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

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

[Barry Song]

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). 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

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

[Valentin Schneider]

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

  !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

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

[Song Bao Hua (Barry Song)]

> -----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

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

[Valentin Schneider]

On 05/03/21 20:25, Song Bao Hua (Barry Song) wrote:
>> -----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).
>

You're right, I read it the wrong way around, sorry about that.

Reviewed-by: Valentin Schneider <Valentin.Schneider@arm.com>