From: Keith Busch <kbusch@kernel.org>
To: Ming Lei <ming.lei@redhat.com>
Cc: Bart Van Assche <bvanassche@acm.org>,
Christoph Hellwig <hch@lst.de>,
linux-block@vger.kernel.org, John Garry <john.garry@huawei.com>,
Hannes Reinecke <hare@suse.com>,
Thomas Gleixner <tglx@linutronix.de>
Subject: Re: blk-mq: improvement CPU hotplug (simplified version) v3
Date: Fri, 22 May 2020 07:47:20 -0700 [thread overview]
Message-ID: <20200522144720.GC3423299@dhcp-10-100-145-180.wdl.wdc.com> (raw)
In-Reply-To: <20200522023923.GC755458@T590>
On Fri, May 22, 2020 at 10:39:23AM +0800, Ming Lei wrote:
> On Thu, May 21, 2020 at 12:15:52PM -0700, Bart Van Assche wrote:
> > On 2020-05-20 21:33, Ming Lei wrote:
> > > No.
> > >
> > > If vector 3 is for covering hw queue 12 ~ 15, the vector shouldn't be
> > > shutdown when cpu 14 is offline.
> > >> Also I am pretty sure that we don't do this way with managed IRQ. And
> > > non-managed IRQ will be migrated to other online cpus during cpu offline,
> > > so not an issue at all. See migrate_one_irq().
> >
> > Thanks for the pointer to migrate_one_irq().
> >
> > However, I'm not convinced the above statement is correct. My
> > understanding is that the block driver knows which interrupt vector has
> > been associated with which hardware queue but the blk-mq core not. It
> > seems to me that patch 6/6 of this series is based on the following
> > assumptions:
> > (a) That the interrupt that is associated with a hardware queue is
> > processed by one of the CPU's in hctx->cpumask.
> > (b) That hardware queues do not share interrupt vectors.
> >
> > I don't think that either assumption is correct.
>
> What the patch tries to do is just:
>
> - when the last cpu of hctx->cpumask is going to become offline, mark
> this hctx as inactive, then drain any inflight IO requests originated
> from this hctx
>
> The correctness is that once we stops to produce request, we can drain
> any in-flight requests before shutdown the last cpu of hctx. Then finally
> this hctx becomes quiesced completely. Do you think this way is wrong?
> If yes, please prove it.
I don't think this applies to what Bart is saying, but there is a
pathological case where things break down: if a driver uses managed
irq's, but doesn't use the same affinity for the hctx's, an offline cpu
may have been the only one providing irq handling for an online hctx.
I feel like that should be a driver bug if it were to set itself up that
way, but I don't find anything enforces that.
next prev parent reply other threads:[~2020-05-22 14:47 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-20 17:06 blk-mq: improvement CPU hotplug (simplified version) v3 Christoph Hellwig
2020-05-20 17:06 ` [PATCH 1/6] blk-mq: remove the bio argument to ->prepare_request Christoph Hellwig
2020-05-20 18:16 ` Bart Van Assche
2020-05-22 9:11 ` Hannes Reinecke
2020-05-20 17:06 ` [PATCH 2/6] blk-mq: simplify the blk_mq_get_request calling convention Christoph Hellwig
2020-05-20 18:22 ` Bart Van Assche
2020-05-22 9:13 ` Hannes Reinecke
2020-05-20 17:06 ` [PATCH 3/6] blk-mq: move more request initialization to blk_mq_rq_ctx_init Christoph Hellwig
2020-05-20 20:10 ` Bart Van Assche
2020-05-20 17:06 ` [PATCH 4/6] blk-mq: open code __blk_mq_alloc_request in blk_mq_alloc_request_hctx Christoph Hellwig
2020-05-22 9:17 ` Hannes Reinecke
2020-05-20 17:06 ` [PATCH 5/6] blk-mq: add blk_mq_all_tag_iter Christoph Hellwig
2020-05-20 20:24 ` Bart Van Assche
2020-05-27 6:05 ` Christoph Hellwig
2020-05-22 9:18 ` Hannes Reinecke
2020-05-20 17:06 ` [PATCH 6/6] blk-mq: drain I/O when all CPUs in a hctx are offline Christoph Hellwig
2020-05-22 9:25 ` Hannes Reinecke
2020-05-25 9:20 ` Ming Lei
2020-05-20 21:46 ` blk-mq: improvement CPU hotplug (simplified version) v3 Bart Van Assche
2020-05-21 2:57 ` Ming Lei
2020-05-21 3:50 ` Bart Van Assche
2020-05-21 4:33 ` Ming Lei
2020-05-21 19:15 ` Bart Van Assche
2020-05-22 2:39 ` Ming Lei
2020-05-22 14:47 ` Keith Busch [this message]
2020-05-23 3:05 ` Ming Lei
2020-05-23 15:19 ` Bart Van Assche
2020-05-25 4:09 ` Ming Lei
2020-05-25 15:32 ` Bart Van Assche
2020-05-25 16:38 ` Keith Busch
2020-05-26 0:37 ` Ming Lei
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