linux-kernel - Re: [PATCH] sched: Move the wakeup tracepoint from ttwu_do_wakeup() to ttwu

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Message-ID: <53682AA7.9060401@cn.fujitsu.com>
Date:	Tue, 6 May 2014 09:19:51 +0900
From:	Dongsheng Yang <yangds.fnst@...fujitsu.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	<peterz@...radead.org>, <bsegall@...gle.com>, <mingo@...hat.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched: Move the wakeup tracepoint from ttwu_do_wakeup()
 to ttwu_activate().

On 05/05/2014 11:00 PM, Steven Rostedt wrote:
> On Mon, 5 May 2014 15:34:07 +0900
> Dongsheng Yang <yangds.fnst@...fujitsu.com> wrote:
>
>> The original design of sched:sched_wakeup is to trace the event inserting
>> a task into run queue. It means we can know that the state of this task is
>> changed from "sleeping" to "waiting_cpu". Then we can calculate the delay time
>> from this task on_rq = 1 to running on cpu in `perf sched latency`.
>>
>> But currently, sched:sched_wakeup event is tracing ttwu_do_wakeup() and this
>> function only set the p->state to TASK_RUNNING. This trace point can tell user
>> very little. When get a event of sched:sched_wakeup, user know that kernel call
>> ttwu_do_wakeup() once, but we can not say that task start to wait cpu from now
>> on. Maybe it did not dequeue at all, in this case we will get a wrong latency
>> time calculated by "sched_in_time - wakeup_time".
>>
>> Anyway, current sched:sched_wakeup event can tell user very little.
>> When we get a sched:sched_wakeup:
>> * We can not say a task is inserted into run queue, it is also used for task
>> which is on_rq and only change the task->state to TASK_RUNNING.
>> * We can not say the task->state is changed from {UN}INTERRUPTABLE to RUNNING,
>> sometimes task->state is already changed to RUNNING by other cpu.
>>
>> As explained above, this patch move the trace point of sched:sched_wakeup from
>> ttwu_do_wakeup() to ttwu_activate(), then when we get an event of sched_wakeup,
>> we can say that a task enqueued and started waiting cpu to run.
>>
> tldr;

Yes, it is really loooong, sorry for my terrible expression in english. :(
>
> ttwu_do_wakeup() is called when the task's state is switched back to
> TASK_RUNNING, whether or not the task actually scheduled out. Tracing
> the wakeup event when the task never scheduled out is quite confusing.
> It should only trace the task wake up if the task actually did go to
> sleep. Move the tracepoint from ttwu_do_wakeup() to ttwu_activate()
> where it is called only if the task is really woken up and not just
> have its state changed.

Thanx for your kind comment here, it looks shorter and more clear to me.
I will update my commit message with your suggestion.

Thank you very much! :)
>
> -- Steve
>
>
>> Signed-off-by: Dongsheng Yang <yangds.fnst@...fujitsu.com>
>> ---
>>   kernel/sched/core.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 9074c6d..0cae994 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -1420,6 +1420,7 @@ static void ttwu_activate(struct rq *rq, struct task_struct *p, int en_flags)
>>   {
>>   	activate_task(rq, p, en_flags);
>>   	p->on_rq = 1;
>> +	trace_sched_wakeup(p, true);
>>   
>>   	/* if a worker is waking up, notify workqueue */
>>   	if (p->flags & PF_WQ_WORKER)
>> @@ -1433,7 +1434,6 @@ static void
>>   ttwu_do_wakeup(struct rq *rq, struct task_struct *p, int wake_flags)
>>   {
>>   	check_preempt_curr(rq, p, wake_flags);
>> -	trace_sched_wakeup(p, true);
>>   
>>   	p->state = TASK_RUNNING;
>>   #ifdef CONFIG_SMP
> .
>

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