Subject: Re: PELT initial task load and wake_up_new_task()
To: Yuyang Du <yuyang.du@intel.com>
References: <566B8009.2090006@linaro.org> <20151213191319.GA28098@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Patrick Bellasi <patrick.bellasi@arm.com>,
        Juri Lelli <Juri.Lelli@arm.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
From: Steve Muckle <steve.muckle@linaro.org>
Message-ID: <566F61AA.4020904@linaro.org>
Date: Mon, 14 Dec 2015 16:41:14 -0800
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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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