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Subject: Re: io_uring vs CPU hotplug, was Re: [PATCH 5/9] blk-mq: don't set
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To:     Thomas Gleixner <tglx@linutronix.de>,
        Christoph Hellwig <hch@lst.de>, Ming Lei <ming.lei@redhat.com>
Cc:     linux-kernel@vger.kernel.org, linux-block@vger.kernel.org,
        John Garry <john.garry@huawei.com>,
        Bart Van Assche <bvanassche@acm.org>,
        Hannes Reinecke <hare@suse.com>, io-uring@vger.kernel.org
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From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <2a12a7aa-c339-1e51-de0d-9bc6ced14c64@kernel.dk>
Date:   Wed, 20 May 2020 14:18:36 -0600
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On 5/20/20 1:41 PM, Thomas Gleixner wrote:
> Jens Axboe <axboe@kernel.dk> writes:
>> On 5/20/20 8:45 AM, Jens Axboe wrote:
>>> It just uses kthread_create_on_cpu(), nothing home grown. Pretty sure
>>> they just break affinity if that CPU goes offline.
>>
>> Just checked, and it works fine for me. If I create an SQPOLL ring with
>> SQ_AFF set and bound to CPU 3, if CPU 3 goes offline, then the kthread
>> just appears unbound but runs just fine. When CPU 3 comes online again,
>> the mask appears correct.
> 
> When exactly during the unplug operation is it unbound?

When the CPU has been fully offlined. I check the affinity mask, it
reports 0. But it's still being scheduled, and it's processing work.
Here's an example, PID 420 is the thread in question:

[root@archlinux cpu3]# taskset -p 420
pid 420's current affinity mask: 8
[root@archlinux cpu3]# echo 0 > online 
[root@archlinux cpu3]# taskset -p 420
pid 420's current affinity mask: 0
[root@archlinux cpu3]# echo 1 > online 
[root@archlinux cpu3]# taskset -p 420
pid 420's current affinity mask: 8

So as far as I can tell, it's working fine for me with the goals
I have for that kthread.

-- 
Jens Axboe