RE: [PATCH] sched/topology: remove redundant cpumask_and in init_overlap_sched_group

["Song Bao Hua (Barry Song)" <song.bao.hua@hisilicon.com>]

> -----Original Message-----
> From: Valentin Schneider [mailto:valentin.schneider@arm.com]
> Sent: Saturday, March 6, 2021 12:49 AM
> To: Song Bao Hua (Barry Song) <song.bao.hua@hisilicon.com>;
> vincent.guittot@linaro.org; mingo@redhat.com; peterz@infradead.org;
> juri.lelli@redhat.com; dietmar.eggemann@arm.com; rostedt@goodmis.org;
> bsegall@google.com; mgorman@suse.de
> Cc: linux-kernel@vger.kernel.org; linuxarm@openeuler.org; Song Bao Hua (Barry
> Song) <song.bao.hua@hisilicon.com>
> Subject: Re: [PATCH] sched/topology: remove redundant cpumask_and in
> init_overlap_sched_group
> 
> On 05/03/21 11:29, Barry Song wrote:
> > mask is built in build_balance_mask() by for_each_cpu(i, sg_span), so
> > it must be a subset of sched_group_span(sg).
> 
> So we should indeed have
> 
>   cpumask_subset(sched_group_span(sg), mask)
> 
> but that doesn't imply
> 
>   cpumask_first(sched_group_span(sg)) == cpumask_first(mask)
> 
> does it? I'm thinking if in your topology of N CPUs, CPUs 0 and N-1 are the
> furthest away, you will most likely hit

It is true:
cpumask_first(sched_group_span(sg)) != cpumask_first(mask)

but 

cpumask_first_and(sched_group_span(sg), mask) = cpumask_first(mask)

since mask is always subset of sched_group_span(sg).

/**
 * cpumask_first_and - return the first cpu from *srcp1 & *srcp2
 * @src1p: the first input
 * @src2p: the second input
 *
 * Returns >= nr_cpu_ids if no cpus set in both.  See also cpumask_next_and().
 */

*srcp2 is subset of *srcp1, so  *srcp1 & *srcp2 = *srcp2

> 
>   !cpumask_equal(sg_pan, sched_domain_span(sibling->child))
>                  ^^^^^^                    ^^^^^^^^^^^^^
>                  CPUN-1                        CPU0
> 
> which should be the case on your Kunpeng920 system.
> 
> > Though cpumask_first_and
> > doesn't lead to a wrong result of balance cpu, it is pointless to do
> > cpumask_and again.
> >
> > Signed-off-by: Barry Song <song.bao.hua@hisilicon.com>
> > ---
> >  kernel/sched/topology.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> > index 12f8058..45f3db2 100644
> > --- a/kernel/sched/topology.c
> > +++ b/kernel/sched/topology.c
> > @@ -934,7 +934,7 @@ static void init_overlap_sched_group(struct sched_domain
> *sd,
> >  	int cpu;
> >
> >  	build_balance_mask(sd, sg, mask);
> > -	cpu = cpumask_first_and(sched_group_span(sg), mask);
> > +	cpu = cpumask_first(mask);
> >
> >  	sg->sgc = *per_cpu_ptr(sdd->sgc, cpu);
> >  	if (atomic_inc_return(&sg->sgc->ref) == 1)
> > --
> > 1.8.3.1

Thanks
Barry