Re: [PATCH v13 5/8] blk-mq: use hk cpus only when isolcpus=io_queue is enabled

[Aaron Tomlin]

On Thu, May 14, 2026 at 12:42:57PM +0200, Daniel Wagner wrote:
> On Wed, May 13, 2026 at 07:30:44PM -0400, Aaron Tomlin wrote:
> > > I think this should also check if the CPU is online. The fallback should
> > > ensure the system will boot.
> > 
> > While I completely agree with the goal of ensuring a safe boot, I suspect
> > that we actually must iterate over for_each_possible_cpu(cpu) here to
> > prevent map boundary violations during CPU hotplug events.
> 
> I have to check this version of the patch what is happening in detail.
> Originally the idea was that the fallback should only be taken if it's
> not possible to create a valid mapping due to $reason. In this case the
> system should still boot and issue a warning. This is also what the
> comments says in the function, pick one online CPU and map all hctx to
> it. Don't bother to be smart.

Hi Daniel,

Understood.

Previously, heavy isolation could starve a device of its minvec
requirement, forcing the block layer into this fallback.

The next iteration of this series has been updated to ensure a minimum
vector safety net. To prevent fatal -ENOSPC device probe aborts on heavily
isolated systems (i.e., where the housekeeping CPU count might be lower
than a device's structural minimum), the final vector calculation is
safeguarded to never drop below minvec. Queues will safely share the
available housekeeping CPUs instead of failing the probe.

Kind regards,
-- 
Aaron Tomlin

Attachment: signature.asc
Description: PGP signature