linux-kernel - Re: [PATCH 1/2] cgroup/cpuset: Make cpuset hotplug processing synchronous

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Date: Wed, 03 Apr 2024 18:17:25 +0200
From: Valentin Schneider <vschneid@...hat.com>
To: Waiman Long <longman@...hat.com>, Michal Koutný
 <mkoutny@...e.com>
Cc: Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>, Johannes
 Weiner <hannes@...xchg.org>, Thomas Gleixner <tglx@...utronix.de>, Peter
 Zijlstra <peterz@...radead.org>, "Rafael J. Wysocki" <rafael@...nel.org>,
 Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>, Shuah Khan
 <shuah@...nel.org>, linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
 linux-pm@...r.kernel.org, linux-kselftest@...r.kernel.org, Frederic
 Weisbecker <frederic@...nel.org>, "Paul E. McKenney" <paulmck@...nel.org>,
 Ingo Molnar <mingo@...nel.org>, Anna-Maria Behnsen
 <anna-maria@...utronix.de>, Alex Shi <alexs@...nel.org>, Vincent Guittot
 <vincent.guittot@...aro.org>, Barry Song <song.bao.hua@...ilicon.com>
Subject: Re: [PATCH 1/2] cgroup/cpuset: Make cpuset hotplug processing
 synchronous

On 03/04/24 10:47, Waiman Long wrote:
> On 4/3/24 10:26, Valentin Schneider wrote:
>> IIUC that was Thomas' suggestion [1], but I can't tell yet how bad it would
>> be to change cgroup_lock() to also do a cpus_read_lock().
>
> Changing the locking order is certainly doable. I have taken a cursory
> look at it and at least the following files need to be changed:
>
>   kernel/bpf/cgroup.c
>   kernel/cgroup/cgroup.c
>   kernel/cgroup/legacy_freezer.c
>   mm/memcontrol.c
>
> That requires a lot more testing to make sure that there won't be a
> regression. Given that the call to cgroup_transfer_tasks() should be
> rare these days as it will only apply in the case of cgroup v1 under
> certain condtion, I am not sure this requirement justifies making such
> extensive changes. So I kind of defer it until we reach a consensus that
> it is the right thing to do.
>

Yeah if we can avoid it initially I'd be up for it.

Just one thing that came to mind - there's no flushing of the
cpuset_migrate_tasks_workfn() work, so the scheduler might move tasks
itself before the cpuset does via:

  balance_push() ->__balance_push_cpu_stop() -> select_fallback_rq()

But, given the current placement of cpuset_wait_for_hotplug(), I believe
that's something we can already have, so we should be good.

> Cheers,
> Longman