linux-kernel - Re: [PATCH v4 8/9] cgroup/cpuset: Documentation update for partition

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Message-ID: <82aeb5aa-7d69-78dd-bb26-60a51dc8a839@redhat.com>
Date:   Mon, 10 Jul 2023 23:24:37 -0400
From:   Waiman Long <longman@...hat.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Shuah Khan <shuah@...nel.org>, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kselftest@...r.kernel.org,
        Juri Lelli <juri.lelli@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Mrunal Patel <mpatel@...hat.com>,
        Ryan Phillips <rphillips@...hat.com>,
        Brent Rowsell <browsell@...hat.com>,
        Peter Hunt <pehunt@...hat.com>, Phil Auld <pauld@...hat.com>
Subject: Re: [PATCH v4 8/9] cgroup/cpuset: Documentation update for partition

On 7/10/23 21:07, Tejun Heo wrote:
> Hello,
>
> On Mon, Jul 10, 2023 at 08:53:18PM -0400, Waiman Long wrote:
>> For local partition, it doesn't make sense to have a cpust.cpus.exclusive
>> that is not the same as cpuset.cpus as it artificially reduce the set of
>> CPUs that can be used in a partition. In the case of a remote partition, the
> Yeah, I was wondering about local partitions. "Automatic but can be
> overridden" behavior becomes confusing if it's difficult for the user to
> easily tell which part is automatic when. I wonder whether it'd be better to
> make the condition static - e.g. for a partition cgroup, cpus.exclusive
> always contains all bits in cpus no matter what value is written to it. Or,
> if we separate out cpus.exclusive and cpus.exclusive.effective, no matter
> what cpus.exclusive is set, a partition root's cpus.exclusive.effective
> always includes all bits in cpus.effective.

With no offline CPUs, cpus.effective should be the same as 
cpus.exclusive.effective for a valid partition root. Here 
cpus.exclusive.effective is a bit different from cpus.effective as it 
can contain offline cpus. It also mean that adding 
cpus.exclusive.effective can be redundant.

As said before, I try to avoid adding new cpuset control file unless 
absolutely necessary. I now have a slight different proposal. Once 
manually set, I can keep cpuset.cpus.exclusive invariant. I do need to 
do a bit more work when enabling a partition root to find out the 
effective set of exclusive CPUs to be used or make the partition invalid 
if no exclusive CPU is available. I still want to do a initial check 
when setting cpuset.cpus.exclusive to make sure that the value is at 
least valid at the beginning.

Do you think this is an acceptable compromise?

Thanks,
Longman