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

[Madhavan Srinivasan]

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 <ynorov@xxxxxxxxxx>
> > > > ---
> > > >    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@xxxxxxxxxxxxx>
> >   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