linux-kernel - Re: [PATCH v2] cgroup/cpuset: Enable memory migration for cpuset v2

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Message-ID: <YRUdYQzxj8rswNyz@cmpxchg.org>
Date:   Thu, 12 Aug 2021 09:08:49 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Jonathan Corbet <corbet@....net>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH v2] cgroup/cpuset: Enable memory migration for cpuset v2

On Wed, Aug 11, 2021 at 03:57:07PM -0400, Waiman Long wrote:
> When a user changes cpuset.cpus, each task in a v2 cpuset will be moved
> to one of the new cpus if it is not there already. For memory, however,
> they won't be migrated to the new nodes when cpuset.mems changes. This is
> an inconsistency in behavior.
> 
> In cpuset v1, there is a memory_migrate control file to enable such
> behavior by setting the CS_MEMORY_MIGRATE flag. Make it the default
> for cpuset v2 so that we have a consistent set of behavior for both
> cpus and memory.
> 
> There is certainly a cost to make memory migration the default, but it
> is a one time cost that shouldn't really matter as long as cpuset.mems
> isn't changed frequenty.  Update the cgroup-v2.rst file to document the
> new behavior and recommend against changing cpuset.mems frequently.
> 
> Since there won't be any concurrent access to the newly allocated cpuset
> structure in cpuset_css_alloc(), we can use the cheaper non-atomic
> __set_bit() instead of the more expensive atomic set_bit().
> 
> Signed-off-by: Waiman Long <longman@...hat.com>

LGTM.

Acked-by: Johannes Weiner <hannes@...xchg.org>