Re: [PATCH V2 RESEND 3/5] blk-mq: stop to handle IO before hctx's all CPUs become offline

From: Ming Lei <ming.lei@redhat.com>
To: Hannes Reinecke <hare@suse.de>
Cc: Jens Axboe <axboe@kernel.dk>,
	linux-block@vger.kernel.org, John Garry <john.garry@huawei.com>,
	Bart Van Assche <bvanassche@acm.org>,
	Hannes Reinecke <hare@suse.com>, Christoph Hellwig <hch@lst.de>,
	Thomas Gleixner <tglx@linutronix.de>,
	Keith Busch <keith.busch@intel.com>
Subject: Re: [PATCH V2 RESEND 3/5] blk-mq: stop to handle IO before hctx's all CPUs become offline
Date: Mon, 7 Oct 2019 23:01:02 +0800	[thread overview]
Message-ID: <20191007150101.GA1668@ming.t460p> (raw)
In-Reply-To: <efd6edfa-cde1-7ad3-d04f-1204770e6b42@suse.de>

On Mon, Oct 07, 2019 at 08:23:29AM +0200, Hannes Reinecke wrote:
> On 10/6/19 4:45 AM, Ming Lei wrote:
> > Most of blk-mq drivers depend on managed IRQ's auto-affinity to setup
> > up queue mapping. Thomas mentioned the following point[1]:
> > 
> > "
> >  That was the constraint of managed interrupts from the very beginning:
> > 
> >   The driver/subsystem has to quiesce the interrupt line and the associated
> >   queue _before_ it gets shutdown in CPU unplug and not fiddle with it
> >   until it's restarted by the core when the CPU is plugged in again.
> > "
> > 
> > However, current blk-mq implementation doesn't quiesce hw queue before
> > the last CPU in the hctx is shutdown. Even worse, CPUHP_BLK_MQ_DEAD is
> > one cpuhp state handled after the CPU is down, so there isn't any chance
> > to quiesce hctx for blk-mq wrt. CPU hotplug.
> > 
> > Add new cpuhp state of CPUHP_AP_BLK_MQ_ONLINE for blk-mq to stop queues
> > and wait for completion of in-flight requests.
> > 
> > [1] https://lore.kernel.org/linux-block/alpine.DEB.2.21.1904051331270.1802@nanos.tec.linutronix.de/
> > 
> > Cc: Bart Van Assche <bvanassche@acm.org>
> > Cc: Hannes Reinecke <hare@suse.com>
> > Cc: Christoph Hellwig <hch@lst.de>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Keith Busch <keith.busch@intel.com>
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > ---
> >  block/blk-mq-tag.c         |  2 +-
> >  block/blk-mq-tag.h         |  2 ++
> >  block/blk-mq.c             | 65 ++++++++++++++++++++++++++++++++++++++
> >  include/linux/blk-mq.h     |  1 +
> >  include/linux/cpuhotplug.h |  1 +
> >  5 files changed, 70 insertions(+), 1 deletion(-)
> > 
> I really don't like the zillions of 'XXX_in_flight()' helper in blk-mq;
> blk_mq_queue_inflight(), blk_mq_in_flight(), blk_mq_in_flight_rw() et al.
> Can't you implement your one on top of the already existing?

This one returns in-flight rqs on specific tags(hctx), so far no such
kind of interface, that is why blk_mq_all_tag_busy_iter is exported out
in this patch.

Thanks,
Ming