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

On 8/8/2026 2:08 AM, Andrew Morton wrote:
> On Fri, 7 Aug 2026 13:06:12 +0530 Vishal Badole <[email protected]> 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
>>
>> QID 9 and QID 11 are the queues whose CPUs were offline when the spread
>> was recomputed. "completion polled" means the commands did finish in
>> hardware, but their interrupts were never delivered to a CPU that was
>> watching the queue, so nothing reaped them until the timeout fired.
>>
>> It happens because commit 89802ca36c96 ("lib/group_cpus: make group CPU
>> cluster aware") derives the cluster groups from topology_cluster_cpumask(),
>> which lists only the cluster siblings that are online when it is called.
>> The resulting partition therefore depends on the transient online mask
>> rather than on the topology alone. Recomputed on resume while the non-boot
>> CPUs are still offline, it no longer matches the boot-time partition, and a
>> queue is left with an affinity that does not cover the CPU it is meant to
>> serve once that CPU comes back online. The dependence is on the online
>> mask, not on any AMD-specific behaviour, so the same stall is reproducible
>> on Intel platforms as well.
> 
> Thanks for the careful description of what is wrong.  It helps.
> 
> And things do sounds very wrong.  Could people@intel please prioritize
> their review and testing of this fix?
> 
>> 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 CPU is online and reuse that view for every later spread.
>> A fully-online view is the complete cluster membership, so the partition
>> derived from it is identical no matter which CPUs are online when the
>> controller is reset, which is exactly the invariance the callers already
>> assume.
>>
>> ...
>>
>> +	/*
>> +	 * Only a fully-online view is the complete cluster membership; if any
> 
> minor: the above sentence is hard to understand.
> 
Thanks for the feedback, I’ll simplify that sentence to make it clearer 
in the next patch revision.

>> +	if (!data_race(cpumask_equal(cpu_possible_mask, cpu_online_mask)))
> 
> AI review suggests using cpu_present_mask here:
> 
> 	https://sashiko.dev/#/patchset/[email protected]
> 
Good catch, I'll switch to cpu_present_mask in the next patch revision.