linux-kernel - Re: [RESEND PATCH V2 3/4] perf/x86/intel: drain PEBS buffer in event read

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Message-ID: <20180118140523.GB2940@krava>
Date:   Thu, 18 Jan 2018 15:05:23 +0100
From:   Jiri Olsa <jolsa@...hat.com>
To:     "Liang, Kan" <kan.liang@...ux.intel.com>
Cc:     peterz@...radead.org, mingo@...hat.com, acme@...nel.org,
        linux-kernel@...r.kernel.org, tglx@...utronix.de,
        eranian@...gle.com, ak@...ux.intel.com
Subject: Re: [RESEND PATCH V2 3/4] perf/x86/intel: drain PEBS buffer in event
 read

On Thu, Jan 18, 2018 at 08:30:30AM -0500, Liang, Kan wrote:
> 
> 
> On 1/18/2018 4:49 AM, Jiri Olsa wrote:
> > On Tue, Jan 16, 2018 at 01:49:13PM -0500, Liang, Kan wrote:
> > > 
> > > 
> > > On 1/11/2018 10:45 AM, Jiri Olsa wrote:
> > > > On Thu, Jan 11, 2018 at 10:21:25AM -0500, Liang, Kan wrote:
> > > > 
> > > > SNIP
> > > > 
> > > > > > 
> > > > > > hum, but the PEBS drain is specific just for
> > > > > > PERF_X86_EVENT_AUTO_RELOAD events, right?
> > > > > 
> > > > > Accurately, PEBS drain is specific for PERF_X86_EVENT_FREERUNNING here.
> > > > > PERF_X86_EVENT_FREERUNNING event must be _AUTO_RELOAD event.
> > > > > But in some cases, _AUTO_RELOAD event cannot be _FREERUNNING event.
> > > > > 
> > > > > Only the event which is both _FREERUNNING and _AUTO_RELOAD need to do PEBS
> > > > > drain in _read().
> > > > > 
> > > > > So it does the check in intel_pmu_pebs_read()
> > > > > +	if (pebs_needs_sched_cb(cpuc))
> > > > > +		return intel_pmu_drain_pebs_buffer();
> > > > > 
> > > > > > 
> > > > > > wrt readability maybe you could add function like:
> > > > > 
> > > > > The existing function pebs_needs_sched_cb() can do the check.
> > > > > We just need to expose it, and also the intel_pmu_drain_pebs_buffer().
> > > > > 
> > > > > But to be honest, I still cannot see a reason for that.
> > > > > It could save a call to intel_pmu_pebs_read(), but _read() is not critical
> > > > > path. It doesn't save much.
> > > > 
> > > > hum, pmu->read is also called for PERF_SAMPLE_READ for sample,
> > > > check perf_output_read
> > > > 
> > > > for non sampling event you shouldn't be able to create PEBS
> > > > event, there's check in x86_pmu_hw_config
> > > > 
> > > > I agree it does not save much, it just confused me while
> > > > I was reading the code, like why is this needed for all
> > > > events with precise_ip
> > > > 
> > > 
> > > 
> > > Sorry for the late response.
> > > 
> > > How about the patch as below?
> > > The patch will be split into two patches in V3. One is to introduce
> > > intel_pmu_large_pebs_read, the other is to introduce intel_pmu_read_event.
> > > 
> > > Thanks,
> > > Kan
> > > 
> > > diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
> > > index 731153a..1610a9d 100644
> > > --- a/arch/x86/events/intel/core.c
> > > +++ b/arch/x86/events/intel/core.c
> > > @@ -2060,6 +2060,14 @@ static void intel_pmu_del_event(struct perf_event
> > > *event)
> > >   		intel_pmu_pebs_del(event);
> > >   }
> > > 
> > > +static void intel_pmu_read_event(struct perf_event *event)
> > > +{
> > > +	if (intel_pmu_large_pebs_read(event))
> > > +		return;
> > 
> > should this be 'if (!intel_pmu_large_pebs_read(event))'
> > 
> 
> NO. For large pebs, the event->count has been updated in drain_pebs(). So it
> doesn't need to do x86_perf_event_update() again.
> 

ok, thanks

jirka