linux-kernel - Re: [PATCH v8 3/6] cpuset: Add cpuset.sched.load

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Message-ID: <20180528124508.GE3452@worktop.programming.kicks-ass.net>
Date:   Mon, 28 May 2018 14:45:08 +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>
Subject: Re: [PATCH v8 3/6] cpuset: Add cpuset.sched.load_balance flag to v2

On Thu, May 24, 2018 at 02:55:25PM -0400, Waiman Long wrote:
> On 05/24/2018 11:43 AM, Peter Zijlstra wrote:

> > I'm confused... why exactly do we have both domain and load_balance ?
> 
> The domain is for partitioning the CPUs only. It doesn't change the load
> balancing state. So the load_balance flag is still need to turn on and
> off load balancing.

OK, so we have to two boolean flags, giving 4 possible states. Lets just
go through them one by on:

A) domain:0 load_balance:0 -- we have no exclusive domain, but have
   load-balancing disabled across them. AFAICT this should be an invalid
   state.

B) domain:0 load_balance:1 -- we have no exclusive domain, but have
   load-balancing enabled. AFAICT this is the default state and is a
   no-op.

C) domain:1 load_balance:0 -- we have an exclusive domain, and have
   load-balancing disabled across it. This is, AFAICT, identical to
   having a bunch of sub/sibling groups each with a single CPU domain.

D) domain:1 load_balance:1 -- we have an exclusive domain, and have
   load-balancing enabled. This is a partition.

Now, I think I've overlooked the fact that load_balance==1 only really
means something when the parent's load_balance==0, but I'm not sure that
really changes anything.

So, afaict, the above only have two useful states: B and D. Which again
raises the question, why two knobs? What useful configurations does it
allow?