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: <20250224122507.GE8144@e132581.arm.com>
Date: Mon, 24 Feb 2025 12:25:07 +0000
From: Leo Yan <leo.yan@....com>
To: "Rob Herring (Arm)" <robh@...nel.org>
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 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.

Thanks,
Leo