linux-kernel - Re: [PATCH v2] sched/fair: Do not set skip buddy up the sched hierarchy

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Message-ID: <edc9f4aa-a6ab-3bab-0a9e-73a155b8a48a@arm.com>
Date:   Mon, 9 Dec 2019 10:18:59 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Josh Don <joshdon@...gle.com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Paul Turner <pjt@...gle.com>
Subject: Re: [PATCH v2] sched/fair: Do not set skip buddy up the sched
 hierarchy

On 06.12.19 23:13, Josh Don wrote:

[...]

> On Thu, Dec 5, 2019 at 11:57 PM Vincent Guittot
> <vincent.guittot@...aro.org> wrote:
>>
>> Hi Josh,
>>
>> On Wed, 4 Dec 2019 at 21:06, Josh Don <joshdon@...gle.com> wrote:
>>>
>>> From: Venkatesh Pallipadi <venki@...gle.com>
>>>
>>> Setting skip buddy all the way up the hierarchy does not play well
>>> with intra-cgroup yield. One typical usecase of yield is when a
>>> thread in a cgroup wants to yield CPU to another thread within the
>>> same cgroup. For such a case, setting the skip buddy all the way up

But with yield_task{_fair}() you have no way to control which other task
gets accelerated. The other task in the taskgroup (cgroup) could be even
on another CPU.

It's not like yield_to_task_fair() which uses next buddy to accelerate
another task p.

What's this typical usecase?

>>> the hierarchy is counter-productive, as that results in CPU being
>>> yielded to a task in some other cgroup.
>>>
>>> So, limit the skip effect only to the task requesting it.

[...]