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From: Ming Lei <ming.lei@redhat.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Christoph Hellwig <hch@lst.de>,
	linux-block@vger.kernel.org, John Garry <john.garry@huawei.com>,
	Bart Van Assche <bvanassche@acm.org>,
	Hannes Reinecke <hare@suse.com>
Subject: Re: [PATCH 5/9] blk-mq: don't set data->ctx and data->hctx in blk_mq_alloc_request_hctx
Date: Mon, 18 May 2020 19:54:54 +0800
Message-ID: <20200518115454.GA46364@T590> (raw)
In-Reply-To: <87imgty15d.fsf@nanos.tec.linutronix.de>

On Mon, May 18, 2020 at 12:42:54PM +0200, Thomas Gleixner wrote:
> Ming Lei <ming.lei@redhat.com> writes:
> > On Mon, May 18, 2020 at 10:32:22AM +0200, Thomas Gleixner wrote:
> >> Christoph Hellwig <hch@lst.de> writes:
> >> Is this absolutely necessary to be a smp function call? That's going to
> >
> > I think it is.
> >
> > Request is bound to the allocation CPU and the hw queue(hctx) which is
> > mapped from the allocation CPU.
> >
> > If request is allocated from one cpu which is going to offline, we can't
> > handle that easily.
> 
> That's a pretty handwavy explanation and does not give any reason why
> this needs to be a smp function call and cannot be solved otherwise,
> e.g. by delegating this to a work queue.

I guess I misunderstood your point, sorry for that.

The requirement is just that the request needs to be allocated on one online
CPU after INACTIVE is set, and we can use a workqueue to do that.

> 
> >> be problematic vs. RT. Same applies to the explicit preempt_disable() in
> >> patch 7.
> >
> > I think it is true and the reason is same too, but the period is quite short,
> > and it is just taken for iterating several bitmaps for finding one free bit.
> 
> And takes spinlocks along the way.... See:
> 
>   https://www.kernel.org/doc/html/latest/locking/locktypes.html
> 
> for a full explanation why this can't work on RT. And that's the same
> reason why the smp function call will fall apart on a RT enabled kernel.

We do want to avoid the cost of any lock, because it is in the fast IO path.

Looks preempt_disable in patch 7 can't be avoided.

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 [this message]
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
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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