linux-kernel - Re: PELT initial task load and wake_up_new

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Message-ID: <56705FE1.5000600@linaro.org>
Date:	Tue, 15 Dec 2015 10:45:53 -0800
From:	Steve Muckle <steve.muckle@...aro.org>
To:	Yuyang Du <yuyang.du@...el.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Patrick Bellasi <patrick.bellasi@....com>,
	Juri Lelli <Juri.Lelli@....com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: PELT initial task load and wake_up_new_task()

On 12/14/2015 06:24 PM, Yuyang Du wrote:
>>> On Fri, Dec 11, 2015 at 06:01:45PM -0800, Steve Muckle wrote:
>>>> In init_entity_runnable_average() the last_update_time is initialized to
>>>> zero. The task is given max load and utilization as a pessimistic
>>>> initial estimate.
>>>>
>>>> But if in wake_up_new_task() the task is placed on a CPU other than
>>>> where it was created, __update_load_avg() will be called via
>>>> set_task_cpu() -> migrate_task_rq_fair() -> remove_entity_load_avg().
>>>>
>>>> Since last_update_time is zero the delta will be huge and the task's
>>>> load will be entirely decayed away before it is enqueued at the
>>>> destination CPU.
>>>  
>>> Since the new task's last_update_time is equal to 0, it will not be decayed.
>>
>> Can you point me to the code for that logic? I don't see anything that
>> prevents the decay when a newly woken task is placed on a different CPU
>> via the call chain I mentioned above. My testing also shows the load
>> being decayed to zero.
>>
> You may search the last_update_time, and see it would be treated differently
> if it is 0. Hope this may be helpful.

Are you referring to the test in enqueue_entity_load_avg()? If so that
isn't called until after remove_entity_load_avg() in this scenario,
which has no check on last_update_time.

thanks,
Steve
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