[v1] sched/topology: Optimize topology_span_sane()

[PATCH 2/2 RESEND] sched/topology: Optimize topology_span_sane()

Posted by Kyle Meyer 1 year, 10 months ago

Optimize topology_span_sane() by removing duplicate comparisons.

The total number of comparisons is reduced from N * (N - 1) to
N * (N - 1) / 2 (per non-NUMA scheduling domain level).

Signed-off-by: Kyle Meyer <kyle.meyer@hpe.com>
Reviewed-by: Yury Norov <yury.norov@gmail.com>
---
 kernel/sched/topology.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 99ea5986038c..b6bcafc09969 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -2347,7 +2347,7 @@ static struct sched_domain *build_sched_domain(struct sched_domain_topology_leve
 static bool topology_span_sane(struct sched_domain_topology_level *tl,
 			      const struct cpumask *cpu_map, int cpu)
 {
-	int i;
+	int i = cpu + 1;
 
 	/* NUMA levels are allowed to overlap */
 	if (tl->flags & SDTL_OVERLAP)
@@ -2359,9 +2359,7 @@ static bool topology_span_sane(struct sched_domain_topology_level *tl,
 	 * breaking the sched_group lists - i.e. a later get_group() pass
 	 * breaks the linking done for an earlier span.
 	 */
-	for_each_cpu(i, cpu_map) {
-		if (i == cpu)
-			continue;
+	for_each_cpu_from(i, cpu_map) {
 		/*
 		 * We should 'and' all those masks with 'cpu_map' to exactly
 		 * match the topology we're about to build, but that can only
-- 
2.44.0

Re: [PATCH 2/2 RESEND] sched/topology: Optimize topology_span_sane()

Posted by Vincent Guittot 1 year, 10 months ago

On Tue, 9 Apr 2024 at 17:54, Kyle Meyer <kyle.meyer@hpe.com> wrote:
>
> Optimize topology_span_sane() by removing duplicate comparisons.
>
> The total number of comparisons is reduced from N * (N - 1) to
> N * (N - 1) / 2 (per non-NUMA scheduling domain level).
>
> Signed-off-by: Kyle Meyer <kyle.meyer@hpe.com>
> Reviewed-by: Yury Norov <yury.norov@gmail.com>

Acked-by: Vincent Guittot <vincent.guittot@linaro.org>

> ---
>  kernel/sched/topology.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 99ea5986038c..b6bcafc09969 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -2347,7 +2347,7 @@ static struct sched_domain *build_sched_domain(struct sched_domain_topology_leve
>  static bool topology_span_sane(struct sched_domain_topology_level *tl,
>                               const struct cpumask *cpu_map, int cpu)
>  {
> -       int i;
> +       int i = cpu + 1;
>
>         /* NUMA levels are allowed to overlap */
>         if (tl->flags & SDTL_OVERLAP)
> @@ -2359,9 +2359,7 @@ static bool topology_span_sane(struct sched_domain_topology_level *tl,
>          * breaking the sched_group lists - i.e. a later get_group() pass
>          * breaks the linking done for an earlier span.
>          */
> -       for_each_cpu(i, cpu_map) {
> -               if (i == cpu)
> -                       continue;
> +       for_each_cpu_from(i, cpu_map) {
>                 /*
>                  * We should 'and' all those masks with 'cpu_map' to exactly
>                  * match the topology we're about to build, but that can only
> --
> 2.44.0
>

Re: [PATCH 2/2 RESEND] sched/topology: Optimize topology_span_sane()

Posted by Andy Shevchenko 1 year, 10 months ago

On Tue, Apr 09, 2024 at 10:52:50AM -0500, Kyle Meyer wrote:
> Optimize topology_span_sane() by removing duplicate comparisons.
> 
> The total number of comparisons is reduced from N * (N - 1) to
> N * (N - 1) / 2 (per non-NUMA scheduling domain level).

...

> -	for_each_cpu(i, cpu_map) {
> -		if (i == cpu)
> -			continue;
> +	for_each_cpu_from(i, cpu_map) {

Hmm... I'm not familiar with the for_each_cpu*(), but from the above
it seems only a single comparison? Or i.o.w. can i ever repeat the value?
And what about i < cpu cases?

-- 
With Best Regards,
Andy Shevchenko

Re: [PATCH 2/2 RESEND] sched/topology: Optimize topology_span_sane()

Posted by Kyle Meyer 1 year, 10 months ago

On Tue, Apr 09, 2024 at 07:25:06PM +0300, Andy Shevchenko wrote:
> On Tue, Apr 09, 2024 at 10:52:50AM -0500, Kyle Meyer wrote:
> > Optimize topology_span_sane() by removing duplicate comparisons.
> > 
> > The total number of comparisons is reduced from N * (N - 1) to
> > N * (N - 1) / 2 (per non-NUMA scheduling domain level).
> 
> ...
> 
> > -	for_each_cpu(i, cpu_map) {
> > -		if (i == cpu)
> > -			continue;
> > +	for_each_cpu_from(i, cpu_map) {
> 
> Hmm... I'm not familiar with the for_each_cpu*(), but from the above
> it seems only a single comparison? Or i.o.w. can i ever repeat the value?

for_each_cpu() is a macro around for_each_set_bit() which iterates over each set
bit in a bitmap starting at zero.

for_each_cpu_from() is a macro around for_each_set_bit_from() which iterates
over each set bit in a bitmap starting at the specified bit.

The above (topology_span_sane()) currently does a "single comparison" for each
CPU in cpu_map, but it's called for each CPU in cpu_map and for each scheduling
domain level (please see build_sched_domains() in kernel/sched/topology.c).

> And what about i < cpu cases?

Those values have already been passed to topology_span_sane(). This patch uses
for_each_cpu_from() starting at cpu + 1 to prevent those duplicate comparisons.

Thanks,
Kyle Meyer

Re: [PATCH 2/2 RESEND] sched/topology: Optimize topology_span_sane()

Posted by Andy Shevchenko 1 year, 10 months ago

On Tue, Apr 09, 2024 at 02:29:09PM -0500, Kyle Meyer wrote:
> On Tue, Apr 09, 2024 at 07:25:06PM +0300, Andy Shevchenko wrote:
> > On Tue, Apr 09, 2024 at 10:52:50AM -0500, Kyle Meyer wrote:
> > > Optimize topology_span_sane() by removing duplicate comparisons.
> > > 
> > > The total number of comparisons is reduced from N * (N - 1) to
> > > N * (N - 1) / 2 (per non-NUMA scheduling domain level).

...

> > > -	for_each_cpu(i, cpu_map) {
> > > -		if (i == cpu)
> > > -			continue;
> > > +	for_each_cpu_from(i, cpu_map) {
> > 
> > Hmm... I'm not familiar with the for_each_cpu*(), but from the above
> > it seems only a single comparison? Or i.o.w. can i ever repeat the value?
> 
> for_each_cpu() is a macro around for_each_set_bit() which iterates over each set
> bit in a bitmap starting at zero.
> 
> for_each_cpu_from() is a macro around for_each_set_bit_from() which iterates
> over each set bit in a bitmap starting at the specified bit.
> 
> The above (topology_span_sane()) currently does a "single comparison" for each
> CPU in cpu_map, but it's called for each CPU in cpu_map and for each scheduling
> domain level (please see build_sched_domains() in kernel/sched/topology.c).
> 
> > And what about i < cpu cases?
> 
> Those values have already been passed to topology_span_sane(). This patch uses
> for_each_cpu_from() starting at cpu + 1 to prevent those duplicate comparisons.

So, it appears to me that commit message has a room to improve / elaborate based
on what you explained to me above.

Thanks!

-- 
With Best Regards,
Andy Shevchenko