Re: Question on handling managed IRQs when hotplugging CPUs

From: Keith Busch
Date: Tue Feb 05 2019 - 09:53:20 EST

On Tue, Feb 05, 2019 at 05:24:11AM -0800, John Garry wrote:
> On 04/02/2019 07:12, Hannes Reinecke wrote:
> Hi Hannes,
> >
> > So, as the user then has to wait for the system to declars 'ready for
> > CPU remove', why can't we just disable the SQ and wait for all I/O to
> > complete?
> > We can make it more fine-grained by just waiting on all outstanding I/O
> > on that SQ to complete, but waiting for all I/O should be good as an
> > initial try.
> > With that we wouldn't need to fiddle with driver internals, and could
> > make it pretty generic.
> I don't fully understand this idea - specifically, at which layer would
> we be waiting for all the IO to complete?

Whichever layer dispatched the IO to a CPU specific context should
be the one to wait for its completion. That should be blk-mq for most
block drivers.