Re: [PATCH v2 4/7] perf, x86: Save/resotre LBR stack during contextswitch
From: Yan, Zheng
Date: Fri Jul 05 2013 - 01:36:42 EST
On 07/04/2013 08:45 PM, Peter Zijlstra wrote:
>
On Mon, Jul 01, 2013 at 03:23:04PM +0800, Yan, Zheng wrote:
>
>
> @@ -2488,25 +2508,31 @@ static void perf_branch_stack_sched_in(struct task_struct *prev,
>
>
>
> list_for_each_entry_rcu(pmu, &pmus, entry) {
>
> cpuctx = this_cpu_ptr(pmu->pmu_cpu_context);
>
> + task_ctx = cpuctx->task_ctx;
>
>
>
> /*
>
> - * check if the context has at least one
>
> - * event using PERF_SAMPLE_BRANCH_STACK
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> + * force flush the branch stack if there are cpu-wide events
>
> + * using PERF_SAMPLE_BRANCH_STACK
>
> + *
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> + * save/restore the branch stack if the task context has
>
> + * at least one event using PERF_SAMPLE_BRANCH_STACK
>
> */
>
> - if (cpuctx->ctx.nr_branch_stack > 0
>
> - && pmu->flush_branch_stack) {
>
> -
>
> + bool force_flush = cpuctx->ctx.nr_branch_stack > 0;
>
> + if (pmu->branch_stack_sched &&
>
> + (force_flush ||
>
> + (task_ctx && task_ctx->nr_branch_stack > 0))) {
>
> pmu = cpuctx->ctx.pmu;
>
>
>
> - perf_ctx_lock(cpuctx, cpuctx->task_ctx);
>
> + perf_ctx_lock(cpuctx, task_ctx);
>
>
>
> perf_pmu_disable(pmu);
>
>
>
> - pmu->flush_branch_stack();
>
> + pmu->branch_stack_sched(task_ctx,
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> + sched_in, force_flush);
>
>
>
> perf_pmu_enable(pmu);
>
>
>
> - perf_ctx_unlock(cpuctx, cpuctx->task_ctx);
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> + perf_ctx_unlock(cpuctx, task_ctx);
>
> }
>
> }
>
>
>
>
I never really like this; and yes I know I wrote part of that. Is there
>
any way we can get rid of this and to it 'properly' through the events
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that get scheduled?
>
>
After all; the LBR usage is through the events, so scheduling the events
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should also manage the LBR state.
>
>
What is missing for that to work?
>
the LBR is shared resource, can be used by multiple events at the same time.
Strictly speakingïLBR is associated with task, not event. One example is
there are 5 events using the LBR stack feature, but there are only 4 counters.
So these events need schedule. Saving/restoring LBR on the basis of event is
clearly wrong.
Regards
Yan, Zheng
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