RE: [PATCH V6 3/6] perf, x86: large PEBS interrupt threshold

[Liang, Kan]

> -----Original Message-----
> From: Peter Zijlstra [mailto:peterz@xxxxxxxxxxxxx]
> Sent: Wednesday, April 15, 2015 1:15 PM
> To: Liang, Kan
> Cc: linux-kernel@xxxxxxxxxxxxxxx; mingo@xxxxxxxxxx;
> acme@xxxxxxxxxxxxx; eranian@xxxxxxxxxx; andi@xxxxxxxxxxxxxx
> Subject: Re: [PATCH V6 3/6] perf, x86: large PEBS interrupt threshold
> 
> On Thu, Apr 09, 2015 at 12:37:43PM -0400, Kan Liang wrote:
> > This patch also make AUTO_RELOAD conditional on large PEBS. Auto
> > reload only be enabled when fix period and large PEBS.
> 
> What's a large PEBS?
>
Should be large PEBS available.
!(event->attr.sample_type & ~PEBS_FREERUNNING_FLAGS)

> > +++ b/arch/x86/kernel/cpu/perf_event.h
> > @@ -87,6 +87,17 @@ struct amd_nb {
> >  #define MAX_PEBS_EVENTS		8
> >
> >  /*
> > + * Flags PEBS can handle without an PMI.
> > + *
> > + * TID can only be handled by flushing at context switch.
> > + */
> > +#define PEBS_FREERUNNING_FLAGS \
> > +	(PERF_SAMPLE_IP | PERF_SAMPLE_TID | PERF_SAMPLE_ADDR | \
> > +	PERF_SAMPLE_ID | PERF_SAMPLE_CPU |
> PERF_SAMPLE_STREAM_ID | \
> > +	PERF_SAMPLE_DATA_SRC | PERF_SAMPLE_IDENTIFIER | \
> > +	PERF_SAMPLE_TRANSACTION)
> > +
> > +/*
> >   * A debug store configuration.
> >   *
> >   * We only support architectures that use 64bit fields.
> > diff --git a/arch/x86/kernel/cpu/perf_event_intel.c
> > b/arch/x86/kernel/cpu/perf_event_intel.c
> > index 0a7b5ca..6c8579a 100644
> > --- a/arch/x86/kernel/cpu/perf_event_intel.c
> > +++ b/arch/x86/kernel/cpu/perf_event_intel.c
> > @@ -2306,7 +2306,9 @@ static int intel_pmu_hw_config(struct
> perf_event *event)
> >  		return ret;
> >
> >  	if (event->attr.precise_ip) {
> > -		if (!event->attr.freq)
> > +		/* only enable auto reload when fix period and large PEBS
> */
> > +		if (!event->attr.freq &&
> > +		    !(event->attr.sample_type &
> ~PEBS_FREERUNNING_FLAGS))
> >  			event->hw.flags |=
> PERF_X86_EVENT_AUTO_RELOAD;
> >  		if (x86_pmu.pebs_aliases)
> >  			x86_pmu.pebs_aliases(event);
> 
> I suspect you meant the above change right?

Yes.

> 
> But this negates part of the benefit of the auto reload; where previously it
> saved an MSR write for pretty much all PEBS usage it now becomes a
> burden for pretty much everyone.
> 
> Why cannot we retain the win for all PEBS users?

The change tries to address your comments.
https://lkml.org/lkml/2015/3/30/294

Yes, we can retain the win. If so, I think we need to introduce another
flag like PERF_X86_EVENT_LARGE_PEBS and check it in pebs_is_enabled.
Or just keep the previous V5 patch unchanged.  

Thanks,
Kan


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