From: Ming Lei <ming.lei@redhat.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Jens Axboe <axboe@kernel.dk>, Christoph Hellwig <hch@lst.de>,
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, Peter Zijlstra <peterz@infradead.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: Thu, 21 May 2020 10:27:46 +0800 [thread overview]
Message-ID: <20200521022746.GA730422@T590> (raw)
In-Reply-To: <87eereuudh.fsf@nanos.tec.linutronix.de>
On Thu, May 21, 2020 at 12:14:18AM +0200, Thomas Gleixner wrote:
> Jens Axboe <axboe@kernel.dk> writes:
>
> > 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.
>
> Works for me is not really useful information and does not answer my
> question:
>
> >> When exactly during the unplug operation is it unbound?
>
> The problem Ming and Christoph are trying to solve requires that the
> thread is migrated _before_ the hardware queue is shut down and
> drained. That's why I asked for the exact point where this happens.
>
> When the CPU is finally offlined, i.e. the CPU cleared the online bit in
> the online mask is definitely too late simply because it still runs on
> that outgoing CPU _after_ the hardware queue is shut down and drained.
IMO, the patch in Christoph's blk-mq-hotplug.2 still works for percpu
kthread.
It is just not optimal in the retrying, but it should be fine. When the
percpu kthread is scheduled on the CPU to be offlined:
- if the kthread doesn't observe the INACTIVE flag, the allocated request
will be drained.
- otherwise, the kthread just retries and retries to allocate & release,
and sooner or later, its time slice is consumed, and migrated out, and the
cpu hotplug handler will get chance to run and move on, then the cpu is
shutdown.
- After the cpu is shutdown, the percpu kthread becomes unbound, and
the allocation from new online cpu will succeed.
Thanks,
Ming
next prev parent reply other threads:[~2020-05-21 2:28 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
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 [this message]
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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