Re: Question on handling managed IRQs when hotplugging CPUs

John Garry
2019-02-05T15:09:28Z

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

>
> .
>