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Message-ID: <02b8e7b3-941d-8bb9-cd0e-992738893ba3@redhat.com>
Date:   Tue, 6 Sep 2022 16:01:06 -0400
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Subject: Re: BUG: HANG_DETECT waiting for migration_cpu_stop() complete
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To:     Tejun Heo <tj@kernel.org>, Jing-Ting Wu <jing-ting.wu@mediatek.com>
Cc:     Mukesh Ojha <quic_mojha@quicinc.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Valentin Schneider <vschneid@redhat.com>,
        wsd_upstream@mediatek.com, linux-kernel@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org,
        linux-mediatek@lists.infradead.org, Jonathan.JMChen@mediatek.com,
        "chris.redpath@arm.com" <chris.redpath@arm.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Vincent Donnefort <vdonnefort@gmail.com>,
        Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Christian Brauner <brauner@kernel.org>,
        cgroups@vger.kernel.org, lixiong.liu@mediatek.com,
        wenju.xu@mediatek.com
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 <b605c3ec-94ab-a55f-5825-9b370d77ecf3@quicinc.com>
 <203d4614c1b2a498a240ace287156e9f401d5395.camel@mediatek.com>
 <YxeR0RoiTdm8sWCJ@slm.duckdns.org>
From:   Waiman Long <longman@redhat.com>
In-Reply-To: <YxeR0RoiTdm8sWCJ@slm.duckdns.org>
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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