Re: [RFC] perf/core: Add an ioctl to get a number of lost samples
From: Jiri Olsa
Date: Wed Aug 11 2021 - 15:57:35 EST
On Wed, Aug 11, 2021 at 12:33:38PM -0700, Stephane Eranian wrote:
> On Wed, Aug 11, 2021 at 8:04 AM Jiri Olsa <jolsa@xxxxxxxxxx> 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@xxxxxxxxxx>
> > > ---
> > > 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@xxxxxxxxxx/
> >
> > 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?
nope..
/*
* Return end-of-file for a read on an event that is in
* error state (i.e. because it was pinned but it couldn't be
* scheduled on to the CPU at some point).
*/
if (event->state == PERF_EVENT_STATE_ERROR)
return 0;
jirka