linux-kernel - Re: [PATCH 09/12] perf_events: add hook to flush branch

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Date:	Wed, 7 Dec 2011 10:25:59 -0800
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, acme@...hat.com,
	ming.m.lin@...el.com, andi@...stfloor.org, robert.richter@....com,
	ravitillo@....gov, will.deacon@....com, paulus@...ba.org,
	benh@...nel.crashing.org, rth@...ddle.net, ralf@...ux-mips.org,
	davem@...emloft.net, lethal@...ux-sh.org
Subject: Re: [PATCH 09/12] perf_events: add hook to flush branch_stack on
 context switch (v2)

On Mon, Dec 5, 2011 at 1:37 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Fri, 2011-10-14 at 14:37 +0200, Stephane Eranian wrote:
>> +               /*
>> +                * check if the context has at least one
>> +                * event using PERF_SAMPLE_BRANCH_STACK
>> +                */
>> +               if (cpuctx->ctx.nr_branch_stack > 0
>> +                   && pmu->flush_branch_stack) {
>> +
>> +                       pmu = cpuctx->ctx.pmu;
>> +
>> +                       perf_ctx_lock(cpuctx, cpuctx->task_ctx);
>> +
>> +                       perf_pmu_disable(pmu);
>> +
>> +                       pmu->flush_branch_stack();
>> +
>> +                       perf_pmu_enable(pmu);
>> +
>> +                       perf_ctx_unlock(cpuctx, cpuctx->task_ctx);
>> +               }
>> +       }
>
> (what whitespace looks funny)
>
> So all PMUs not supporting this branch stuff will fail to create a
> has_branch_stack() event, right? Thus all ctx with !0 nr_branch_stack
> support it. Doesn't this make the test for pmu->flush_branch_stack
> redundant?
>
>
No, nr_branch_stack counts the number of active events with
branch_stack. It's like the ctx->nr_cgroups. Processors which
do not support branch_stack will always have this field to 0.
It's not because a processor supports branch_stack that we
need to call flush_branch_stack(), i.e., we use a lazy approach.
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