Re: Question on handling managed IRQs when hotplugging CPUs

[John Garry]

On 29/01/2019 15:44, Keith Busch wrote:
> On Tue, Jan 29, 2019 at 03:25:48AM -0800, John Garry wrote:
> > Hi,
> >
> > I have a question on $subject which I hope you can shed some light on.
> >
> > According to commit c5cb83bb337c25 ("genirq/cpuhotplug: Handle managed
> > IRQs on CPU hotplug"), if we offline the last CPU in a managed IRQ
> > affinity mask, the IRQ is shutdown.
> >
> > The reasoning is that this IRQ is thought to be associated with a
> > specific queue on a MQ device, and the CPUs in the IRQ affinity mask are
> > the same CPUs associated with the queue. So, if no CPU is using the
> > queue, then no need for the IRQ.
> >
> > However how does this handle scenario of last CPU in IRQ affinity mask
> > being offlined while IO associated with queue is still in flight?
> >
> > Or if we make the decision to use queue associated with the current CPU,
> > and then that CPU (being the last CPU online in the queue's IRQ
> > afffinity mask) goes offline and we finish the delivery with another CPU?
> >
> > In these cases, when the IO completes, it would not be serviced and timeout.
> >
> > I have actually tried this on my arm64 system and I see IO timeouts.
>
> Hm, we used to freeze the queues with CPUHP_BLK_MQ_PREPARE callback,
> which would reap all outstanding commands before the CPU and IRQ are
> taken offline. That was removed with commit 4b855ad37194f ("blk-mq:
> Create hctx for each present CPU"). It sounds like we should bring
> something like that back, but make more fine grain to the per-cpu context.

Seems reasonable. But we would need it to deal with drivers where they 
only expose a single queue to BLK MQ, but use many queues internally. I 
think megaraid sas does this, for example.

I would also be slightly concerned with commands being issued from the 
driver unknown to blk mq, like SCSI TMF.

Thanks,
John

> .