Re: [PATCH v2] lib/group_cpus: rotate extra groups to avoid IRQ stacking

[Naman Jain <namjain@linux.microsoft.com>]

On 9/2/2026 10:27 PM, Michael Kelley wrote:
> From: Naman Jain <namjain@linux.microsoft.com> Sent: Tuesday, September 1, 2026 10:03 PM
>>
>> On 9/1/2026 5:32 AM, Michael Kelley wrote:
>>> From: Naman Jain <namjain@linux.microsoft.com> Sent: Sunday, August 9, 2026 11:22 PM
> 
> [snip]
> 
>>>> @@ -270,11 +279,15 @@ static void assign_cpus_to_groups(unsigned int ncpus,
>>>>    	for (v = 0; v < nv->ngroups; v++, *curgrp += 1) {
>>>>    		cpus_per_grp = ncpus / nv->ngroups;
>>>>
>>>> -		/* Account for extra groups to compensate rounding errors */
>>>> -		if (extra_grps) {
>>>> +		/*
>>>> +		 * Rotate which groups get the extra CPU so that
>>>> +		 * successive callers produce different mappings,
>>>> +		 * avoiding IRQ stacking when multiple devices
>>>> +		 * share the same CPU topology.
>>>> +		 */
>>>> +		if (extra_grps &&
>>>
>>> Explicitly testing extra_grps for zero isn't necessary. If it is
>>> zero, the "less than" test below will always be false anyway.
>>>
>>
>> That is true, but the intention was to improve readability of this
>> block. Unless you really feel that this should be removed, I would
>> prefer to retain this.
>>
> 
> Fair enough. It's your call unless Thomas Gleixner as the maintainer
> says otherwise. I flag these cases because I spend time looking for
> some subtle reason why the test is actually needed. For example,
> if the left side of the "less than" test below has side-effects, then
> checking extra_gaps for zero prevents the side-effects from occurring.
> To me the redundant test introduces ambiguity -- is there a reason
> it is needed, or is it superfluous?
> 

In my head, I was reading it like this: If there are extra groups and 
these groups satisfy this condition, do this. Other than that, it did 
not add any value.

I should remove it.

> Interestingly, I had an identical conversation in another patch
> review [1] three days ago.
>

:) Sorry for the trouble and thank you again for reviewing the patch.

Regards,
Naman