Re: [PATCH] perf_events: fix time tracking in samples

[Peter Zijlstra <peterz@infradead.org>]

On Tue, 2010-10-19 at 19:01 +0200, Stephane Eranian wrote:
> On Tue, Oct 19, 2010 at 6:52 PM, Peter Zijlstra <peterz@infradead.org> wrote:
> > On Tue, 2010-10-19 at 18:47 +0200, Stephane Eranian wrote:
> >> This patch corrects time tracking in samples. Without this patch
> >> both time_enabled and time_running may be reported as zero when
> >> user asks for PERF_SAMPLE_READ.
> >>
> >> You use PERF_SAMPLE_READ when you want to sample the values of
> >> other counters in each sample. Because of multiplexing, it is
> >> necessary to know both time_enable, time_running to be able
> >> to scale counts correctly.
> >>
> >> We defer updating timing until we know it is really needed, i.e.,
> >> only when we have PERF_SAMPLE_READ.
> >>
> >> With this patch, the libpfm4 example task_smpl now reports
> >> correct counts (shown on 2.4GHz Core 2):
> >>
> >> $ task_smpl -p 2400000000 -e unhalted_core_cycles:u,instructions_retired:u,baclears  noploop 5
> >> noploop for 5 seconds
> >> IIP:0x000000004006d6 PID:5596 TID:5596 TIME:466,210,211,430 STREAM_ID:33 PERIOD:2,400,000,000 ENA=1,010,157,814 RUN=1,010,157,814 NR=3
> >>       2,400,000,254 unhalted_core_cycles:u (33)
> >>       2,399,273,744 instructions_retired:u (34)
> >>       53,340 baclears (35)
> >>
> >> Signed-off-by: Stephane Eranian <eranian@google.com>
> >>
> >> ---
> >>
> >> diff --git a/kernel/perf_event.c b/kernel/perf_event.c
> >> index f309e80..04611dd 100644
> >> --- a/kernel/perf_event.c
> >> +++ b/kernel/perf_event.c
> >> @@ -3494,6 +3494,9 @@ static void perf_output_read_group(struct perf_output_handle *handle,
> >>  static void perf_output_read(struct perf_output_handle *handle,
> >>                            struct perf_event *event)
> >>  {
> >> +     update_context_time(event->ctx);
> >> +     update_event_times(event);
> >> +
> >>       if (event->attr.read_format & PERF_FORMAT_GROUP)
> >>               perf_output_read_group(handle, event);
> >>       else
> >
> >
> > Right, except that this can actually corrupt the time measurements... :/
> >
> > Usually context times are updated under ctx->lock, and this is called
> > from NMI context, which can interrupt ctx->lock..
> >
> Ok, I missed that. But I don't understand why you need the lock to
> udpate the time. The lower-level clock is lockless if I recall. Can't you
> use an atomic ops in update_context_time()?

atomic ops would slow down those code paths, also, I don't think you can
fully get the ordering between ->tstamp_$foo and ->total_time_$foo just
right.

> > I was thinking about updating a local copy of the times, in that case
> > you can only get funny times from samples, but it won't corrupt the
> > actual running data.
> >
> You want time to be correct in every sample How would you detect
> bogus timing?

Not sure, but barring 64bit atomics for all these, 32bit archs and NMI
are going to be 'interesting'