linux-kernel - Re: Task group cleanups and optimizations (was: Re: [RFC 00/60] Coscheduling for Linux)

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Date:   Tue, 4 Dec 2018 14:23:37 +0100
From:   Jan H. Schönherr <jschoenh@...zon.de>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        Paul Turner <pjt@...gle.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Rik van Riel <riel@...riel.com>
Subject: Re: Task group cleanups and optimizations (was: Re: [RFC 00/60]
 Coscheduling for Linux)

On 23/11/2018 17.51, Frederic Weisbecker wrote:
> On Tue, Sep 18, 2018 at 03:22:13PM +0200, Jan H. Schönherr wrote:
>> On 09/17/2018 11:48 AM, Peter Zijlstra wrote:
>>> Right, so the whole bandwidth thing becomes a pain; the simplest
>>> solution is to detect the throttle at task-pick time, dequeue and try
>>> again. But that is indeed quite horrible.
>>>
>>> I'm not quite sure how this will play out.
>>>
>>> Anyway, if we pull off this flattening feat, then you can no longer use
>>> the hierarchy for this co-scheduling stuff.
>>
>> Yeah. I might be a bit biased towards keeping or at least not fully throwing away
>> the nesting of CFS runqueues. ;)
> 
> One detail here, is that hierarchical task group a strong requirement for cosched
> or could you live with it flattened in the end?

Currently, it is a strong requirement.

As mentioned at the bottom of https://lkml.org/lkml/2018/10/19/859 it should be
possible to pull the hierarchical aspect out of CFS and implement it one level
higher. But that would be a major re-design of everything.

I use the hierarchical aspect to a) keep coscheduled groups in separate sets of runqeues,
so that it is easy to select/balance tasks within a particular group; and b) to implement
per-core, per-node, per-system runqueues that represent larger fractions of the system,
which then fan out into per-CPU runqueues (eventually).

Regards
Jan