From: Jens Axboe <axboe@kernel.dk>
To: Valentin Schneider <valentin.schneider@arm.com>,
Peter Zijlstra <peterz@infradead.org>,
Thomas Gleixner <tglx@linutronix.de>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>,
linux-kernel@vger.kernel.org, Qian Cai <cai@redhat.com>,
Vincent Donnefort <vincent.donnefort@arm.com>,
Dexuan Cui <decui@microsoft.com>,
Lai Jiangshan <laijs@linux.alibaba.com>,
Paul McKenney <paulmck@kernel.org>,
Vincent Guittot <vincent.guittot@linaro.org>,
Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH -tip V3 0/8] workqueue: break affinity initiatively
Date: Tue, 12 Jan 2021 07:57:26 -0700 [thread overview]
Message-ID: <f5b0f650-5dc1-c67d-f548-340e7ca54d41@kernel.dk> (raw)
In-Reply-To: <jhj7doj1dr1.mognet@arm.com>
On 1/11/21 12:21 PM, Valentin Schneider wrote:
> On 11/01/21 18:16, Peter Zijlstra wrote:
>> Sadly it appears like io_uring() uses kthread_create_on_cpu() without
>> then having any hotplug crud on, so that needs additinoal frobbing.
>>
>
> I noticed that as well sometime ago, and I believed then (still do) this
> usage is broken. I don't think usage of kthread_create_on_cpu() outside
> of smpboot makes sense, because without any hotplug step to park the
> thread, its affinity can end up being reset after its dedicated CPU gets
> offlined.
>
> I'm clueless about io_uring, but if it *actually* has a good reason to
> use some pcpu kthreads (it seems it doesn't have to be on all CPUs?),
> then it needs to register some hotplug step to park them / do something
> sensible on hotplug.
For io_uring, it's purely used by the SQPOLL mode, which sets aside a
kernel thread for submissions so the application doesn't have to do
anything but write new SQE entries to submit. The thread then notices
these, and submits them. There's an option to affinitize that thread
to a single CPU, which often makes sense for setups like that. Think
of it like a strong hint.
Things aren't going to break if this CPU goes away and we end up being
affinitized to some other CPU, though it is suboptimal. So I guess we
might need some notifiers to ensure that we reset the CPU back again
if it's gone offline+online again? I can take a look at that.
--
Jens Axboe
next prev parent reply other threads:[~2021-01-12 14:58 UTC|newest]
Thread overview: 43+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-26 2:51 Lai Jiangshan
2020-12-26 2:51 ` [PATCH -tip V3 1/8] workqueue: use cpu_possible_mask instead of cpu_active_mask to break affinity Lai Jiangshan
2020-12-26 2:51 ` [PATCH -tip V3 2/8] workqueue: Manually break affinity on pool detachment Lai Jiangshan
2020-12-26 2:51 ` [PATCH -tip V3 3/8] workqueue: introduce wq_online_cpumask Lai Jiangshan
2021-01-04 13:56 ` Peter Zijlstra
2021-01-05 2:41 ` Lai Jiangshan
2021-01-05 2:53 ` Lai Jiangshan
2021-01-05 8:23 ` Lai Jiangshan
2021-01-05 13:17 ` Peter Zijlstra
2021-01-05 14:37 ` Lai Jiangshan
2021-01-05 14:40 ` Lai Jiangshan
2021-01-05 16:24 ` Peter Zijlstra
2020-12-26 2:51 ` [PATCH -tip V3 4/8] workqueue: use wq_online_cpumask in restore_unbound_workers_cpumask() Lai Jiangshan
2020-12-26 2:51 ` [PATCH -tip V3 5/8] workqueue: Manually break affinity on hotplug for unbound pool Lai Jiangshan
[not found] ` <20201226101631.5448-1-hdanton@sina.com>
2020-12-27 14:04 ` Lai Jiangshan
2020-12-26 2:51 ` [PATCH -tip V3 6/8] workqueue: reorganize workqueue_online_cpu() Lai Jiangshan
2020-12-26 2:51 ` [PATCH -tip V3 7/8] workqueue: reorganize workqueue_offline_cpu() unbind_workers() Lai Jiangshan
2020-12-26 2:51 ` [PATCH -tip V3 8/8] workqueue: Fix affinity of kworkers when attaching into pool Lai Jiangshan
[not found] ` <20201229100639.2086-1-hdanton@sina.com>
2020-12-29 10:13 ` Lai Jiangshan
2021-01-08 11:46 ` [PATCH -tip V3 0/8] workqueue: break affinity initiatively Peter Zijlstra
2021-01-11 10:07 ` Thomas Gleixner
2021-01-11 11:01 ` Peter Zijlstra
2021-01-11 15:00 ` Paul E. McKenney
2021-01-11 17:16 ` Peter Zijlstra
2021-01-11 18:09 ` Paul E. McKenney
2021-01-11 21:50 ` Paul E. McKenney
2021-01-12 17:14 ` Paul E. McKenney
2021-01-12 23:53 ` Paul E. McKenney
2021-01-15 9:11 ` Peter Zijlstra
2021-01-15 13:04 ` Peter Zijlstra
2021-01-16 6:00 ` Lai Jiangshan
2021-01-11 19:21 ` Valentin Schneider
2021-01-11 20:23 ` Peter Zijlstra
2021-01-11 22:47 ` Valentin Schneider
2021-01-12 4:33 ` Lai Jiangshan
2021-01-12 14:53 ` Peter Zijlstra
2021-01-12 15:38 ` Lai Jiangshan
2021-01-13 11:10 ` Peter Zijlstra
2021-01-13 12:00 ` Lai Jiangshan
2021-01-13 12:57 ` Lai Jiangshan
2021-01-12 17:52 ` Valentin Schneider
2021-01-12 14:57 ` Jens Axboe [this message]
2021-01-12 15:51 ` Peter Zijlstra
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