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From:   Sonny Rao <sonnyrao@google.com>
Date:   Thu, 26 Mar 2020 15:03:42 -0700
Message-ID: <CAPz6YkXVYJBow-6G023eaDEKFORPh3AKQb6gfrM8XX8=bdXL2A@mail.gmail.com>
Subject: Re: [PATCH RFC] cpuset: Make cpusets get restored on hotplug
To:     Waiman Long <longman@redhat.com>
Cc:     Joel Fernandes <joel@joelfernandes.org>, Tejun Heo <tj@kernel.org>,
        linux-kernel@vger.kernel.org, Dmitry Shmidt <dimitrysh@google.com>,
        Amit Pundir <amit.pundir@linaro.org>, kernel-team@android.com,
        Jesse Barnes <jsbarnes@google.com>, vpillai@digitalocean.com,
        Peter Zijlstra <peterz@infradead.org>,
        Guenter Roeck <groeck@chromium.org>,
        Greg Kerr <kerrnel@google.com>, cgroups@vger.kernel.org,
        Johannes Weiner <hannes@cmpxchg.org>,
        Li Zefan <lizefan@huawei.com>
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On Thu, Mar 26, 2020 at 2:47 PM Waiman Long <longman@redhat.com> wrote:
>
> On 3/26/20 4:05 PM, Sonny Rao wrote:
> >> I think Tejun is concerned about a change in the default behavior of
> >> cpuset v1.
> >>
> >> There is a special v2 mode for cpuset that is enabled by the mount
> >> option "cpuset_v2_mode". This causes the cpuset v1 to adopt some of the
> >> v2 behavior. I introduced this v2 mode a while back to address, I think,
> >> a similar concern. Could you try that to see if it is able to address
> >> your problem? If not, you can make some code adjustment within the
> >> framework of the v2 mode. As long as it is an opt-in, I think we are
> >> open to further change.
> > I am surprised if anyone actually wants this behavior, we (Chrome OS)
> > 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?
> >
> I am a bit confused. The v2 mode make cpuset v1 behaves more like cpuset
> v2. The original v1 behavior has some obvious issue that was fixed in
> v2. So what v2 option are you talking about?

I was merely pointing out the behavior of the v1 implementation is so
undesirable that it wasn't kept at all in v2.  IMHO, it's a bug that
should be fixed, and I think it's possible to keep the old behavior if
all cpus are offlined, but since you've added this option we can use
it instead.

>
> Regards,
> Longman
>