Re: [PATCH V7 3/6] perf, x86: handle multiple records in PEBS buffer

Peter Zijlstra
2015-05-05T13:07:51Z

On Mon, Apr 20, 2015 at 04:07:47AM -0400, Kan Liang wrote:
> From: Yan, Zheng <zheng.z.yan@intel.com>

<snip>

> Here lists some possible ways you may get a lot of collision.

This is the first time the world 'collisions' is used; either define
what you mean by it or avoid using it.

> - when you count the same thing multiple times. But it is not a useful
> configuration.
> - you can be unfortunate if you measure with a userspace only PEBS
> event along with either a kernel or unrestricted PEBS event. Imagine
> the event triggering and setting the overflow flag right before
> entering the kernel. Then all kernel side events will end up with
> multiple bits set.
>
> Here are some numbers about collisions.
> Four frequently occurring events
> (cycles:p,instructions:p,branches:p,mem-stores:p) are tested
>
> Test events which are sampled together collision rate
> cycles:p,instructions:p 0.25%
> cycles:p,instructions:p,branches:p 0.30%
> cycles:p,instructions:p,branches:p,mem-stores:p 0.35%
>
> cycles:p,cycles:p 98.52%

It would be good if you can illustrate this with the new PREF_RECORD and
the perf tool itself.

> Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
> Signed-off-by: Kan Liang <kan.liang@intel.com>
> ---

> --- a/arch/x86/kernel/cpu/perf_event_intel_ds.c
> +++ b/arch/x86/kernel/cpu/perf_event_intel_ds.c

> @@ -958,19 +961,97 @@ static void setup_pebs_sample_data(struct perf_event *event,
> data->br_stack = &cpuc->lbr_stack;
> }
>
> +static void perf_log_lost(struct perf_event *event)
> +{
> + struct perf_output_handle handle;
> + struct perf_sample_data sample;
> + int ret;
> +
> + struct {
> + struct perf_event_header header;
> + u64 id;
> + u64 lost;
> + } lost_event = {
> + .header = {
> + .type = PERF_RECORD_LOST,
> + .misc = 0,
> + .size = sizeof(lost_event),
> + },
> + .id = event->id,
> + .lost = 1,
> + };
> +
> + perf_event_header__init_id(&lost_event.header, &sample, event);
> +
> + ret = perf_output_begin(&handle, event,
> + lost_event.header.size);
> + if (ret)
> + return;
> +
> + perf_output_put(&handle, lost_event);
> + perf_event__output_id_sample(event, &handle, &sample);
> + perf_output_end(&handle);
> +}

RECORDs are generic, and should live in the core code.

Also, you should introduce this RECORD in a separate patch.

Ideally, you'd also update the tools side to parse this and modify
perf-record to show the number of dropped events as a percentage, going
warn/error when >1%/>5% or so?