Re: Question on handling managed IRQs when hotplugging CPUs

[Keith Busch]

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.