Re: Question on handling managed IRQs when hotplugging CPUs

[John Garry]

On 29/01/2019 17:20, Keith Busch wrote:
> On Tue, Jan 29, 2019 at 05:12:40PM +0000, John Garry wrote:
> > On 29/01/2019 15:44, Keith Busch wrote:
> > > 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.
>
> I don't think either of those descriptions sound like good candidates
> for using managed IRQ affinities.

I wouldn't say that this behaviour is obvious to the developer. I can't 
see anything in Documentation/PCI/MSI-HOWTO.txt

It also seems that this policy to rely on upper layer to flush+freeze 
queues would cause issues if managed IRQs are used by drivers in other 
subsystems. Networks controllers may have multiple queues and 
unsoliciated interrupts.

Thanks,
John

> .