Re: [PATCH v7 08/10] blk-mq: use hk cpus only when isolcpus=io_queue is enabled

[Christoph Hellwig <hch@lst.de>]

On Fri, Jul 04, 2025 at 11:00:56AM +0200, Daniel Wagner wrote:
> > I'm no expert on the housekeeping stuff, but why isn't the
> > housekeeping_enabled check done in housekeeping_cpumask directly so
> > that the drivers could use housekeeping_cpumask without a blk-mq
> > wrapper?
> 
> Yes, housekeeping_cpumask will return cpu_possible_mask when housekeping
> is disabled. Though some drivers want cpu_online_mask instead. If all
> drivers would agree on one version of the mask it should allow to drop
> to these helpers (maybe we the houskeeping API needs to be extended then
> though)

Drivers don't get cpu hotplug notifications, so cpu_possible_mask is
the only valid answer right now.  That could change if we ever implement
notifications to the drivers.

> This is also what Hannes brought up. If the number of supported hardware
> queues for a device is less than cpu_possible_mask, it really makes
> sense to distribute the hardware queues only between the online cpus. I
> think the only two drivers which are interested in the cpu_possible_mask
> are nvme-pci and virtio.

That's the only two drivers that get it right :(