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Subject: Re: [PATCH] perf_events: fix time tracking in samples
From: Peter Zijlstra <peterz@infradead.org>
To: eranian@google.com
Cc: linux-kernel@vger.kernel.org, mingo@elte.hu, paulus@samba.org,
        davem@davemloft.net, fweisbec@gmail.com, perfmon2-devel@lists.sf.net,
        eranian@gmail.com, robert.richter@amd.com
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Date: Tue, 19 Oct 2010 18:52:58 +0200
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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.