Re: [PATCH v4] sched/core: Use zero length to reset cpumasks in sched_setaffinity()

[Peter Zijlstra]

On Wed, Oct 04, 2023 at 11:23:41AM +0200, Ingo Molnar wrote:

> >  	if (user_mask) {
> > -		cpumask_copy(user_mask, in_mask);
> > +		/*
> > +		 * All-set user cpumask resets affinity and drops the explicit
> > +		 * user mask.
> > +		 */
> > +		cpumask_and(user_mask, in_mask, cpu_possible_mask);
> > +		if (cpumask_equal(user_mask, cpu_possible_mask)) {
> > +			kfree(user_mask);
> > +			user_mask = NULL;
> > +		}
> 
> Question: is there any observable behavioral difference between current 
> (old) all-set cpumask calls and the patched (new) one?

Very little I think -- the main difference is that we no longer carry
the ->user_cpus_ptr mask around, and that saves a little masking.