linux-kernel - Re: [PATCH v9 3/7] cpuset: Add cpuset.sched.load

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Date:   Thu, 31 May 2018 17:20:50 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Ingo Molnar <mingo@...hat.com>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        kernel-team@...com, pjt@...gle.com, luto@...capital.net,
        Mike Galbraith <efault@....de>, torvalds@...ux-foundation.org,
        Roman Gushchin <guro@...com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v9 3/7] cpuset: Add cpuset.sched.load_balance flag to v2

On Thu, May 31, 2018 at 09:54:27AM -0400, Waiman Long wrote:
> On 05/31/2018 08:26 AM, Peter Zijlstra wrote:

> > I still find all that a bit weird.
> >
> > So load_balance=0 basically changes a partition into a
> > 'fully-partitioned partition' with the seemingly random side-effect that
> > now sub-partitions are allowed to consume all CPUs.
> 
> Are you suggesting that we should allow sub-partition to consume all the
> CPUs no matter the load balance state? I can live with that if you think
> it is more logical.

I'm on the fence myself; the only thing I'm fairly sure of is that tying
this particular behaviour to the load-balance knob seems off.

> > The rationale, only given in the Changelog above, seems to be to allow
> > 'easy' emulation of isolcpus.
> >
> > I'm still not convinced this is a useful knob to have. You can do
> > fully-partitioned by simply creating a lot of 1 cpu parititions.
> 
> That is certainly true. However, I think there are some additional
> overhead in the scheduler side in maintaining those 1-cpu partitions. Right?

cpuset-controller as such doesn't have much overhead scheduler wise,
cpu-controller OTOH does, and there depth is the predominant factor, so
many sibling groups should not matter there either.

> > So this one knob does two separate things, both of which seem, to me,
> > redundant.
> >
> > Can we please get better rationale for this?
> 
> I am fine getting rid of the load_balance flag if this is the consensus.
> However, we do need to come up with a good migration story for those
> users that need the isolcpus capability. I think Mike was the one asking
> for supporting isolcpus. So Mike, what is your take on that.

So I don't strictly mind having a knob that does the 'fully-partitioned
partition' thing -- however odd that sounds -- but I feel we should have
a solid use-case for it.

I also think we should not mix the 'consume all' thing with the
'fully-partitioned' thing, as they are otherwise unrelated.