Re: [RFC] perf/core: Add an ioctl to get a number of lost samples

[Stephane Eranian <eranian@google.com>]

On Wed, Aug 11, 2021 at 8:04 AM Jiri Olsa <jolsa@redhat.com> wrote:
>
> On Tue, Aug 10, 2021 at 11:21:35PM -0700, Namhyung Kim wrote:
> > Sometimes we want to know an accurate number of samples even if it's
> > lost.  Currenlty PERF_RECORD_LOST is generated for a ring-buffer which
> > might be shared with other events.  So it's hard to know per-event
> > lost count.
> >
> > Add event->lost_samples field and PERF_EVENT_IOC_LOST_SAMPLES to
> > retrieve it from userspace.
> >
> > Signed-off-by: Namhyung Kim <namhyung@kernel.org>
> > ---
> >  include/linux/perf_event.h      | 2 ++
> >  include/uapi/linux/perf_event.h | 1 +
> >  kernel/events/core.c            | 9 +++++++++
> >  kernel/events/ring_buffer.c     | 5 ++++-
> >  4 files changed, 16 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> > index f5a6a2f069ed..44d72079c77a 100644
> > --- a/include/linux/perf_event.h
> > +++ b/include/linux/perf_event.h
> > @@ -756,6 +756,8 @@ struct perf_event {
> >       struct pid_namespace            *ns;
> >       u64                             id;
> >
> > +     atomic_t                        lost_samples;
> > +
> >       u64                             (*clock)(void);
> >       perf_overflow_handler_t         overflow_handler;
> >       void                            *overflow_handler_context;
> > diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
> > index bf8143505c49..24397799127d 100644
> > --- a/include/uapi/linux/perf_event.h
> > +++ b/include/uapi/linux/perf_event.h
> > @@ -505,6 +505,7 @@ struct perf_event_query_bpf {
> >  #define PERF_EVENT_IOC_PAUSE_OUTPUT          _IOW('$', 9, __u32)
> >  #define PERF_EVENT_IOC_QUERY_BPF             _IOWR('$', 10, struct perf_event_query_bpf *)
> >  #define PERF_EVENT_IOC_MODIFY_ATTRIBUTES     _IOW('$', 11, struct perf_event_attr *)
> > +#define PERF_EVENT_IOC_LOST_SAMPLES          _IOR('$', 12, __u64 *)
>
> would it be better to use the read syscall for that?
>   https://lore.kernel.org/lkml/20210622153918.688500-5-jolsa@kernel.org/
>
> that patchset ended up on me not having a way to reproduce the
> issue you guys wanted the fix for ;-) the lost count is there
> as well
>
Does the read format approach succeed even when the event is in error state?

> jirka
>
> >
> >  enum perf_event_ioc_flags {
> >       PERF_IOC_FLAG_GROUP             = 1U << 0,
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index 0e125ae2fa92..a4d6736b6594 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -5664,6 +5664,15 @@ static long _perf_ioctl(struct perf_event *event, unsigned int cmd, unsigned lon
> >
> >               return perf_event_modify_attr(event,  &new_attr);
> >       }
> > +
> > +     case PERF_EVENT_IOC_LOST_SAMPLES: {
> > +             u64 lost = atomic_read(&event->lost_samples);
> > +
> > +             if (copy_to_user((void __user *)arg, &lost, sizeof(lost)))
> > +                     return -EFAULT;
> > +             return 0;
> > +     }
> > +
> >       default:
> >               return -ENOTTY;
> >       }
> > diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
> > index 52868716ec35..06d7dacb05da 100644
> > --- a/kernel/events/ring_buffer.c
> > +++ b/kernel/events/ring_buffer.c
> > @@ -172,8 +172,10 @@ __perf_output_begin(struct perf_output_handle *handle,
> >               goto out;
> >
> >       if (unlikely(rb->paused)) {
> > -             if (rb->nr_pages)
> > +             if (rb->nr_pages) {
> >                       local_inc(&rb->lost);
> > +                     atomic_inc(&event->lost_samples);
> > +             }
> >               goto out;
> >       }
> >
> > @@ -254,6 +256,7 @@ __perf_output_begin(struct perf_output_handle *handle,
> >
> >  fail:
> >       local_inc(&rb->lost);
> > +     atomic_inc(&event->lost_samples);
> >       perf_output_put_handle(handle);
> >  out:
> >       rcu_read_unlock();
> > --
> > 2.32.0.605.g8dce9f2422-goog
> >
>