linux-kernel - Re: [PATCH] sched: fix lose fair sleeper bonus in switch_to

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Date:	Tue, 8 Sep 2015 14:55:20 +0800
From:	Wanpeng Li <wanpeng.li@...mail.com>
To:	Byungchul Park <byungchul.park@....com>
CC:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched: fix lose fair sleeper bonus in switch_to_fair()

On 9/8/15 10:10 AM, Byungchul Park wrote:
> hello wanpeng,
>
> On Mon, Sep 07, 2015 at 05:45:20PM +0800, Wanpeng Li wrote:
>> The sleeper task will be normalized when moved from fair_sched_class, in
>> order that vruntime will be adjusted either the task is running or sleeping
>> when moved back. The nomalization in switch_to_fair for sleep task will
>> result in lose fair sleeper bonus in place_entity() once the vruntime -
>> cfs_rq->min_vruntime is big when moved from fair_sched_class.
> it is nothing to do with normalization.
>
> if vruntime - cfs_rq->min_vruntime is big even though place_entity() was
> called when moved from fair class, then we actually expect that it still has
> a big vruntime when moved back to fair class.
>
> if we don't expect that it still has a big vruntime when moved back to fair
> class, we need to consider other approaches e.g. to move a position calling
> place_entity() or to add place_entity() call properly ..
>
> however we should not touch normalization logic. in other words, if we
> normalized the vruntime when leaved, then we should necessarily restore the
> vruntime to a non-normalized value when moved back.

Not about vruntime - cfs_rq->min_vruntime is big, I think my patch 
description above is confusing, and what's wrong I found is explained in 
the mail which reply to Peterz.

>
>> This patch fix it by adjusting vruntime just during migrating as original
>> codes since the vruntime of the task has usually NOT been normalized in
>> this case.
> could you explain this in detail? when is a vruntime not normalized?
>

The comments in task_move_group_fair() which you removed in your commit:

|  * When !queued, vruntime of the task has usually NOT been normalized

Regards,
Wanpeng Li
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