linux-kernel - Re: [PATCH RFC] cpuset: Make cpusets get restored on hotplug

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Date:   Thu, 26 Mar 2020 21:26:51 -0400
From:   Waiman Long <longman@...hat.com>
To:     Joel Fernandes <joel@...lfernandes.org>, Tejun Heo <tj@...nel.org>
Cc:     Sonny Rao <sonnyrao@...gle.com>, linux-kernel@...r.kernel.org,
        Dmitry Shmidt <dimitrysh@...gle.com>,
        Amit Pundir <amit.pundir@...aro.org>, kernel-team@...roid.com,
        Jesse Barnes <jsbarnes@...gle.com>, vpillai@...italocean.com,
        Peter Zijlstra <peterz@...radead.org>,
        Guenter Roeck <groeck@...omium.org>,
        Greg Kerr <kerrnel@...gle.com>, cgroups@...r.kernel.org,
        Johannes Weiner <hannes@...xchg.org>,
        Li Zefan <lizefan@...wei.com>
Subject: Re: [PATCH RFC] cpuset: Make cpusets get restored on hotplug

On 3/26/20 4:23 PM, Joel Fernandes wrote:
> On Thu, Mar 26, 2020 at 04:18:59PM -0400, Tejun Heo wrote: >> On Thu, Mar 26, 2020 at 01:05:04PM -0700, Sonny Rao wrote: >>> I am
surprised if anyone actually wants this behavior, we (Chrome >>> OS) >>
>> The behavior is silly but consistent in that it doesn't allow empty
>> active cpusets and it has been like that for many many years now. >>
>>> found out about it accidentally, and then found that Android had >>>
been carrying a patch to fix it. And if it were a desirable >>> behavior
then why isn't it an option in v2? >> >> Nobody is saying it's a good
behavior (hence the change in cgroup2) >> and there are situations where
changing things like this is >> justifiable, but, here: >> >> * The
proposed change makes the interface inconsistent and does so >>
unconditionally on what is now a mostly legacy interface. >> >> * There
already is a newer version of the interface which includes >> the
desired behavior. >> >> * I forgot but as Waiman pointed out, you can
even opt-in to the >> new behavior, which is more comprehensive without
the >> inconsitencies, while staying on cgroup1. > > Thank you Tejun,
Waiman and Sonny. I confirmed the cgroup_v2_mode > option fixes cgroup
v1's broken behavior. > > We will use this mount option on our systems
to fix it.
I am glad that it works for you.

I think the problem is that the v2_mode mount option is not that well
documented. Maybe I should send a patch to add some some description
about it in cgroup-v2.rst or cgroup-v1/cpusets.rst.

Cheers,
Longman