Re: [PATCH v2] perf/core: Avoid context switch overheads

[æåç]

2017-08-08 18:37 GMT+08:00 Peter Zijlstra <peterz@xxxxxxxxxxxxx>:
> On Tue, Aug 08, 2017 at 06:00:45PM +0800, çç wrote:
>
>> diff --git a/kernel/events/core.c b/kernel/events/core.c
>> index 426c2ff..3d86695 100644
>> --- a/kernel/events/core.c
>> +++ b/kernel/events/core.c
>> @@ -3180,6 +3180,13 @@ static void perf_event_context_sched_in(struct perf_event_context *ctx,
>>               return;
>>
>>       perf_ctx_lock(cpuctx, ctx);
>> +     /*
>> +      * We must check ctx->nr_events while holding ctx->lock, such
>> +      * that we serialize against perf_install_in_context().
>> +      */
>> +     if (!cpuctx->task_ctx && !ctx->nr_events)
>> +             goto unlock;
>
> Do we really need the cpuctx->task_ctx test? I think that task_ctx is
> 'tight' these days. We never have it set unless there are events
> scheduled for that context.
>
> I even think the cpuctx->task_ctx == ctx test right above here is
> superfluous these days. That could only happen when the
> perf_install_in_context() IPI came before perf_event_task_sched_in(),
> but we removed the arch option to do context switches with IRQs enabled.
>

It looks that cpuctx->task_ctx exists somewhere else, so I thought it was
conservative making this patch.

For a centain, during my process of debugging I didn't figure out any value
of cpuctx->task_ctx. I shall make a v3.

Thanks

>> +
>>       perf_pmu_disable(ctx->pmu);
>>       /*
>>        * We want to keep the following priority order:
>> @@ -3193,6 +3200,8 @@ static void perf_event_context_sched_in(struct perf_event_context *ctx,
>>               cpu_ctx_sched_out(cpuctx, EVENT_FLEXIBLE);
>>       perf_event_sched_in(cpuctx, ctx, task);
>>       perf_pmu_enable(ctx->pmu);
>> +
>> +unlock:
>>       perf_ctx_unlock(cpuctx, ctx);
>>  }
>>
>> --
>> 2.8.4.31.g9ed660f
>>