
> Move event->hw.interrupts = MAX_INTERRUPTS before the stop(). It makes
> the order the same as perf_event_unthrottle(). Except the patch, no one
> checks the hw.interrupts in the stop(). There is no impact from the
> order change.
> 
> When stops in the throttle, the event should not be updated,
> stop(event, 0).

I am confused for this conclusion. When a CPU or task clock event is
stopped by throttling, should it also be updated? Otherwise, we will
lose accouting for the period prior to the throttling.

I saw you exchanged with Alexei for a soft lockup issue, the reply [1]
shows that skipping event update on throttling does not help to
resolve the lockup issue.

Could you elaberate why we don't need to update clock events when
throttling?

Thanks,
Leo

[1] https://lore.kernel.org/linux-perf-users/CAADnVQKRJKsG08KkEriuBQop0LgDr+c9rkNE6MUh_n3rzZoXVQ@xxxxxxxxxxxxxx/

> But the cpu_clock_event_stop() doesn't handle the flag.
> In logic, it's wrong. But it didn't bring any problems with the old
> code, because the stop() was not invoked when handling the throttle.
> Checking the flag before updating the event.
> 
> Reported-by: Leo Yan <leo.yan@xxxxxxx>
> Reported-by: Aishwarya TCV <aishwarya.tcv@xxxxxxx>
> Closes: https://lore.kernel.org/lkml/20250527161656.GJ2566836@xxxxxxxxxxxxxxx/
> Reported-by: Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx>
> Closes: https://lore.kernel.org/lkml/djxlh5fx326gcenwrr52ry3pk4wxmugu4jccdjysza7tlc5fef@ktp4rffawgcw/
> Reported-by: Venkat Rao Bagalkote <venkat88@xxxxxxxxxxxxx>
> Closes: https://lore.kernel.org/lkml/8e8f51d8-af64-4d9e-934b-c0ee9f131293@xxxxxxxxxxxxx/
> Reported-by: Vince Weaver <vincent.weaver@xxxxxxxxx>
> Closes: https://lore.kernel.org/lkml/4ce106d0-950c-aadc-0b6a-f0215cd39913@xxxxxxxxx/
> Reviewed-by: Ian Rogers <irogers@xxxxxxxxxx>
> Signed-off-by: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> ---
> 
> The patch is to fix the issue introduced by
> 
>   9734e25fbf5a perf: Fix the throttle logic for a group
> 
> It is still in the tip.git, I'm not sure if the commit ID is valid. So
> the Fixes tag is not added.
> 
> There are some discussions regarding to a soft lockup issue.
> That is an existing issue, which are not introduced by the above change.
> It should be fixed separately.
> https://lore.kernel.org/lkml/CAADnVQ+Lx0HWEM8xdLC80wco3BTUPAD_2dQ-3oZFiECZMcw2aQ@xxxxxxxxxxxxxx/
> 
> Changes since V3:
> - Check before update in event_stop()
> - Add Reviewed-by from Ian
> 
>  kernel/events/core.c | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index f34c99f8ce8f..cc77f127e11a 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -2656,8 +2656,8 @@ static void perf_event_unthrottle(struct perf_event *event, bool start)
>  
>  static void perf_event_throttle(struct perf_event *event)
>  {
> -	event->pmu->stop(event, 0);
>  	event->hw.interrupts = MAX_INTERRUPTS;
> +	event->pmu->stop(event, 0);
>  	if (event == event->group_leader)
>  		perf_log_throttle(event, 0);
>  }
> @@ -11749,7 +11749,12 @@ static void perf_swevent_cancel_hrtimer(struct perf_event *event)
>  {
>  	struct hw_perf_event *hwc = &event->hw;
>  
> -	if (is_sampling_event(event)) {
> +	/*
> +	 * The throttle can be triggered in the hrtimer handler.
> +	 * The HRTIMER_NORESTART should be used to stop the timer,
> +	 * rather than hrtimer_cancel(). See perf_swevent_hrtimer()
> +	 */
> +	if (is_sampling_event(event) && (hwc->interrupts != MAX_INTERRUPTS)) {
>  		ktime_t remaining = hrtimer_get_remaining(&hwc->hrtimer);
>  		local64_set(&hwc->period_left, ktime_to_ns(remaining));
>  
> @@ -11804,7 +11809,8 @@ static void cpu_clock_event_start(struct perf_event *event, int flags)
>  static void cpu_clock_event_stop(struct perf_event *event, int flags)
>  {
>  	perf_swevent_cancel_hrtimer(event);
> -	cpu_clock_event_update(event);
> +	if (flags & PERF_EF_UPDATE)
> +		cpu_clock_event_update(event);
>  }
>  
>  static int cpu_clock_event_add(struct perf_event *event, int flags)
> @@ -11882,7 +11888,8 @@ static void task_clock_event_start(struct perf_event *event, int flags)
>  static void task_clock_event_stop(struct perf_event *event, int flags)
>  {
>  	perf_swevent_cancel_hrtimer(event);
> -	task_clock_event_update(event, event->ctx->time);
> +	if (flags & PERF_EF_UPDATE)
> +		task_clock_event_update(event, event->ctx->time);
>  }
>  
>  static int task_clock_event_add(struct perf_event *event, int flags)
> -- 
> 2.38.1
>