[PATCH 0/2] sched/topology: optimize topology_span_sane()

[PATCH 0/2] sched/topology: optimize topology_span_sane()

[Yury Norov]

Pre-compute pre-compute topology_span_sane() loop params and optimize
the functtion to avoid calling cpumask_equal() when masks are the same.

This series follows up comments from here:

https://lore.kernel.org/lkml/ZqqV5OxZPHUgjhag@LeoBras/T/#md6b2b6bdd09e63740bbf010530211842a79b5f57


Yury Norov (2):
  sched/topology: pre-compute topology_span_sane() loop params
  sched/topology: optimize topology_span_sane()

 kernel/sched/topology.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

-- 
2.43.0

[PATCH 1/2] sched/topology: pre-compute topology_span_sane() loop params

[Yury Norov]

tl->mask(cpu) is used unchanged in the loop, and tl->mask(i) in worst
case may be calculated twice as parameters for cpumask_equal() and
cpumask_intersects(). So, precalculate both.

Suggested-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Suggested-by: Leonardo Bras <leobras@redhat.com>
Signed-off-by: Yury Norov <yury.norov@gmail.com>
---
 kernel/sched/topology.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 76504b776d03..754ad5fa3c99 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -2353,6 +2353,7 @@ 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)
 {
+	const struct cpumask *mi, *mc = tl->mask(cpu);
 	int i = cpu + 1;
 
 	/* NUMA levels are allowed to overlap */
@@ -2366,14 +2367,15 @@ static bool topology_span_sane(struct sched_domain_topology_level *tl,
 	 * breaks the linking done for an earlier span.
 	 */
 	for_each_cpu_from(i, cpu_map) {
+		mi = tl->mask(i);
+
 		/*
 		 * We should 'and' all those masks with 'cpu_map' to exactly
 		 * match the topology we're about to build, but that can only
 		 * remove CPUs, which only lessens our ability to detect
 		 * overlaps
 		 */
-		if (!cpumask_equal(tl->mask(cpu), tl->mask(i)) &&
-		    cpumask_intersects(tl->mask(cpu), tl->mask(i)))
+		if (!cpumask_equal(mc, mi) && cpumask_intersects(mc, mi))
 			return false;
 	}
 
-- 
2.43.0

Re: [PATCH 1/2] sched/topology: pre-compute topology_span_sane() loop params

[Chen Yu]

On 2024-08-02 at 10:57:42 -0700, Yury Norov wrote:
> tl->mask(cpu) is used unchanged in the loop, and tl->mask(i) in worst
> case may be calculated twice as parameters for cpumask_equal() and
> cpumask_intersects(). So, precalculate both.
> 
> Suggested-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
> Suggested-by: Leonardo Bras <leobras@redhat.com>
> Signed-off-by: Yury Norov <yury.norov@gmail.com>
> ---
>  kernel/sched/topology.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 76504b776d03..754ad5fa3c99 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -2353,6 +2353,7 @@ 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)
>  {
> +	const struct cpumask *mi, *mc = tl->mask(cpu);

Could it avoid the calculation by putting
mc = tl->mask(cpu) after the
if (tl->flags & SDTL_OVERLAP)?

Other than that,

Reviewed-by: Chen Yu <yu.c.chen@intel.com>

thanks,
Chenyu

[PATCH 2/2] sched/topology: optimize topology_span_sane()

[Yury Norov]

The function may call cpumask_equal with tl->mask(cpu) == tl->mask(i),
even when cpu != i. In such case, cpumask_equal() would always return
true, and we can proceed to the next iteration immediately.

Signed-off-by: Yury Norov <yury.norov@gmail.com>
---
 kernel/sched/topology.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 754ad5fa3c99..571759606954 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -2368,6 +2368,8 @@ static bool topology_span_sane(struct sched_domain_topology_level *tl,
 	 */
 	for_each_cpu_from(i, cpu_map) {
 		mi = tl->mask(i);
+		if (mi == mc)
+			continue;
 
 		/*
 		 * We should 'and' all those masks with 'cpu_map' to exactly
-- 
2.43.0

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

[Valentin Schneider]

On 02/08/24 10:57, Yury Norov wrote:
> The function may call cpumask_equal with tl->mask(cpu) == tl->mask(i),
> even when cpu != i.

For which architecture have you observed this? AFAIA all implementations of
tl->sched_domain_mask_f are built on a per-CPU cpumask.

e.g. for the default topology, we have:

  cpu_smt_mask() -> topology_sibling_cpumask()

which is implemented as:

  arch/loongarch/include/asm/topology.h:35:#define topology_sibling_cpumask(cpu)		(&cpu_sibling_map[cpu])
  arch/mips/include/asm/topology.h:18:#define topology_sibling_cpumask(cpu)		(&cpu_sibling_map[cpu])
  arch/powerpc/include/asm/topology.h:139:#define topology_sibling_cpumask(cpu)	(per_cpu(cpu_sibling_map, cpu))
  arch/s390/include/asm/topology.h:31:#define topology_sibling_cpumask(cpu)	  (&cpu_topology[cpu].thread_mask)
  arch/sparc/include/asm/topology_64.h:50:#define topology_sibling_cpumask(cpu)		(&per_cpu(cpu_sibling_map, cpu))
  arch/x86/include/asm/topology.h:186:#define topology_sibling_cpumask(cpu)		(per_cpu(cpu_sibling_map, cpu))
  include/linux/arch_topology.h:91:#define topology_sibling_cpumask(cpu)	(&cpu_topology[cpu].thread_sibling)
  include/linux/topology.h:218:#define topology_sibling_cpumask(cpu)		cpumask_of(cpu)

and ditto for cpu_coregroup_mask() & cpu_cpu_mask().

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

[Yury Norov]

On Tue, Aug 06, 2024 at 05:50:23PM +0200, Valentin Schneider wrote:
> On 02/08/24 10:57, Yury Norov wrote:
> > The function may call cpumask_equal with tl->mask(cpu) == tl->mask(i),
> > even when cpu != i.
> 
> For which architecture have you observed this? AFAIA all implementations of
> tl->sched_domain_mask_f are built on a per-CPU cpumask.

x86_64, qemu emulating 16 CPUs in 4 nodes, Linux 6.10, approximately
defconfig.

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

[Valentin Schneider]

On 06/08/24 11:00, Yury Norov wrote:
> On Tue, Aug 06, 2024 at 05:50:23PM +0200, Valentin Schneider wrote:
>> On 02/08/24 10:57, Yury Norov wrote:
>> > The function may call cpumask_equal with tl->mask(cpu) == tl->mask(i),
>> > even when cpu != i.
>>
>> For which architecture have you observed this? AFAIA all implementations of
>> tl->sched_domain_mask_f are built on a per-CPU cpumask.
>
> x86_64, qemu emulating 16 CPUs in 4 nodes, Linux 6.10, approximately
> defconfig.

For the default_topology:
cpu_smt_mask() # SMT
  (per_cpu(cpu_sibling_map, cpu))

cpu_clustergroup_mask() # CLS
  per_cpu(cpu_l2c_shared_map, cpu);

cpu_coregroup_mask() # MC
  per_cpu(cpu_llc_shared_map, cpu);

cpu_cpu_mask() # PKG
  cpumask_of_node(cpu_to_node(cpu));

Ok so PKG can potentially hit that condition, and so can any
sched_domain_mask_f that relies on the node masks...

I'm thinking ideally we should have checks in place to ensure all
node_to_cpumask_map[] masks are disjoint, then we could entirely skip the levels
that use these masks in topology_span_sane(), but there's unfortunately no
nice way to flag them... Also there would cases where there's no real
difference between PKG and NODE other than NODE is still based on a per-cpu
cpumask and PKG isn't, so I don't see a nicer way to go about this.

Please add something like the following to the changelog, and with that:
Reviewed-by: Valentin Schneider <vschneid@redhat.com>

"""
Some topology levels (e.g. PKG in default_topology[]) have a
sched_domain_mask_f implementation that reuses the same mask for several
CPUs (in PKG's case, one mask for all CPUs in the same NUMA node).

For such topology levels, repeating cpumask_equal() checks is wasteful -
check that the tl->mask(i) pointers aren't the same first.
"""

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

[Yury Norov]

On Wed, Aug 07, 2024 at 03:53:18PM +0200, Valentin Schneider wrote:
> On 06/08/24 11:00, Yury Norov wrote:
> > On Tue, Aug 06, 2024 at 05:50:23PM +0200, Valentin Schneider wrote:
> >> On 02/08/24 10:57, Yury Norov wrote:
> >> > The function may call cpumask_equal with tl->mask(cpu) == tl->mask(i),
> >> > even when cpu != i.
> >>
> >> For which architecture have you observed this? AFAIA all implementations of
> >> tl->sched_domain_mask_f are built on a per-CPU cpumask.
> >
> > x86_64, qemu emulating 16 CPUs in 4 nodes, Linux 6.10, approximately
> > defconfig.
> 
> For the default_topology:
> cpu_smt_mask() # SMT
>   (per_cpu(cpu_sibling_map, cpu))
> 
> cpu_clustergroup_mask() # CLS
>   per_cpu(cpu_l2c_shared_map, cpu);
> 
> cpu_coregroup_mask() # MC
>   per_cpu(cpu_llc_shared_map, cpu);
> 
> cpu_cpu_mask() # PKG
>   cpumask_of_node(cpu_to_node(cpu));
> 
> Ok so PKG can potentially hit that condition, and so can any
> sched_domain_mask_f that relies on the node masks...
> 
> I'm thinking ideally we should have checks in place to ensure all
> node_to_cpumask_map[] masks are disjoint, then we could entirely skip the levels
> that use these masks in topology_span_sane(), but there's unfortunately no
> nice way to flag them... Also there would cases where there's no real
> difference between PKG and NODE other than NODE is still based on a per-cpu
> cpumask and PKG isn't, so I don't see a nicer way to go about this.
> 
> Please add something like the following to the changelog, and with that:
> Reviewed-by: Valentin Schneider <vschneid@redhat.com>

Sure, will do.

> """
> Some topology levels (e.g. PKG in default_topology[]) have a
> sched_domain_mask_f implementation that reuses the same mask for several
> CPUs (in PKG's case, one mask for all CPUs in the same NUMA node).
> 
> For such topology levels, repeating cpumask_equal() checks is wasteful -
> check that the tl->mask(i) pointers aren't the same first.
> """

[PATCH v2 0/2] sched/topology: optimize topology_span_sane()

[Yury Norov]

The function may call cpumask_equal with tl->mask(cpu) == tl->mask(i),
even when cpu != i. In such case, cpumask_equal() would always return
true, and we can proceed to the next iteration immediately.

Valentin Schneider shares on it:

  PKG can potentially hit that condition, and so can any
  sched_domain_mask_f that relies on the node masks...
  
  I'm thinking ideally we should have checks in place to
  ensure all node_to_cpumask_map[] masks are disjoint,
  then we could entirely skip the levels that use these
  masks in topology_span_sane(), but there's unfortunately
  no nice way to flag them... Also there would be cases
  where there's no real difference between PKG and NODE
  other than NODE is still based on a per-cpu cpumask and
  PKG isn't, so I don't see a nicer way to go about this.

v1: https://lore.kernel.org/lkml/ZrJk00cmVaUIAr4G@yury-ThinkPad/T/
v2:
 - defer initialization of 'mc' in patch #1 @Chen Yu;
 - more comments from Valentin Schneider.


Yury Norov (2):
  sched/topology: pre-compute topology_span_sane() loop params
  sched/topology: optimize topology_span_sane()

 kernel/sched/topology.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

-- 
2.43.0

[PATCH 2/2] sched/topology: optimize topology_span_sane()

[Yury Norov]

The function may call cpumask_equal with tl->mask(cpu) == tl->mask(i),
even though cpu != i. In such case, cpumask_equal() would always return
true, and we can proceed to the next iteration immediately.

Comment is provided by Valentin Schneider.

Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Signed-off-by: Yury Norov <yury.norov@gmail.com>
---
 kernel/sched/topology.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 8af3b48da458..3661d4173d1f 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -2370,6 +2370,18 @@ static bool topology_span_sane(struct sched_domain_topology_level *tl,
 	 */
 	for_each_cpu_from(i, cpu_map) {
 		mi = tl->mask(i);
+		/*
+		 * Some topology levels (e.g. PKG in default_topology[])
+		 * have a sched_domain_mask_f implementation that reuses
+		 * the same mask for several CPUs (in PKG's case, one mask
+		 * for all CPUs in the same NUMA node).
+		 *
+		 * For such topology levels, repeating cpumask_equal()
+		 * checks is wasteful. Instead, we first check that the
+		 * tl->mask(i) pointers aren't the same.
+		 */
+		if (mi == mc)
+			continue;
 
 		/*
 		 * We should 'and' all those masks with 'cpu_map' to exactly
-- 
2.43.0

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

[Yury Norov]

On Tue, Mar 19, 2024 at 01:51:48PM -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).
> 
> 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