linux-kernel - Re: [PATCH v3 02/12] sched: remove a wake

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Message-ID: <53BCB1BE.7060301@redhat.com>
Date:	Tue, 08 Jul 2014 23:06:38 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Vincent Guittot <vincent.guittot@...aro.org>, peterz@...radead.org,
	mingo@...nel.org, linux-kernel@...r.kernel.org,
	linux@....linux.org.uk, linux-arm-kernel@...ts.infradead.org
CC:	preeti@...ux.vnet.ibm.com, Morten.Rasmussen@....com, efault@....de,
	nicolas.pitre@...aro.org, linaro-kernel@...ts.linaro.org,
	daniel.lezcano@...aro.org, dietmar.eggemann@....com
Subject: Re: [PATCH v3 02/12] sched: remove a wake_affine condition

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On 06/30/2014 12:05 PM, Vincent Guittot wrote:
> I have tried to understand the meaning of the condition : 
> (this_load <= load && this_load + target_load(prev_cpu, idx) <=
> tl_per_task) but i failed to find a use case that can take
> advantage of it and i haven't found clear description in the
> previous commits' log. Futhermore, the comment of the condition
> refers to the task_hot function that was used before being replaced
> by the current condition: /* * This domain has SD_WAKE_AFFINE and *
> p is cache cold in this domain, and * there is no bad imbalance. 
> */
> 
> If we look more deeply the below condition this_load +
> target_load(prev_cpu, idx) <= tl_per_task
> 
> When sync is clear, we have : tl_per_task = runnable_load_avg /
> nr_running this_load = max(runnable_load_avg, cpuload[idx]) 
> target_load =  max(runnable_load_avg', cpuload'[idx])
> 
> It implies that runnable_load_avg' == 0 and nr_running <= 1 in
> order to match the condition. This implies that runnable_load_avg
> == 0 too because of the condition: this_load <= load but if this
> _load is null, balanced is already set and the test is redundant.
> 
> If sync is set, it's not as straight forward as above (especially
> if cgroup are involved) but the policy should be similar as we have
> removed a task that's going to sleep in order to get a more
> accurate load and this_load values.
> 
> The current conclusion is that these additional condition don't
> give any benefit so we can remove them.
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>

Acked-by: Rik van Riel <riel@...hat.com>

- -- 
All rights reversed
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