Subject: Re: [PATCH 2/2] powerpc/xive: rework xive_find_target_in_mask()

From: Madhavan Srinivasan <[email-masked]>
Date: Tue, 31 Mar 2026 11:00:07 +0530

On 3/31/26 8:29 AM, Shrikanth Hegde wrote:
> Hi Yury,
>
> On 3/30/26 10:34 PM, Yury Norov wrote:
>> On Sun, Mar 29, 2026 at 02:43:27PM +0530, Shrikanth Hegde wrote:
>>> Hi Yury.
>>>
>>> On 3/19/26 9:06 AM, Yury Norov wrote:
>>>> Switch the function to using modern cpumask API and drop most of the
>>>> housekeeping code.
>>>>
>>>> Notice, if first >= nr_cpu_ids, for_each_cpu_wrap() iterator 
>>>> behaves just
>>>> like for_each_cpu(), i.e. begins from 0. So even if WARN_ON() is 
>>>> triggered,
>>>> no special handling is needed.
>>>>
>>>> Signed-off-by: Yury Norov <[email-masked]>
>>>> ---
>>>>    arch/powerpc/sysdev/xive/common.c | 31 
>>>> ++++++-------------------------
>>>>    1 file changed, 6 insertions(+), 25 deletions(-)
>>>>
>>>> diff --git a/arch/powerpc/sysdev/xive/common.c 
>>>> b/arch/powerpc/sysdev/xive/common.c
>>>> index e91ec9036ad8..4e05f678e171 100644
>>>> --- a/arch/powerpc/sysdev/xive/common.c
>>>> +++ b/arch/powerpc/sysdev/xive/common.c
>>>> @@ -548,40 +548,21 @@ static void xive_dec_target_count(int cpu)
>>>>    static int xive_find_target_in_mask(const struct cpumask *mask,
>>>>                        unsigned int fuzz)
>>>>    {
>>>> -    int cpu, first, num, i;
>>>> +    int cpu, first;
>>>>        /* Pick up a starting point CPU in the mask based on fuzz */
>>>> -    num = cpumask_weight(mask);
>>>> -    first = fuzz % num;
>>>> -
>>>> -    /* Locate it */
>>>> -    cpu = cpumask_first(mask);
>>>> -    for (i = 0; i < first && cpu < nr_cpu_ids; i++)
>>>> -        cpu = cpumask_next(cpu, mask);
>>>> -
>>>> -    /* Sanity check */
>>>> -    if (WARN_ON(cpu >= nr_cpu_ids))
>>>> -        cpu = cpumask_first(cpu_online_mask);
>>>> -
>>>> -    /* Remember first one to handle wrap-around */
>>>> -    first = cpu;
>>>> +    fuzz %= cpumask_weight(mask);
>>>> +    first = cpumask_nth(fuzz, mask);
>>>> +    WARN_ON(first >= nr_cpu_ids);
>>>>        /*
>>>>         * Now go through the entire mask until we find a valid
>>>>         * target.
>>>>         */
>>>> -    do {
>>>> -        /*
>>>> -         * We re-check online as the fallback case passes us
>>>> -         * an untested affinity mask
>>>> -         */
>>>> +    for_each_cpu_wrap(cpu, mask, first) {
>>>>            if (cpu_online(cpu) && xive_try_pick_target(cpu))
>>>>                return cpu;
>>>> -        cpu = cpumask_next(cpu, mask);
>>>> -        /* Wrap around */
>>>> -        if (cpu >= nr_cpu_ids)
>>>> -            cpu = cpumask_first(mask);
>>>> -    } while (cpu != first);
>>>> +    }
>>>>        return -1;
>>>>    }
>>>
>>> Only concern i have, (which could potentially leads to while(1) loop
>>> today if it), is if mask is empty. atleast for_each_cpu_wrap will not
>>> be a while(1) loop.
>>>
>>> So, IMO this is better than what we have today.
>>>
>>> nit: maybe a good to add a WARN_ON(cpu >= nr_cpu_ids) in the end.
>>>
>>> Reviewed-by: Shrikanth Hegde <[email-masked]>
>>   The iterator of for_each_cpu() is always >= nr_cpu_ids when it 
>> exits the
>> loop because it's the exit condition. If you want to warn user before
>> returning -1, just do __WARN().
>
> Ok.
>
> It might take a while a while to send the next version out since i am
> working on getting the next version of that paravirt series out soon.
OK, I am pulling this patchset as-is for next merge. I will expect
a followup patch  to add a __WARN().

Maddy