Re: [PATCH 1/2] cpumask: Add for_each_cpu_from()

Re: [PATCH 1/2] cpumask: Add for_each_cpu_from()

[Yury Norov]

On Tue, Mar 19, 2024 at 01:51:47PM -0500, Kyle Meyer wrote:
> Add for_each_cpu_from() as a generic cpumask macro.
> 
> for_each_cpu_from() is the same as for_each_cpu(), except it starts at
> @cpu instead of zero.
> 
> Signed-off-by: Kyle Meyer <kyle.meyer@hpe.com>

Acked-by: Kyle Meyer <kyle.meyer@hpe.com>

> ---
>  include/linux/cpumask.h | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
> index 1c29947db848..655211db38ff 100644
> --- a/include/linux/cpumask.h
> +++ b/include/linux/cpumask.h
> @@ -368,6 +368,16 @@ unsigned int __pure cpumask_next_wrap(int n, const struct cpumask *mask, int sta
>  #define for_each_cpu_or(cpu, mask1, mask2)				\
>  	for_each_or_bit(cpu, cpumask_bits(mask1), cpumask_bits(mask2), small_cpumask_bits)
>  
> +/**
> + * for_each_cpu_from - iterate over every cpu present in @mask, starting at @cpu
> + * @cpu: the (optionally unsigned) integer iterator
> + * @mask: the cpumask pointer
> + *
> + * After the loop, cpu is >= nr_cpu_ids.
> + */
> +#define for_each_cpu_from(cpu, mask)				\
> +	for_each_set_bit_from(cpu, cpumask_bits(mask), small_cpumask_bits)
> +
>  /**
>   * cpumask_any_but - return a "random" in a cpumask, but not this one.
>   * @mask: the cpumask to search
> -- 
> 2.44.0

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

Re: [PATCH 1/2] cpumask: Add for_each_cpu_from()

[Yury Norov]

On Wed, Mar 20, 2024 at 11:31:18AM -0700, Yury Norov wrote:
> On Tue, Mar 19, 2024 at 01:51:47PM -0500, Kyle Meyer wrote:
> > Add for_each_cpu_from() as a generic cpumask macro.
> > 
> > for_each_cpu_from() is the same as for_each_cpu(), except it starts at
> > @cpu instead of zero.
> > 
> > Signed-off-by: Kyle Meyer <kyle.meyer@hpe.com>
> 
> Acked-by: Kyle Meyer <kyle.meyer@hpe.com>

Sorry, please read: 

Acked-by: Yury Norov <yury.norov@gmail.com>

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

[Valentin Schneider]

On 19/03/24 13:51, Kyle Meyer wrote:
> A soft lockup is being detected in build_sched_domains() on 32 socket
> Sapphire Rapids systems with 3840 processors.
>
> topology_span_sane(), called by build_sched_domains(), checks that each
> processor's non-NUMA scheduling domains are completely equal or
> completely disjoint. If a non-NUMA scheduling domain partially overlaps
> another, scheduling groups can break.
>
> This series adds for_each_cpu_from() as a generic cpumask macro to
> 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), decreasing the
> boot time by approximately 20 seconds and preventing the soft lockup on
> the mentioned systems.
>
> Kyle Meyer (2):
>   cpumask: Add for_each_cpu_from()
>   sched/topology: Optimize topology_span_sane()

I somehow never got 2/2, and it doesn't show up on lore.kernel.org
either. I can see it from Yury's reply and it looks OK to me, but you'll
have to resend it for maintainers to be able to pick it up.

>
>  include/linux/cpumask.h | 10 ++++++++++
>  kernel/sched/topology.c |  6 ++----
>  2 files changed, 12 insertions(+), 4 deletions(-)
>
> -- 
> 2.44.0

[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 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 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