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

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Date:   Thu, 31 May 2018 18:08:57 +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 11:36:39AM -0400, Waiman Long wrote:
> > 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 main reason for doing it this way is that I don't want to have
> load-balanced partition with no cpu in it. How about we just don't allow
> consume-all at all. Each partition must have at least 1 cpu.

I suspect that might be sufficient. It certainly is for the use-cases
I'm aware of. You always want a system/control set which runs the
regular busy work of running a system.

Then you have one (or more) partitions to run your 'important' work.

> > I also think we should not mix the 'consume all' thing with the
> > 'fully-partitioned' thing, as they are otherwise unrelated.
>
> The "consume all" and "fully-partitioned" look the same to me. Are you
> talking about allocating all the CPUs in a partition to sub-partitions
> so that there is no CPU left in the parent partition?

Not sure what you're asking. "consume all" is allowing sub-partitions to
allocate all CPUs of the parent, such that there are none left.

"fully-partitioned" is N cpus but no load-balancing, also equivalent to
N 1 CPU parititions.

They are distinct things. Disabling load-balancing should not affect how
many CPUs can be allocated to sub-partitions, the moment you hit 1 CPU
the load balancing is effectively off already. Going down to 0 CPUs
isn't a problem for the load-balancer, it wasn't doing anything anyway.

So the question is if someone really needs the one partition without
balancing over N separate paritions.