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    SubjectRe: [PATCH] perf_events: fix time tracking in samples
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    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..

    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.



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