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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