linux-kernel - Re: [PATCH 1/2] sched: normalize sleeper's vruntime during group change

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Date:	Mon, 4 Oct 2010 12:18:20 -0700
From:	Dima Zavin <dima@...roid.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Mike Galbraith <efault@....de>,
	Arve Hjønnevåg <arve@...roid.com>
Subject: Re: [PATCH 1/2] sched: normalize sleeper's vruntime during group change

>> > Please explain this stuff..
>>
>> The situation today is quite bad for sleeping tasks. Currently, when
>> you move a sleeping thread between cgroups, the thread can retain its
>> old vruntime value if the old group was far ahead of the new group
>> since it essentially does a max(se->vruntime, new_vruntime) in
>> place_entity. This can prevent the task from running for a very long
>> time. That is what this patch was trying to address. It normalizes the
>> sleeper thread's vruntime before moving it to the new group.
>>
>>
>
> Hrm,.. ok, I tend to not use this cgroup gunk more that I absolutely
> have to, so I'll take your word for it.
>
> But doesn't normal cross-cpu task migration already solve this problem?
> Therefore wouldn't it be possible to adapt/extend that code to also deal
> with this particular issue?

It does, but from what I can tell it does so lazily for sleeping
tasks, i.e. the logic is in try_to_wake_up(). The cgroup attach moves
the task immediately, so when we attempt to wake it up it will already
be too late for the wake_up code to do the right thing since the task
has the new cpu_rq assigned from sched_move_task(). The wakeup logic
will not have the old group info.

--Dima
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