Re: [PATCH v4 2/2] perf/core: Fix incorrect time diff in tick adjust period

[Liang, Kan]

On 2024-08-21 9:42 a.m., Luo Gengkun wrote:
> Perf events has the notion of sampling frequency which is implemented in
> software by dynamically adjusting the counter period so that samples occur
> at approximately the target frequency.  Period adjustment is done in 2
> places:
>  - when the counter overflows (and a sample is recorded)
>  - each timer tick, when the event is active
> The later case is slightly flawed because it assumes that the time since
> the last timer-tick period adjustment is 1 tick, whereas the event may not
> have been active (e.g. for a task that is sleeping).
>

Do you have a real-world example to demonstrate how bad it is if the
algorithm doesn't take sleep into account?

I'm not sure if introducing such complexity in the critical path is
worth it.

> Fix by using jiffies to determine the elapsed time in that case.
> 
> Signed-off-by: Luo Gengkun <luogengkun@xxxxxxxxxxxxxxx>
> ---
>  include/linux/perf_event.h |  1 +
>  kernel/events/core.c       | 11 ++++++++---
>  2 files changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index 1a8942277dda..d29b7cf971a1 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -265,6 +265,7 @@ struct hw_perf_event {
>  	 * State for freq target events, see __perf_event_overflow() and
>  	 * perf_adjust_freq_unthr_context().
>  	 */
> +	u64				freq_tick_stamp;
>  	u64				freq_time_stamp;
>  	u64				freq_count_stamp;
>  #endif
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index a9395bbfd4aa..86e80e3ef6ac 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -55,6 +55,7 @@
>  #include <linux/pgtable.h>
>  #include <linux/buildid.h>
>  #include <linux/task_work.h>
> +#include <linux/jiffies.h>
>  
>  #include "internal.h"
>  
> @@ -4120,7 +4121,7 @@ static void perf_adjust_freq_unthr_events(struct list_head *event_list)
>  {
>  	struct perf_event *event;
>  	struct hw_perf_event *hwc;
> -	u64 now, period = TICK_NSEC;
> +	u64 now, period, tick_stamp;
>  	s64 delta;
>  
>  	list_for_each_entry(event, event_list, active_list) {
> @@ -4148,6 +4149,10 @@ static void perf_adjust_freq_unthr_events(struct list_head *event_list)
>  		 */
>  		event->pmu->stop(event, PERF_EF_UPDATE);
>  
> +		tick_stamp = jiffies64_to_nsecs(get_jiffies_64());

Seems it only needs to retrieve the time once at the beginning, not for
each event.

There is a perf_clock(). It's better to use it for the consistency.

Thanks,
Kan
> +		period = tick_stamp - hwc->freq_tick_stamp;
> +		hwc->freq_tick_stamp = tick_stamp;
> +
>  		now = local64_read(&event->count);
>  		delta = now - hwc->freq_count_stamp;
>  		hwc->freq_count_stamp = now;
> @@ -4157,9 +4162,9 @@ static void perf_adjust_freq_unthr_events(struct list_head *event_list)
>  		 * reload only if value has changed
>  		 * we have stopped the event so tell that
>  		 * to perf_adjust_period() to avoid stopping it
> -		 * twice.
> +		 * twice. And skip if it is the first tick adjust period.
>  		 */
> -		if (delta > 0)
> +		if (delta > 0 && likely(period != tick_stamp))
>  			perf_adjust_period(event, period, delta, false);>
>  		event->pmu->start(event, delta > 0 ? PERF_EF_RELOAD : 0);