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

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Date:	Mon, 14 Dec 2015 16:41:14 -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()

Hi Yuyang,

On 12/13/2015 11:13 AM, Yuyang Du wrote:
> Hi Steve,
> 
> 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.

thanks
Steve
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