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

Ming Lei
2019-05-29T10:10:53Z

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.

Thanks,
Ming