Re: [PATCH v2 4/7] perf, x86: Save/resotre LBR stack during context switch

[Peter Zijlstra <peterz@infradead.org>]

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.