Re: [PATCH v5 2/2] perf/core: Fix incorrect time diff in tick adjust period

[Peter Zijlstra <peterz@infradead.org>]

On Sat, Aug 31, 2024 at 07:43:16AM +0000, 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).
> 
> Fix by using jiffies to determine the elapsed time in that case.
> 
> Signed-off-by: Luo Gengkun <luogengkun@huaweicloud.com>
> Reviewed-by: Adrian Hunter <adrian.hunter@intel.com>
> ---
>  include/linux/perf_event.h |  1 +
>  kernel/events/core.c       | 12 +++++++++---
>  2 files changed, 10 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..183291e0d070 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"
>  
> @@ -4120,9 +4121,11 @@ 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;
>  
> +	tick_stamp = jiffies64_to_nsecs(get_jiffies_64());
> +
>  	list_for_each_entry(event, event_list, active_list) {
>  		if (event->state != PERF_EVENT_STATE_ACTIVE)
>  			continue;
> @@ -4148,6 +4151,9 @@ static void perf_adjust_freq_unthr_events(struct list_head *event_list)
>  		 */
>  		event->pmu->stop(event, PERF_EF_UPDATE);
>  
> +		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 +4163,9 @@ 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);

This one I'm less happy with.. that condition 'period != tick_stamp'
doesn't make sense to me. That's only false if hwc->freq_tick_stamp ==
0, which it will only be once after event creation. Even through the
Changelog babbles about event scheduling.

Also, that all should then be written something like:

	if (delta > 0 && ...) {
		perf_adjust_period(...);
		adjusted = true;
	}

	event->pmu->start(event, adjusted ? PERF_EF_RELOAD : 0);