linux-kernel - Re: [PATCH V11 05/10] arm64/perf: Add branch stack support in ARMV8 PMU

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Date:   Tue, 6 Jun 2023 11:41:17 +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 V11 05/10] arm64/perf: Add branch stack support in ARMV8
 PMU

On Tue, Jun 06, 2023 at 04:04:25PM +0530, Anshuman Khandual wrote:
> On 6/5/23 17:35, Mark Rutland wrote:
> > On Wed, May 31, 2023 at 09:34:23AM +0530, Anshuman Khandual wrote:
> >>  static int armv8pmu_probe_pmu(struct arm_pmu *cpu_pmu)
> >> @@ -1145,12 +1162,24 @@ static int armv8pmu_probe_pmu(struct arm_pmu *cpu_pmu)
> >>  	};
> >>  	int ret;
> >>  
> >> +	ret = armv8pmu_private_alloc(cpu_pmu);
> >> +	if (ret)
> >> +		return ret;
> >> +
> >>  	ret = smp_call_function_any(&cpu_pmu->supported_cpus,
> >>  				    __armv8pmu_probe_pmu,
> >>  				    &probe, 1);
> >>  	if (ret)
> >>  		return ret;
> >>  
> >> +	if (arm_pmu_branch_stack_supported(cpu_pmu)) {
> >> +		ret = branch_records_alloc(cpu_pmu);
> >> +		if (ret)
> >> +			return ret;
> >> +	} else {
> >> +		armv8pmu_private_free(cpu_pmu);
> >> +	}
> > 
> > I see from the next patch that "private" is four ints, so please just add that
> > to struct arm_pmu under an ifdef CONFIG_ARM64_BRBE. That'll simplify this, and
> > if we end up needing more space in future we can consider factoring it out.
> 
> struct arm_pmu {
> 	........................................
>         /* Implementation specific attributes */
>         void            *private;
> }
> 
> private pointer here creates an abstraction for given pmu implementation
> to hide attribute details without making it known to core arm pmu layer.
> Although adding ifdef CONFIG_ARM64_BRBE solves the problem as mentioned
> above, it does break that abstraction. Currently arm_pmu layer is aware
> about 'branch records' but not about BRBE in particular which the driver
> adds later on. I suggest we should not break that abstraction.

I understand the rationale, but I think it's simpler for now to break that
abstraction. We can always refactor it later.

> Instead a global 'static struct brbe_hw_attr' in drivers/perf/arm_brbe.c
> can be initialized into arm_pmu->private during armv8pmu_branch_probe(),
> which will also solve the allocation-free problem. 

IIUC that's not going to work for big.LITTLE systems where the BRBE support
varies, as we need this data per arm_pmu.

> Also similar helpers armv8pmu_task_ctx_alloc()/free() could be defined to
> manage task context cache i.e arm_pmu->pmu.task_ctx_cache independently.
> 
> But now armv8pmu_task_ctx_alloc() can be called after pmu probe confirms
> to have arm_pmu->has_branch_stack.

I think those are different, and should be kept.

Thanks,
Mark.