Re: [PATCH V2] perf: Fix the throttle error of some clock events

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

On 28/05/25 11:28 pm, kan.liang@linux.intel.com wrote:
> From: Kan Liang <kan.liang@linux.intel.com>
>
> The Arm CI reports RCU stall, which can be reproduced by the below perf
> command.
>    perf record -a -e cpu-clock -- sleep 2


This issue is reported on IBM CI as well. But the issue is hit, while 
running perf test (specifically while running  76: perf list tests)

>
> The cpu-clock and task_clock are two special SW events, which rely on
> the hrtimer. Instead of invoking the stop(), the HRTIMER_NORESTART is
> returned to stop the timer. Because the hrtimer interrupt handler cannot
> cancel itself, which causes infinite loop.
>
> There may be two ways to fix it.
> - Add a check of MAX_INTERRUPTS in the event_stop. Return immediately if
> the stop is invoked by the throttle.
> - Introduce a PMU flag to track the case. Avoid the event_stop in
> perf_event_throttle() if the flag is detected.
>
> The latter looks more generic. It may be used if there are more other
> cases that want to avoid the stop later. The latter is implemented.


Tested this patch by applying on the linux-mainline kernel on IBM Power9 
system, and it fixes the issue. Hence,


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



Regards,

Venkat.

>
> 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/
> Tested-by: Leo Yan <leo.yan@arm.com>
> Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
> ---
>
> Changes since V1:
> - Rebase on top of the latest tip.git
> - Add Tested-by from Leo
>
>   include/linux/perf_event.h |  1 +
>   kernel/events/core.c       | 23 ++++++++++++++++++++---
>   2 files changed, 21 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index 52dc7cfab0e0..97a747a97a50 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -305,6 +305,7 @@ struct perf_event_pmu_context;
>   #define PERF_PMU_CAP_EXTENDED_HW_TYPE	0x0100
>   #define PERF_PMU_CAP_AUX_PAUSE		0x0200
>   #define PERF_PMU_CAP_AUX_PREFER_LARGE	0x0400
> +#define PERF_PMU_CAP_NO_THROTTLE_STOP	0x0800
>   
>   /**
>    * pmu::scope
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index f34c99f8ce8f..abd19bb571e3 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -2656,7 +2656,22 @@ 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);
> +	/*
> +	 * Some PMUs, e.g., cpu-clock and task_clock, may rely on
> +	 * a special mechanism (hrtimer) to manipulate counters.
> +	 * The regular stop doesn't work, since the hrtimer interrupt
> +	 * handler cannot cancel itself.
> +	 *
> +	 * The stop should be avoided for such cases. Let the
> +	 * driver-specific code handle it.
> +	 *
> +	 * The counters will eventually be disabled in the driver-specific
> +	 * code. In unthrottle, they still need to be re-enabled.
> +	 * There is no handling for PERF_PMU_CAP_NO_THROTTLE_STOP in
> +	 * the perf_event_unthrottle().
> +	 */
> +	if (!(event->pmu->capabilities & PERF_PMU_CAP_NO_THROTTLE_STOP))
> +		event->pmu->stop(event, 0);
>   	event->hw.interrupts = MAX_INTERRUPTS;
>   	if (event == event->group_leader)
>   		perf_log_throttle(event, 0);
> @@ -11848,7 +11863,8 @@ static int cpu_clock_event_init(struct perf_event *event)
>   static struct pmu perf_cpu_clock = {
>   	.task_ctx_nr	= perf_sw_context,
>   
> -	.capabilities	= PERF_PMU_CAP_NO_NMI,
> +	.capabilities	= PERF_PMU_CAP_NO_NMI |
> +			  PERF_PMU_CAP_NO_THROTTLE_STOP,
>   	.dev		= PMU_NULL_DEV,
>   
>   	.event_init	= cpu_clock_event_init,
> @@ -11930,7 +11946,8 @@ static int task_clock_event_init(struct perf_event *event)
>   static struct pmu perf_task_clock = {
>   	.task_ctx_nr	= perf_sw_context,
>   
> -	.capabilities	= PERF_PMU_CAP_NO_NMI,
> +	.capabilities	= PERF_PMU_CAP_NO_NMI |
> +			  PERF_PMU_CAP_NO_THROTTLE_STOP,
>   	.dev		= PMU_NULL_DEV,
>   
>   	.event_init	= task_clock_event_init,