From: Jens Axboe <axboe@kernel.dk>
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
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
Date: Wed, 20 May 2020 09:20:50 -0600 [thread overview]
Message-ID: <448d3660-0d83-889b-001f-a09ea53fa117@kernel.dk> (raw)
In-Reply-To: <8f893bb8-66a9-d311-ebd8-d5ccd8302a0d@kernel.dk>
On 5/20/20 8:45 AM, Jens Axboe wrote:
> 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.
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.
So don't think there's anything wrong on that side. The affinity is a
performance optimization, not a correctness issue. Really not much we
can do if the chosen CPU is offlined, apart from continue to chug along.
--
Jens Axboe
next prev parent reply other threads:[~2020-05-20 15:23 UTC|newest]
Thread overview: 47+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-18 6:39 blk-mq: improvement CPU hotplug (simplified version) v2 Christoph Hellwig
2020-05-18 6:39 ` [PATCH 1/9] blk-mq: split out a __blk_mq_get_driver_tag helper Christoph Hellwig
2020-05-18 6:39 ` [PATCH 2/9] blk-mq: remove the bio argument to ->prepare_request Christoph Hellwig
2020-05-18 6:39 ` [PATCH 3/9] blk-mq: simplify the blk_mq_get_request calling convention Christoph Hellwig
2020-05-18 6:39 ` [PATCH 4/9] blk-mq: merge blk_mq_rq_ctx_init into __blk_mq_alloc_request Christoph Hellwig
2020-05-18 6:39 ` [PATCH 5/9] blk-mq: don't set data->ctx and data->hctx in blk_mq_alloc_request_hctx Christoph Hellwig
2020-05-18 8:32 ` Thomas Gleixner
2020-05-18 9:31 ` Ming Lei
2020-05-18 10:42 ` Thomas Gleixner
2020-05-18 11:54 ` Ming Lei
2020-05-18 13:16 ` Christoph Hellwig
2020-05-18 14:11 ` Ming Lei
2020-05-18 16:56 ` Christoph Hellwig
2020-05-18 18:38 ` Thomas Gleixner
2020-05-18 18:45 ` Christoph Hellwig
2020-05-18 18:59 ` Thomas Gleixner
2020-05-19 1:54 ` Ming Lei
2020-05-19 15:30 ` Christoph Hellwig
2020-05-20 1:18 ` Ming Lei
2020-05-20 3:04 ` Ming Lei
2020-05-20 8:03 ` io_uring vs CPU hotplug, was " Christoph Hellwig
2020-05-20 14:45 ` Jens Axboe
2020-05-20 15:20 ` Jens Axboe [this message]
2020-05-20 15:31 ` Christoph Hellwig
2020-05-20 19:41 ` Thomas Gleixner
2020-05-20 20:18 ` Jens Axboe
2020-05-20 22:14 ` Thomas Gleixner
2020-05-20 22:40 ` Jens Axboe
2020-05-21 2:27 ` Ming Lei
2020-05-21 8:13 ` Thomas Gleixner
2020-05-21 9:23 ` Ming Lei
2020-05-21 18:39 ` Thomas Gleixner
2020-05-21 18:45 ` Jens Axboe
2020-05-21 20:00 ` Thomas Gleixner
2020-05-22 1:57 ` Ming Lei
2020-05-18 18:47 ` Thomas Gleixner
2020-05-18 13:18 ` Thomas Gleixner
2020-05-18 6:39 ` [PATCH 6/9] blk-mq: don't set data->ctx and data->hctx in __blk_mq_alloc_request Christoph Hellwig
2020-05-18 6:39 ` [PATCH 7/9] blk-mq: disable preemption during allocating request tag Christoph Hellwig
2020-05-18 6:39 ` [PATCH 8/9] blk-mq: add blk_mq_all_tag_iter Christoph Hellwig
2020-05-18 6:39 ` [PATCH 9/9] blk-mq: drain I/O when all CPUs in a hctx are offline Christoph Hellwig
2020-05-18 8:42 ` John Garry
2020-05-18 9:21 ` Ming Lei
2020-05-18 11:49 ` blk-mq: improvement CPU hotplug (simplified version) v2 John Garry
2020-05-19 15:30 ` Christoph Hellwig
2020-05-19 17:17 ` John Garry
2020-05-20 14:35 ` John Garry
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