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References: <20201023101158.088940906@infradead.org> <20201023102347.067278757@infradead.org> <ff62e3ee994efb3620177bf7b19fab16f4866845.camel@redhat.com> <jhjpn4bwznx.mognet@arm.com> <c2c013282faf278ee6e0fc66deefbab165ff4e88.camel@redhat.com>
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From:   Valentin Schneider <valentin.schneider@arm.com>
To:     Qian Cai <qcai@redhat.com>
Cc:     Peter Zijlstra <peterz@infradead.org>, tglx@linutronix.de,
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        mgorman@suse.de, bristot@redhat.com, vincent.donnefort@arm.com,
        tj@kernel.org, ouwen210@hotmail.com
Subject: Re: [PATCH v4 11/19] sched/core: Make migrate disable and CPU hotplug cooperative
In-reply-to: <c2c013282faf278ee6e0fc66deefbab165ff4e88.camel@redhat.com>
Date:   Mon, 07 Dec 2020 19:27:56 +0000
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On 04/12/20 21:19, Qian Cai wrote:
> On Tue, 2020-11-17 at 19:28 +0000, Valentin Schneider wrote:
>> We did have some breakage in that area, but all the holes I was aware of
>> have been plugged. What would help here is to see which tasks are still
>> queued on that outgoing CPU, and their recent activity.
>>
>> Something like
>> - ftrace_dump_on_oops on your kernel cmdline
>> - trace-cmd start -e 'sched:*'
>>  <start the test here>
>>
>> ought to do it. Then you can paste the (tail of the) ftrace dump.
>>
>> I also had this laying around, which may or may not be of some help:
>
> Okay, your patch did not help, since it can still be reproduced using this,
>
> https://github.com/linux-test-project/ltp/blob/master/testcases/kernel/hotplug/cpu_hotplug/functional/cpuhotplug04.sh
>
> # while :; do cpuhotplug04.sh -l 1; done
>

Ok, can reproduce this on a TX2 on next-20201207. I didn't use your config,
I oldconfig'd my distro config and only modified it to CONFIG_PREEMPT_NONE.
Interestingly the BUG happens on CPU127 here too...

I'll start digging.