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

[Ming Lei <tom.leiming@gmail.com>]

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