Messages in this thread | | | Date | Fri, 5 Jul 2013 10:15:16 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2 4/7] perf, x86: Save/resotre LBR stack during context switch |
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On Fri, Jul 05, 2013 at 01:36:24PM +0800, Yan, Zheng wrote: > 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.
Yeah so? There's tons of shared resources in the PMU already.
> Strictly speaking,LBR is associated with task, not event.
Wrong!, it _is_ associated with events. Events is all there is. Event can be associated with tasks, but that's completely irrelevant.
> 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.
Different scheduling and you're wrong. Look at perf_rotate_context(), we'd disable everything at perf_pmu_disable() and enable the entire thing at perf_pmu_enable(), on both sides we'd have the LBR running.
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