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

[Venkat Rao Bagalkote <venkat88@linux.ibm.com>]

On 07/06/25 12:55 am, kan.liang@linux.intel.com wrote:
> From: Kan Liang <kan.liang@linux.intel.com>
>
> Both ARM and IBM CI reports RCU stall, which can be reproduced by the
> below perf command.
>    perf record -a -e cpu-clock -- sleep 2
>
> The issue is introduced by the generic throttle patch set, which
> unconditionally invoke the event_stop() when throttle is triggered.
>
> The cpu-clock and task-clock are two special SW events, which rely on
> the hrtimer. The throttle is invoked in the hrtimer handler. The
> event_stop()->hrtimer_cancel() waits for the handler to finish, which is
> a deadlock. Instead of invoking the stop(), the HRTIMER_NORESTART should
> be used to stop the timer.
>
> 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@linux.intel.com/
>    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). 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@arm.com>
> Reported-by: Aishwarya TCV <aishwarya.tcv@arm.com>
> Closes: https://lore.kernel.org/lkml/20250527161656.GJ2566836@e132581.arm.com/
> Reported-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
> Closes: https://lore.kernel.org/lkml/djxlh5fx326gcenwrr52ry3pk4wxmugu4jccdjysza7tlc5fef@ktp4rffawgcw/
> Reported-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
> Closes: https://lore.kernel.org/lkml/8e8f51d8-af64-4d9e-934b-c0ee9f131293@linux.ibm.com/
> Reported-by: Vince Weaver <vincent.weaver@maine.edu>
> Closes: https://lore.kernel.org/lkml/4ce106d0-950c-aadc-0b6a-f0215cd39913@maine.edu/
> Reviewed-by: Ian Rogers <irogers@google.com>
> Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
> ---


Tested this patch by applying on top of 6.16.0-rc1, and it fixes the 
reported issue. Hence,


Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>


Regards,

Venkat.

>
> 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@mail.gmail.com/
>
> 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)