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

[Ben Gainey]

On Mon, 2024-04-22 at 14:08 +0100, James Clark wrote:
> 
> 
> 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))
> 

I don't really have a strong opinion; in all my time I've never seen an
Arm PMU produce a precise and constant period anyway, so this may be
more useful in the case the architecture is able to support precise
sampling. In any case it's is likely to be specific to a particular
workload / configuration anyway.

The main downside I can see for making it configurable is that the
compiler cannot then optimise the get_random call as well as for a
constant, which may be undesirable on this code path.


> >  /*
> >   * 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);