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 [thread overview]
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
next prev parent reply other threads:[~2020-05-18 11:55 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 [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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