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

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Message-ID: <20151213191319.GA28098@intel.com>
Date:	Mon, 14 Dec 2015 03:13:20 +0800
From:	Yuyang Du <yuyang.du@...el.com>
To:	Steve Muckle <steve.muckle@...aro.org>
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 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.
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