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 09:09:43 +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 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
[...]

Regards,
Wanpeng Li