Re: Question on handling managed IRQs when hotplugging CPUs

[John Garry]

On 29/01/2019 16:27, Thomas Gleixner wrote:
> On Tue, 29 Jan 2019, John Garry wrote:
> > On 29/01/2019 12:01, Thomas Gleixner wrote:
> > > If the last CPU which is associated to a queue (and the corresponding
> > > interrupt) goes offline, then the subsytem/driver code has to make sure
> > > that:
> > >
> > >    1) No more requests can be queued on that queue
> > >
> > >    2) All outstanding of that queue have been completed or redirected
> > >       (don't know if that's possible at all) to some other queue.
> >
> > This may not be possible. For the HW I deal with, we have symmetrical delivery
> > and completion queues, and a command delivered on DQx will always complete on
> > CQx. Each completion queue has a dedicated IRQ.
>
> So you can stop queueing on DQx and wait for all outstanding ones to come
> in on CQx, right?

Right, and this sounds like what Keith Busch mentioned in his reply.

> > > That has to be done in that order obviously. Whether any of the
> > > subsystems/drivers actually implements this, I can't tell.
> >
> > Going back to c5cb83bb337c25, it seems to me that the change was made with the
> > idea that we can maintain the affinity for the IRQ as we're shutting it down
> > as no interrupts should occur.
> >
> > However I don't see why we can't instead keep the IRQ up and set the affinity
> > to all online CPUs in offline path, and restore the original affinity in
> > online path. The reason we set the queue affinity to specific CPUs is for
> > performance, but I would not say that this matters for handling residual IRQs.
>
> Oh yes it does. The problem is especially on x86, that if you have a large
> number of queues and you take a large number of CPUs offline, then you run
> into vector space exhaustion on the remaining online CPUs.
>
> In the worst case a single CPU on x86 has only 186 vectors available for
> device interrupts. So just take a quad socket machine with 144 CPUs and two
> multiqueue devices with a queue per cpu. ---> FAIL
>
> It probably fails already with one device because there are lots of other
> devices which have regular interrupt which cannot be shut down.

OK, understood.

Thanks,
John

> Thanks,
>
> 	tglx
>
>
> .