Messages in this thread | | | Date | Tue, 5 May 2015 15:07:23 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH V7 3/6] perf, x86: handle multiple records in PEBS buffer |
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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?
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