Re: [Xen-devel] [PATCH] xen/sched: avoid cpumasks on stack in sched/core.c

[Jürgen Groß <jgross@xxxxxxxx>]

On 24.01.20 01:01, Dario Faggioli wrote:
> On Thu, 2020-01-23 at 10:03 +0100, Juergen Gross wrote:
> > There are still several instances of cpumask_t on the stack in
> > scheduling code. Avoid them as far as possible.
> >
> > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> Reviewed-by: Dario Faggioli <dfaggioli@xxxxxxxx>
>
> Just curious...
>
> > --- a/xen/common/sched/core.c
> > +++ b/xen/common/sched/core.c
> > @@ -2586,11 +2582,11 @@ static void schedule(void)
> >   
> >       if ( gran > 1 )
> >       {
> > -        cpumask_t mask;
> > +        cpumask_t *mask = cpumask_scratch_cpu(cpu);
> >   
> >           prev->rendezvous_in_cnt = gran;
> > -        cpumask_andnot(&mask, sr->cpus, cpumask_of(cpu));
> > -        cpumask_raise_softirq(&mask, SCHED_SLAVE_SOFTIRQ);
> > +        cpumask_andnot(mask, sr->cpus, cpumask_of(cpu));
> > +        cpumask_raise_softirq(mask, SCHED_SLAVE_SOFTIRQ);
> ... why are we keeping the temporary variable (mask) ?
per_cpu accesses are more expensive than those to local variables.
mask is used twice.


Juergen

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