linux-kernel - Re: [PATCH v12 9/9] cpuset: Support forced turning off of partition flag

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Message-ID: <20181002200627.GI270328@devbig004.ftw2.facebook.com>
Date:   Tue, 2 Oct 2018 13:06:27 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Peter Zijlstra <peterz@...radead.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>
Subject: Re: [PATCH v12 9/9] cpuset: Support forced turning off of partition
 flag

Hello, Waiman.

My apologies for the delay.

On Mon, Aug 27, 2018 at 01:50:18PM -0400, Waiman Long wrote:
> My current code has explicitly assumed the following relationship for
> partition root.
> 
>     cpus_allowed = effective_cpus + reserved_cpus
> 
> Also effective_cpus cannot be empty. Specifically, cpus_allowed has to
> be equal to effective_cpus before a cpuset can be made a partition root.
> 
> Any changes that break the above conditions will turn off the partition
> flag forcefully. The only exception is cpu offlining where cpus_allowed
> > effective_cpus + reserved_cpus can happen.
> 
> One reason for doing so is because reserved_cpus is hidden. So the main
> way to infer that is to do cpus_allowed - effective_cpus.
> 
> It is probably doable to make cpus_allowed >= effective_cpus +
> reserved_cpus in general, but we may need to expose reserved_cpus as a
> read-only file, for instance. There may also be other complications that
> we will need to take care of if this is supported. My current preference
> is to not doing that unless there is compelling reason to do so.

So, if we're gonna make this hierarchical, I think it probably would
be better to go in all the way.  It's kinda weird to mix the two
approaches - the normal cpuset operation following the usual
convention (it'd be really great to fix the removal part too) and
parition code doing something else.

I think adding another interface file should be fine here.

Thanks.

-- 
tejun