From: Ming Lei <tom.leiming@gmail.com>
To: Ming Lei <ming.lei@redhat.com>
Cc: John Garry <john.garry@huawei.com>, Jens Axboe <axboe@kernel.dk>,
"Martin K . Petersen" <martin.petersen@oracle.com>,
linux-block <linux-block@vger.kernel.org>,
James Bottomley <James.Bottomley@hansenpartnership.com>,
Linux SCSI List <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 23:33:44 +0800 [thread overview]
Message-ID: <CACVXFVODeFDPHxWkdnY5CZoOJ0did4mi_ap-aXk0oo+Cp05aUQ@mail.gmail.com> (raw)
In-Reply-To: <20190529101028.GA15496@ming.t460p>
On Wed, May 29, 2019 at 6:11 PM Ming Lei <ming.lei@redhat.com> wrote:
>
> On Wed, May 29, 2019 at 10:42:00AM +0100, John Garry wrote:
> > On 29/05/2019 03:42, Ming Lei wrote:
> > > 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.
> > >
> >
> > Even if the IRQ is shutdown later, all CPUs would still be dead, so none
> > available to receive the interrupt or do the work for draining the queue.
> >
> > > I need to think of it further.
> >
> > It would seem that we just need to be informed of CPU offlining earlier, and
> > plug the drain in there.
>
> Yes, looks blk-mq has to be notified before unplugging CPU for this
> issue.
>
> And we should be careful to handle the multiple reply queue case, given the queue
> shouldn't be stopped or quieseced because other reply queues are still active.
>
> The new CPUHP state for blk-mq should be invoked after the to-be-offline
> CPU is quiesced and before it becomes offline.
Hi John,
Thinking of this issue further, so far, one doable solution is to
expose reply queues
as blk-mq hw queues, as done by the following patchset:
https://lore.kernel.org/linux-block/20180205152035.15016-1-ming.lei@redhat.com/
In which global host-wide tags are shared for all blk-mq hw queues.
Also we can remove all the reply_map stuff in drivers, then solve the problem of
draining in-flight requests during unplugging CPU in a generic approach.
Last time, it was reported that the patchset causes performance regression,
which is actually caused by duplicated io accounting in
blk_mq_queue_tag_busy_iter(),
which should be fixed easily.
What do you think of this approach?
Thanks,
Ming Lei
next prev parent reply other threads:[~2019-05-29 15:33 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-27 15:02 [PATCH V2 0/5] blk-mq: Wait for for hctx inflight requests on CPU unplug Ming Lei
2019-05-27 15:02 ` [PATCH V2 1/5] scsi: select reply queue from request's CPU Ming Lei
2019-05-28 5:43 ` Hannes Reinecke
2019-05-28 10:33 ` John Garry
2019-05-29 2:36 ` Ming Lei
2019-05-27 15:02 ` [PATCH V2 2/5] blk-mq: introduce .complete_queue_affinity Ming Lei
2019-05-27 15:02 ` [PATCH V2 3/5] scsi: core: implement callback of .complete_queue_affinity Ming Lei
2019-05-27 15:02 ` [PATCH V2 4/5] scsi: implement .complete_queue_affinity Ming Lei
2019-05-27 15:02 ` [PATCH V2 5/5] blk-mq: Wait for for hctx inflight requests on CPU unplug Ming Lei
2019-05-28 16:50 ` John Garry
2019-05-29 2:28 ` Ming Lei
2019-05-29 2:42 ` Ming Lei
2019-05-29 9:42 ` John Garry
2019-05-29 10:10 ` Ming Lei
2019-05-29 15:33 ` Ming Lei [this message]
2019-05-29 16:10 ` John Garry
2019-05-30 2:28 ` Ming Lei
2019-05-30 4:11 ` Ming Lei
2019-05-30 9:31 ` John Garry
2019-05-30 9:45 ` Ming Lei
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