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

Shrikanth Hegde
2026-03-31T02:59:53Z

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 <ynorov@nvidia.com>
>>> ---
>>> 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 <sshegde@linux.ibm.com>
>
> 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.