Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
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
To:     Christoph Hellwig <hch@lst.de>, Ming Lei <ming.lei@redhat.com>
Cc:     linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        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
References: <20200518093155.GB35380@T590>
 <87imgty15d.fsf@nanos.tec.linutronix.de> <20200518115454.GA46364@T590>
 <20200518131634.GA645@lst.de> <20200518141107.GA50374@T590>
 <20200518165619.GA17465@lst.de> <20200519015420.GA70957@T590>
 <20200519153000.GB22286@lst.de> <20200520011823.GA415158@T590>
 <20200520030424.GI416136@T590> <20200520080357.GA4197@lst.de>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <8f893bb8-66a9-d311-ebd8-d5ccd8302a0d@kernel.dk>
Date:   Wed, 20 May 2020 08:45:23 -0600
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101
 Thunderbird/68.7.0
MIME-Version: 1.0
In-Reply-To: <20200520080357.GA4197@lst.de>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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