linux-kernel - Re: [PATCH] Revert "perf/x86: Allow zero PEBS status with only single active event"

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Date:   Wed, 3 Mar 2021 21:21:31 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Liang, Kan" <kan.liang@...ux.intel.com>
Cc:     Vince Weaver <vincent.weaver@...ne.edu>, mingo@...hat.com,
        linux-kernel@...r.kernel.org, acme@...nel.org,
        mark.rutland@....com, alexander.shishkin@...ux.intel.com,
        jolsa@...hat.com, namhyung@...nel.org, eranian@...gle.com,
        ak@...ux.intel.com, stable@...r.kernel.org
Subject: Re: [PATCH] Revert "perf/x86: Allow zero PEBS status with only
 single active event"

On Wed, Mar 03, 2021 at 02:53:00PM -0500, Liang, Kan wrote:
> On 3/3/2021 1:59 PM, Peter Zijlstra wrote:
> > On Wed, Mar 03, 2021 at 05:42:18AM -0800, kan.liang@...ux.intel.com wrote:

> > > +++ b/arch/x86/events/intel/ds.c
> > > @@ -2000,18 +2000,6 @@ static void intel_pmu_drain_pebs_nhm(struct pt_regs *iregs, struct perf_sample_d
> > >   			continue;
> > >   		}
> > > -		/*
> > > -		 * On some CPUs the PEBS status can be zero when PEBS is
> > > -		 * racing with clearing of GLOBAL_STATUS.
> > > -		 *
> > > -		 * Normally we would drop that record, but in the
> > > -		 * case when there is only a single active PEBS event
> > > -		 * we can assume it's for that event.
> > > -		 */
> > > -		if (!pebs_status && cpuc->pebs_enabled &&
> > > -			!(cpuc->pebs_enabled & (cpuc->pebs_enabled-1)))
> > > -			pebs_status = cpuc->pebs_enabled;
> > 
> > Wouldn't something like:
> > 
> > 			pebs_status = p->status = cpus->pebs_enabled;
> > 
> 
> I didn't consider it as a potential solution in this patch because I don't
> think it's a proper way that SW modifies the buffer, which is supposed to be
> manipulated by the HW.

Right, but then HW was supposed to write sane values and it doesn't do
that either ;-)

> It's just a personal preference. I don't see any issue here. We may try it.

So I mostly agree with you, but I think it's a shame to unsupport such
chips, HSW is still a plenty useable chip today.