linux-kernel - Re: [PATCH] sched/cpufreq: don't trigger cpufreq update w/o real rt/deadline tasks running

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Date:	Thu, 21 Apr 2016 20:12:59 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Wanpeng Li <wanpeng.li@...mail.com>,
	Linux PM list <linux-pm@...r.kernel.org>,
	Steve Muckle <steve.muckle@...aro.org>
Subject: Re: [PATCH] sched/cpufreq: don't trigger cpufreq update w/o real
 rt/deadline tasks running

2016-04-21 19:11 GMT+08:00 Rafael J. Wysocki <rafael.j.wysocki@...el.com>:
> On 4/21/2016 3:09 AM, Wanpeng Li wrote:
>>
>> 2016-04-21 6:28 GMT+08:00 Rafael J. Wysocki <rafael.j.wysocki@...el.com>:
>>>
>>> On 4/21/2016 12:24 AM, Wanpeng Li wrote:
>>>>
>>>> 2016-04-20 22:01 GMT+08:00 Peter Zijlstra <peterz@...radead.org>:
>>>>>
>>>>> On Wed, Apr 20, 2016 at 02:32:35AM +0200, Rafael J. Wysocki wrote:
>>>>>>
>>>>>> On Monday, April 18, 2016 01:51:24 PM Wanpeng Li wrote:
>>>>>>>
>>>>>>> Sometimes update_curr() is called w/o tasks actually running, it is
>>>>>>> captured by:
>>>>>>>       u64 delta_exec = rq_clock_task(rq) - curr->se.exec_start;
>>>>>>> We should not trigger cpufreq update in this case for rt/deadline
>>>>>>> classes, and this patch fix it.
>>>>>>>
>>>>>>> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
>>>>>>
>>>>>> The signed-off-by tag should agree with the From: header.  One way to
>>>>>> achieve
>>>>>> that is to add an extra From: line at the start of the changelog.
>>>>>>
>>>>>> That said, this looks like a good catch that should go into 4.6 to me.
>>>>>>
>>>>>> Peter, what do you think?
>>>>>
>>>>> I'm confused by the Changelog. *what* ?
>>>>
>>>> Sometimes .update_curr hook is called w/o tasks actually running, it is
>>>> captured by:
>>>>
>>>>           u64 delta_exec = rq_clock_task(rq) - curr->se.exec_start;
>>>>
>>>> We should not trigger cpufreq update in this case for rt/deadline
>>>> classes, and this patch fix it.
>>>
>>>
>>> That's what you wrote in the changelog, no need to repeat that.
>>>
>>> I guess Peter is asking for more details, though.  I actually would like
>>> to
>>> get some more details here too.  Like an example of when the situation in
>>> question actually happens.
>>
>> I add a print to print when delta_exec is zero for rt class, something
>> like below:
>>
>>        watchdog/5-48    [005] d...   568.449095: update_curr_rt: rt
>> delta_exec is zero
>>        watchdog/5-48    [005] d...   568.449104: <stack trace>
>>   => pick_next_task_rt
>>   => __schedule
>>   => schedule
>>   => smpboot_thread_fn
>>   => kthread
>>   => ret_from_fork
>>        watchdog/5-48    [005] d...   568.449105: update_curr_rt: rt
>> delta_exec is zero
>>        watchdog/5-48    [005] d...   568.449111: <stack trace>
>>   => put_prev_task_rt
>>   => pick_next_task_idle
>>   => __schedule
>>   => schedule
>>   => smpboot_thread_fn
>>   => kthread
>>   => ret_from_fork
>>        watchdog/6-56    [006] d...   568.510094: update_curr_rt: rt
>> delta_exec is zero
>>        watchdog/6-56    [006] d...   568.510103: <stack trace>
>>   => pick_next_task_rt
>>   => __schedule
>>   => schedule
>>   => smpboot_thread_fn
>>   => kthread
>>   => ret_from_fork
>>        watchdog/6-56    [006] d...   568.510105: update_curr_rt: rt
>> delta_exec is zero
>>        watchdog/6-56    [006] d...   568.510111: <stack trace>
>>   => put_prev_task_rt
>>   => pick_next_task_idle
>>   => __schedule
>>   => schedule
>>   => smpboot_thread_fn
>>   => kthread
>>   => ret_from_fork
>> [...]
>
>
> And the statement in your changelog follows from this I suppose. How does it
> follow, exactly?

For example, rt task A will go to sleep, an rt task B is the next
candidate to run.

__schedule()
    -> deactivate_task(A, DEQUEUE_SLEEP)
        -> dequeue_task_rt()
            -> update_curr_rt()
                -> cpufreq_trigger_update()
                -> delta_exec = rq_clock_task(rq) - curr->se.exec_start;
    [...]
    -> pick_next_task_rt()
        -> update_curr_rt()          =>   rq->curr is still A currently
            -> cpufreq_trigger_update()
            -> delta_exec = rq_clock_task(rq) - curr->se.exec_start;
  => delta == 0, actually A is not running between these two updates
    if (likely(prev != next)) {
        rq->curr = B;
       [...]
     }

Regards,
Wanpeng Li