Re: [RFC PATCH v2 2/4] perf: Allow adding fixed random jitter to the alternate sampling period

[James Clark]

On 22/04/2024 11:49, Ben Gainey wrote:
> This change modifies the core perf overflow handler, adding some small
> random jitter to each sample period whenever an event switches between the
> two alternate sample periods. A new flag is added to perf_event_attr to
> opt into this behaviour.
> 
> This change follows the discussion in [1], where it is recognized that it
> may be possible for certain patterns of execution to end up with biased
> results.
> 
> [1] https://lore.kernel.org/linux-perf-users/Zc24eLqZycmIg3d2@tassilo/
> 
> Signed-off-by: Ben Gainey <ben.gainey@xxxxxxx>
> ---
>  include/uapi/linux/perf_event.h |  3 ++-
>  kernel/events/core.c            | 11 ++++++++++-
>  2 files changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
> index 5c1701d091cf..dd3697a4b300 100644
> --- a/include/uapi/linux/perf_event.h
> +++ b/include/uapi/linux/perf_event.h
> @@ -461,7 +461,8 @@ struct perf_event_attr {
>  				inherit_thread :  1, /* children only inherit if cloned with CLONE_THREAD */
>  				remove_on_exec :  1, /* event is removed from task on exec */
>  				sigtrap        :  1, /* send synchronous SIGTRAP on event */
> -				__reserved_1   : 26;
> +				jitter_alternate_period : 1, /* add a limited amount of jitter on each alternate period */
> +				__reserved_1   : 25;
>  
>  	union {
>  		__u32		wakeup_events;	  /* wakeup every n events */
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 07f1f931e18e..079ae520e836 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -15,6 +15,7 @@
>  #include <linux/idr.h>
>  #include <linux/file.h>
>  #include <linux/poll.h>
> +#include <linux/random.h>
>  #include <linux/slab.h>
>  #include <linux/hash.h>
>  #include <linux/tick.h>
> @@ -9546,6 +9547,8 @@ static inline bool sample_is_allowed(struct perf_event *event, struct pt_regs *r
>  	return true;
>  }
>  
> +# define MAX_ALT_SAMPLE_PERIOD_JITTER 16
> +

Is 16 enough to make a difference with very large alternate periods? I'm
wondering if it's worth making it customisable and instead of adding the
boolean option add a 16 bit jitter field. Or the option could still be a
boolean but the jitter value is some ratio of the alt sample period, like:

  get_random_u32_below(max(16, alternative_sample_period >> 4))

>  /*
>   * Generic event overflow handling, sampling.
>   */
> @@ -9573,7 +9576,10 @@ static int __perf_event_overflow(struct perf_event *event,
>  	if (event->attr.alternative_sample_period) {
>  		bool using_alt = hwc->using_alternative_sample_period;
>  		u64 sample_period = (using_alt ? event->attr.sample_period
> -					       : event->attr.alternative_sample_period);
> +					       : event->attr.alternative_sample_period)
> +				  + (event->attr.jitter_alternate_period
> +					? get_random_u32_below(MAX_ALT_SAMPLE_PERIOD_JITTER)
> +					: 0);
>  
>  		hwc->sample_period = sample_period;
>  		hwc->using_alternative_sample_period = !using_alt;
> @@ -12503,6 +12509,9 @@ SYSCALL_DEFINE5(perf_event_open,
>  		}
>  	}
>  
> +	if (attr.jitter_alternate_period && !attr.alternative_sample_period)
> +		return -EINVAL;
> +
>  	/* Only privileged users can get physical addresses */
>  	if ((attr.sample_type & PERF_SAMPLE_PHYS_ADDR)) {
>  		err = perf_allow_kernel(&attr);