linux-kernel - Re: BUG: HANG_DETECT waiting for migration_cpu

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Date:   Tue, 6 Sep 2022 16:01:06 -0400
From:   Waiman Long <longman@...hat.com>
To:     Tejun Heo <tj@...nel.org>, Jing-Ting Wu <jing-ting.wu@...iatek.com>
Cc:     Mukesh Ojha <quic_mojha@...cinc.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Valentin Schneider <vschneid@...hat.com>,
        wsd_upstream@...iatek.com, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org, Jonathan.JMChen@...iatek.com,
        "chris.redpath@....com" <chris.redpath@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Vincent Donnefort <vdonnefort@...il.com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Christian Brauner <brauner@...nel.org>,
        cgroups@...r.kernel.org, lixiong.liu@...iatek.com,
        wenju.xu@...iatek.com
Subject: Re: BUG: HANG_DETECT waiting for migration_cpu_stop() complete

On 9/6/22 14:30, Tejun Heo wrote:
> Hello,
>
> (cc'ing Waiman in case he has a better idea)
>
> On Mon, Sep 05, 2022 at 04:22:29PM +0800, Jing-Ting Wu wrote:
>> https://lore.kernel.org/lkml/YvrWaml3F+x9Dk+T@slm.duckdns.org/ is for
>> fix cgroup_threadgroup_rwsem <-> cpus_read_lock() deadlock.
>> But this issue is cgroup_threadgroup_rwsem <-> cpuset_rwsem deadlock.
> If I'm understanding what you're writing correctly, this isn't a deadlock.
> The cpuset_hotplug_workfn simply isn't being woken up while holding
> cpuset_rwsem and others are just waiting for that lock to be released.

I believe it is probably a bug in the scheduler core code. 
__set_cpus_allowed_ptr_locked() calls affine_move_task() to move to a 
random cpu within the new set allowable CPUs. However, if migration is 
disabled, it shouldn't call affine_move_task() at all. Instead, I would 
suggest that if the current cpu is within the new allowable cpus, it 
should just skip doing affine_move_task(). Otherwise, it should fail 
__set_cpus_allowed_ptr_locked().

My 2 cents.

Cheers,
Longman