linux-kernel - Re: [PATCH 3/3] sched: Implement interface for cgroup unified hierarchy

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Date:	Mon, 24 Aug 2015 14:15:37 -0700
From:	Paul Turner <pjt@...gle.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Austin S Hemmelgarn <ahferroin7@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>, lizefan@...wei.com,
	cgroups <cgroups@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	kernel-team <kernel-team@...com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 3/3] sched: Implement interface for cgroup unified hierarchy

On Mon, Aug 24, 2015 at 2:12 PM, Tejun Heo <tj@...nel.org> wrote:
> Hello, Paul.
>
> On Mon, Aug 24, 2015 at 02:00:54PM -0700, Paul Turner wrote:
>> > Hmmm... I'm trying to understand the usecases where having hierarchy
>> > inside a process are actually required so that we don't end up doing
>> > something complex unnecessarily.  So far, it looks like an easy
>> > alternative for qemu would be teaching it to manage priorities of its
>> > threads given that the threads are mostly static - vcpus going up and
>> > down are explicit operations which can trigger priority adjustments if
>> > necessary, which is unlikely to begin with.
>>
>> What you're proposing is both unnecessarily complex and imprecise.
>> Arbitrating competition between groups of threads is exactly why we
>> support sub-hierarchies within cpu.
>
> Sure, and to make that behave half-way acceptable, we'll have to take
> on significant amount of effort and likely complexity and I'm trying
> to see whether the usecases are actually justifiable.  I get that
> priority based solution will be less precise and more complex on the
> application side but by how much and does the added precision enough
> to justify the extra facilities to support that?  If it is, sure,
> let's get to it but it'd be great if the concrete prolem cases are
> properly identified and understood.  I'll continue on the other reply.
>

No problem, I think the conversation is absolutely
constructive/important to have and am happy to help drill down.

> Thanks.
>
> --
> tejun
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