Re: Question on handling managed IRQs when hotplugging CPUs

[John Garry]

On 05/02/2019 14:52, Keith Busch wrote:
> 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.

For SCSI devices, unfortunately not all IO sent to the HW originates 
from blk-mq or any other single entity.

Thanks,
John

> .