linux-kernel - Re: [PATCH 01/10] cgroup/cpuset: Fix race between newly created partition and dying one

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Message-ID: <Z-xFqkBsh640l5j0@mtj.duckdns.org>
Date: Tue, 1 Apr 2025 09:59:38 -1000
From: Tejun Heo <tj@...nel.org>
To: Waiman Long <llong@...hat.com>
Cc: Johannes Weiner <hannes@...xchg.org>,
	Michal Koutný <mkoutny@...e.com>,
	Shuah Khan <shuah@...nel.org>, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 01/10] cgroup/cpuset: Fix race between newly created
 partition and dying one

Hello, Waiman.

On Mon, Mar 31, 2025 at 11:12:06PM -0400, Waiman Long wrote:
> The problem is the RCU delay between the time a cgroup is killed and is in a
> dying state and when the partition is deactivated when cpuset_css_offline()
> is called. That delay can be rather lengthy depending on the current
> workload.

If we don't have to do it too often, synchronize_rcu_expedited() may be
workable too. What do you think?

> Another alternative that I can think of is to scan the remote partition list
> for remote partition and sibling cpusets for local partition whenever some
> kind of conflicts are detected when enabling a partition. When a dying
> cpuset partition is detected, deactivate it immediately to resolve the
> conflict. Otherwise, the dying partition will still be deactivated at
> cpuset_css_offline() time.
> 
> That will be a bit more complex and I think can still get the problem solved
> without adding a new method. What do you think? If you are OK with that, I
> will send out a new patch later this week.

If synchronize_rcu_expedited() won't do, let's go with the original patch.
The operation does make general sense in that it's for a distinctive step in
the destruction process although I'm a bit curious why it's called before
DYING is set.

Thanks.

-- 
tejun