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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 17:23:40 +0800
Message-ID: <20200521092340.GA751297@T590> (raw)
In-Reply-To: <87367tvh6g.fsf@nanos.tec.linutronix.de>

Hi Thomas,

On Thu, May 21, 2020 at 10:13:59AM +0200, Thomas Gleixner wrote:
> Ming Lei <ming.lei@redhat.com> writes:
> > On Thu, May 21, 2020 at 12:14:18AM +0200, Thomas Gleixner wrote:
> >> 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.
> 
> 1) This is based on the assumption that the kthread is in the SCHED_OTHER
>    scheduling class. Is that really a valid assumption?

Given it is unlikely path, we can add msleep() before retrying when INACTIVE bit
is observed by current thread, and this way can avoid spinning and should work
for other schedulers.

> 
> 2) What happens in the following scenario:
> 
>    unplug
> 
>      mq_offline
>        set_ctx_inactive()
>        drain_io()
>        
>    io_kthread()
>        try_queue()
>        wait_on_ctx()
> 
>    Can this happen and if so what will wake up that thread?

drain_io() releases all tag of this hctx, then wait_on_ctx() will be waken up
after any tag is released.

If wait_on_ctx() waits for other generic resource, it will be waken up
after this resource is available.

thanks,
Ming


  reply index

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
2020-05-21  8:13                                       ` Thomas Gleixner
2020-05-21  9:23                                         ` Ming Lei [this message]
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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