linux-kernel - Re: [PATCH V17 3/9] drivers: perf: arm_pmu: Add infrastructure for branch stack sampling

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Date: Tue, 21 May 2024 14:44:49 +0100
From: Mark Rutland <mark.rutland@....com>
To: Anshuman Khandual <anshuman.khandual@....com>
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	will@...nel.org, catalin.marinas@....com,
	Mark Brown <broonie@...nel.org>, James Clark <james.clark@....com>,
	Rob Herring <robh@...nel.org>, Marc Zyngier <maz@...nel.org>,
	Suzuki Poulose <suzuki.poulose@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	linux-perf-users@...r.kernel.org
Subject: Re: [PATCH V17 3/9] drivers: perf: arm_pmu: Add infrastructure for
 branch stack sampling

On Fri, Apr 05, 2024 at 08:16:33AM +0530, Anshuman Khandual wrote:
> In order to support the Branch Record Buffer Extension (BRBE), we need to
> extend the arm_pmu framework with some basic infrastructure for the branch
> stack sampling which arm_pmu drivers can opt-in using a new feature flag
> called 'has_branch_stack'. Subsequent patches will use this to add support
> for BRBE in the PMUv3 driver.

Please, just use ther *exact* wording I asked for last time:

| In order to support the Branch Record Buffer Extension (BRBE), we need to
| extend the arm_pmu framework with some basic infrastructure for branch stack
| sampling which arm_pmu drivers can opt-in to using. Subsequent patches will
| use this to add support for BRBE in the PMUv3 driver.

At this point in the commit message, the 'has_branch_stack' flag doesn't
matter, and dropping the 'to' after 'opt-in' makes this painful to read.

> Branch stack sampling support i.e capturing branch records during execution
> in core perf, rides along with normal HW events being scheduled on the PMU.
> This prepares ARMV8 PMU framework for branch stack support on relevant PMUs
> with required HW implementation.

Please delete this paragraph.

> With BRBE, the hardware records branches into a hardware FIFO, which will
> be sampled by software when perf events overflow. A task may be context-
> switched an arbitrary number of times between overflows, and to avoid
> losing samples we need to save the current records when a task is context-
> switched out. To do these we'll need to use the pmu::sched_task() callback,
> and we'll also need to allocate some per-task storage space via event flag
> PERF_ATTACH_TASK_DATA.

[...]

>  /* The events for a given PMU register set. */
>  struct pmu_hw_events {
>  	/*
> @@ -66,6 +78,17 @@ struct pmu_hw_events {
>  	struct arm_pmu		*percpu_pmu;
>  
>  	int irq;
> +
> +	struct branch_records	*branches;
> +
> +	/* Active context for task events */
> +	void			*branch_context;
> +
> +	/* Active events requesting branch records */
> +	unsigned int		branch_users;
> +
> +	/* Active branch sample type filters */
> +	unsigned long		branch_sample_type;
>  };

At this point in the series I understand why we have the 'branches' and
'branch_users' fields, but the 'branch_context' and 'branch_sample_type'
fields haven't been introduced and are not obvious.

What exactly is branch_context, and why is that a 'void *' ?

I can understand if that's a PMU-specific structure to track the active
branch records, but if so I don't understand why 'branch_sample_type'
isn't folded into that.

Mark.