Re: [PATCH v4] lib/group_cpus: Snapshot cluster masks to keep grouping hotplug invariant

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 26 Aug 2026 22:45:37 +0530 Vishal Badole <Vishal.Badole@amd.com> wrote:

> group_cpus_evenly() builds the managed-IRQ affinity spread used by
> multi-queue devices such as NVMe. That spread is meant to be a property
> of the static CPU topology: it walks cpu_present_mask and then
> cpu_possible_mask so every hardware queue owns a fixed set of CPUs,
> including CPUs that are offline at the time. A driver depends on that
> partition staying stable across re-computation - the CPUs a queue is
> given at probe must still describe the same queue after the device is
> later reset and its affinity recomputed.
> 
> On an AMD system that stability breaks across an s2idle cycle. With CPUs
> 3-11 offlined and only CPUs 0-2 left online, the machine is suspended to
> s2idle and resumed. The NVMe controller uses the simple-suspend quirk, so
> resume fully re-initialises it and recomputes the affinity spread. The
> system then hangs for roughly two minutes and stays sluggish afterwards,
> the controller only making progress through its command-timeout poll:
> 
>   nvme nvme0: I/O tag 898 (3382) QID 9 timeout, completion polled
>   nvme nvme0: I/O tag 398 (618e) QID 11 timeout, completion polled
> 
> ...
> 
> Make the cluster grouping depend on the complete cluster topology rather
> than on whichever CPUs happen to be online. Snapshot the cluster masks
> once while every present CPU is online and reuse that view for every
> later spread. Every spread then groups from the same masks, so the
> partition computed when the controller is reset matches the one computed
> at probe and each queue's IRQ still covers the CPUs it serves. If the
> snapshot was never taken, the cluster path is skipped and the plain
> present/possible spread is used.

Thanks, I'll queue this for testing in linux-next while awaiting
further review and testing input.