linux-kernel - Re: [PATCH v20 11/11] perf: arm_pmuv3: Add support for the Branch Record Buffer Extension (BRBE)

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Message-ID: <CAL_Jsq+0fZ2uasgAam7qGTdCeDBQxXeyL-J1_suyxy6GE_ERTg@mail.gmail.com>
Date: Mon, 24 Feb 2025 06:46:35 -0600
From: Rob Herring <robh@...nel.org>
To: Leo Yan <leo.yan@....com>
Cc: Will Deacon <will@...nel.org>, Mark Rutland <mark.rutland@....com>, 
	Catalin Marinas <catalin.marinas@....com>, Jonathan Corbet <corbet@....net>, Marc Zyngier <maz@...nel.org>, 
	Oliver Upton <oliver.upton@...ux.dev>, Joey Gouly <joey.gouly@....com>, 
	Suzuki K Poulose <suzuki.poulose@....com>, Zenghui Yu <yuzenghui@...wei.com>, 
	James Clark <james.clark@...aro.org>, Anshuman Khandual <anshuman.khandual@....com>, 
	linux-arm-kernel@...ts.infradead.org, linux-perf-users@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org, 
	kvmarm@...ts.linux.dev
Subject: Re: [PATCH v20 11/11] perf: arm_pmuv3: Add support for the Branch
 Record Buffer Extension (BRBE)

On Mon, Feb 24, 2025 at 6:25 AM Leo Yan <leo.yan@....com> wrote:
>
> On Tue, Feb 18, 2025 at 02:40:06PM -0600, Rob Herring (Arm) wrote:
> >
> > From: Anshuman Khandual <anshuman.khandual@....com>
>
> [...]
>
> > BRBE records are invalidated whenever events are reconfigured, a new
> > task is scheduled in, or after recording is paused (and the records
> > have been recorded for the event). The architecture allows branch
> > records to be invalidated by the PE under implementation defined
> > conditions. It is expected that these conditions are rare.
>
> [...]
>
> > +static void armv8pmu_sched_task(struct perf_event_pmu_context *pmu_ctx, bool sched_in)
> > +{
> > +       struct arm_pmu *armpmu = *this_cpu_ptr(&cpu_armpmu);
> > +       struct pmu_hw_events *hw_events = this_cpu_ptr(armpmu->hw_events);
> > +
> > +       if (!hw_events->branch_users)
> > +               return;
> > +
> > +       if (sched_in)
> > +               brbe_invalidate();
> > +}
>
> Just a minor concern.  I don't see any handling for task migration.
> E.g., for a task is migrated from one CPU to another CPU, I expect we
> need to save and restore branch records based on BRBE injection.  So
> far, the driver simply invalidates all records.
>
> I think this topic is very likely discussed before.  If this is the
> case, please ignore my comment.  Except this, the code looks good
> to me.

Not really discussed on the list, but that was present in v18 (though
not functional because .sched_task() hook wasn't actually enabled) and
Mark removed it. His work is here[1].The only comment was:

Note: saving/restoring at context-switch doesn't interact well with
event rotation (e.g. if filters change)

Rob

[1] https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/commit/?h=arm64/brbe&id=642985af34d2d6f54e76995380cf24d512078c56