Messages in this thread | | | Date | Fri, 05 Jul 2013 13:36:24 +0800 | From | "Yan, Zheng" <> | Subject | Re: [PATCH v2 4/7] perf, x86: Save/resotre LBR stack during context switch |
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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 >> + * force flush the branch stack if there are cpu-wide events >> + * using PERF_SAMPLE_BRANCH_STACK >> + * >> + * 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, >> + sched_in, force_flush); >> >> perf_pmu_enable(pmu); >> >> - perf_ctx_unlock(cpuctx, cpuctx->task_ctx); >> + 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 > that get scheduled? > > After all; the LBR usage is through the events, so scheduling the events > 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 speaking,LBR 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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