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

[Shrikanth Hegde <sshegde@linux.ibm.com>]

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>