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

["Yan, Zheng" <zheng.z.yan@intel.com>]

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