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

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Date:   Thu, 11 Jan 2018 16:45:22 +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 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

jirka