linux-kernel - Re: io_uring vs CPU hotplug, was Re: [PATCH 5/9] blk-mq: don't set data->ctx and data->hctx in blk_mq_alloc_request

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Message-ID: <8f893bb8-66a9-d311-ebd8-d5ccd8302a0d@kernel.dk>
Date:   Wed, 20 May 2020 08:45:23 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Christoph Hellwig <hch@....de>, Ming Lei <ming.lei@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        linux-block@...r.kernel.org, John Garry <john.garry@...wei.com>,
        Bart Van Assche <bvanassche@....org>,
        Hannes Reinecke <hare@...e.com>, io-uring@...r.kernel.org
Subject: Re: io_uring vs CPU hotplug, was Re: [PATCH 5/9] blk-mq: don't set
 data->ctx and data->hctx in blk_mq_alloc_request_hctx

On 5/20/20 2:03 AM, Christoph Hellwig wrote:
> On Wed, May 20, 2020 at 11:04:24AM +0800, Ming Lei wrote:
>> On Wed, May 20, 2020 at 09:18:23AM +0800, Ming Lei wrote:
>>> On Tue, May 19, 2020 at 05:30:00PM +0200, Christoph Hellwig wrote:
>>>> On Tue, May 19, 2020 at 09:54:20AM +0800, Ming Lei wrote:
>>>>> As Thomas clarified, workqueue hasn't such issue any more, and only other
>>>>> per CPU kthreads can run until the CPU clears the online bit.
>>>>>
>>>>> So the question is if IO can be submitted from such kernel context?
>>>>
>>>> What other per-CPU kthreads even exist?
>>>
>>> I don't know, so expose to wider audiences.
>>
>> One user is io uring with IORING_SETUP_SQPOLL & IORING_SETUP_SQ_AFF, see
>> io_sq_offload_start(), and it is a IO submission kthread.
> 
> As far as I can tell that code is buggy, as it still needs to migrate
> the thread away when the cpu is offlined.  This isn't a per-cpu kthread
> in the sene of having one for each CPU.
> 
> Jens?

It just uses kthread_create_on_cpu(), nothing home grown. Pretty sure
they just break affinity if that CPU goes offline.

-- 
Jens Axboe