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

[Yury Norov <ynorov@nvidia.com>]

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().