Re: [PATCH V2 5/5] blk-mq: Wait for for hctx inflight requests on CPU unplug

From: Ming Lei <ming.lei@redhat.com>
To: John Garry <john.garry@huawei.com>
Cc: Jens Axboe <axboe@kernel.dk>,
	"Martin K . Petersen" <martin.petersen@oracle.com>,
	linux-block@vger.kernel.org,
	James Bottomley <James.Bottomley@hansenpartnership.com>,
	linux-scsi@vger.kernel.org, Bart Van Assche <bvanassche@acm.org>,
	Hannes Reinecke <hare@suse.com>,
	Keith Busch <keith.busch@intel.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Don Brace <don.brace@microsemi.com>,
	Kashyap Desai <kashyap.desai@broadcom.com>,
	Sathya Prakash <sathya.prakash@broadcom.com>,
	Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH V2 5/5] blk-mq: Wait for for hctx inflight requests on CPU unplug
Date: Wed, 29 May 2019 10:42:02 +0800	[thread overview]
Message-ID: <20190529024200.GC21398@ming.t460p> (raw)
In-Reply-To: <20190529022852.GA21398@ming.t460p>

On Wed, May 29, 2019 at 10:28:52AM +0800, Ming Lei wrote:
> On Tue, May 28, 2019 at 05:50:40PM +0100, John Garry wrote:
> > On 27/05/2019 16:02, Ming Lei wrote:
> > > Managed interrupts can not migrate affinity when their CPUs are offline.
> > > If the CPU is allowed to shutdown before they're returned, commands
> > > dispatched to managed queues won't be able to complete through their
> > > irq handlers.
> > > 
> > > Wait in cpu hotplug handler until all inflight requests on the tags
> > > are completed or timeout. Wait once for each tags, so we can save time
> > > in case of shared tags.
> > > 
> > > Based on the following patch from Keith, and use simple delay-spin
> > > instead.
> > > 
> > > https://lore.kernel.org/linux-block/20190405215920.27085-1-keith.busch@intel.com/
> > > 
> > > Some SCSI devices may have single blk_mq hw queue and multiple private
> > > completion queues, and wait until all requests on the private completion
> > > queue are completed.
> > 
> > Hi Ming,
> > 
> > I'm a bit concerned that this approach won't work due to ordering: it seems
> > that the IRQ would be shutdown prior to the CPU dead notification for the
> 
> Managed IRQ shutdown is run in irq_migrate_all_off_this_cpu(), which is
> called in the callback of takedown_cpu(). And the CPU dead notification
> is always sent after that CPU becomes offline, see cpuhp_invoke_callback().

Hammm, looks we both say same thing.

Yeah, it is too late to drain requests in the cpu hotplug DEAD handler,
maybe we can try to move managed IRQ shutdown after sending the dead
notification.

I need to think of it further.

Thanks,
Ming