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

[Leo Yan <leo.yan@arm.com>]

On Wed, May 28, 2025 at 07:48:23AM -0700, 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
> 
> 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.
> 
> Reported-by: Leo Yan <leo.yan@arm.com>
> Reported-by: Aishwarya TCV <aishwarya.tcv@arm.com>

Thanks for adding me and my colleague's name!

> Closes: https://lore.kernel.org/lkml/20250527161656.GJ2566836@e132581.arm.com/
> Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
> ---
>  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 947ad12dfdbe..66f02f46595c 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -303,6 +303,7 @@ struct perf_event_pmu_context;
>  #define PERF_PMU_CAP_AUX_OUTPUT			0x0080
>  #define PERF_PMU_CAP_EXTENDED_HW_TYPE		0x0100
>  #define PERF_PMU_CAP_AUX_PAUSE			0x0200
> +#define PERF_PMU_CAP_NO_THROTTLE_STOP		0x0400

When applying the patch on the top of tip/sched/core, I found it is
conflict with a new added PERF_PMU_CAP_AUX_PREFER_LARGE.  So need to
change the PERF_PMU_CAP_NO_THROTTLE_STOP to 0x0800.

With the change, I tested on Arm64 board with two commands:

  perf record -a -e cpu-clock -- sleep 2
  perf test "perftool-testsuite_report"

Tested-by: Leo Yan <leo.yan@arm.com>

Thanks for quick fixing.

>  /**
>   * pmu::scope
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 8327ab0ee641..4df274705038 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -2655,7 +2655,22 @@ 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;
>  	perf_log_throttle(event, 0);
>  }
> @@ -11846,7 +11861,8 @@ 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,
> @@ -11928,7 +11944,8 @@ 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,
> -- 
> 2.38.1
>