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

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

[Kyle Meyer]

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 on each non-NUMA scheduling domain level, decreasing
the boot time by approximately 20 seconds and preventing the soft lockup
on the mentioned systems.

Changes in v2:
  * 1/2: Change for_each_cpu()'s description.
  * 2/2: Add more information to the commit message.
  * https://lore.kernel.org/linux-kernel/20240409155250.3660517-1-kyle.meyer@hpe.com/T/

Kyle Meyer (2):
  cpumask: Add for_each_cpu_from()
  sched/topology: Optimize topology_span_sane()

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

-- 
2.44.0

[PATCH v2 1/2] cpumask: Add for_each_cpu_from()

[Kyle Meyer]

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: Yury Norov <yury.norov@gmail.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..d75060fbd058 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 CPUs present in @mask, from @cpu to the end of @mask.
+ * @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 v2 1/2] cpumask: Add for_each_cpu_from()

[Valentin Schneider]

On 10/04/24 16:33, 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: Yury Norov <yury.norov@gmail.com>

Reviewed-by: Valentin Schneider <vschneid@redhat.com>

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

[Kyle Meyer]

Optimize topology_span_sane() by removing duplicate comparisons.

Since topology_span_sane() is called inside of for_each_cpu(), each
pervious CPU has already been compared against every other CPU. The
current CPU only needs to be compared against higher-numbered CPUs.

The total number of comparisons is reduced from N * (N - 1) to
N * (N - 1) / 2 on each 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 v2 2/2] sched/topology: Optimize topology_span_sane()

[Andy Shevchenko]

On Wed, Apr 10, 2024 at 04:33:11PM -0500, Kyle Meyer wrote:
> Optimize topology_span_sane() by removing duplicate comparisons.
> 
> Since topology_span_sane() is called inside of for_each_cpu(), each
> pervious CPU has already been compared against every other CPU. The

previous

> current CPU only needs to be compared against higher-numbered CPUs.
> 
> The total number of comparisons is reduced from N * (N - 1) to
> N * (N - 1) / 2 on each non-NUMA scheduling domain level.

Thank you, now it makes sense.

-- 
With Best Regards,
Andy Shevchenko

Re: [PATCH v2 2/2] sched/topology: Optimize topology_span_sane()

[Kyle Meyer]

On Thu, Apr 11, 2024 at 01:38:46PM +0300, Andy Shevchenko wrote:
> On Wed, Apr 10, 2024 at 04:33:11PM -0500, Kyle Meyer wrote:
> > Optimize topology_span_sane() by removing duplicate comparisons.
> > 
> > Since topology_span_sane() is called inside of for_each_cpu(), each
> > pervious CPU has already been compared against every other CPU. The
> 
> previous

Thank you for pointing that out. Should I send an updated version or can
a maintainer correct my mistake?

> > current CPU only needs to be compared against higher-numbered CPUs.
> > 
> > The total number of comparisons is reduced from N * (N - 1) to
> > N * (N - 1) / 2 on each non-NUMA scheduling domain level.
> 
> Thank you, now it makes sense.

Thanks,
Kyle Meyer

Re: [PATCH v2 2/2] sched/topology: Optimize topology_span_sane()

[Andy Shevchenko]

On Thu, Apr 11, 2024 at 04:55:11PM -0500, Kyle Meyer wrote:
> On Thu, Apr 11, 2024 at 01:38:46PM +0300, Andy Shevchenko wrote:
> > On Wed, Apr 10, 2024 at 04:33:11PM -0500, Kyle Meyer wrote:

...

> > > Since topology_span_sane() is called inside of for_each_cpu(), each
> > > pervious CPU has already been compared against every other CPU. The
> > 
> > previous
> 
> Thank you for pointing that out. Should I send an updated version or can
> a maintainer correct my mistake?

Depends on the maintainer. I'm not the one here, don't expect answer from me.

> > > current CPU only needs to be compared against higher-numbered CPUs.

-- 
With Best Regards,
Andy Shevchenko

Re: [PATCH v2 2/2] sched/topology: Optimize topology_span_sane()

[Yury Norov]

On Fri, Apr 12, 2024 at 05:25:26PM +0300, Andy Shevchenko wrote:
> On Thu, Apr 11, 2024 at 04:55:11PM -0500, Kyle Meyer wrote:
> > On Thu, Apr 11, 2024 at 01:38:46PM +0300, Andy Shevchenko wrote:
> > > On Wed, Apr 10, 2024 at 04:33:11PM -0500, Kyle Meyer wrote:
> 
> ...
> 
> > > > Since topology_span_sane() is called inside of for_each_cpu(), each
> > > > pervious CPU has already been compared against every other CPU. The
> > > 
> > > previous
> > 
> > Thank you for pointing that out. Should I send an updated version or can
> > a maintainer correct my mistake?
> 
> Depends on the maintainer. I'm not the one here, don't expect answer from me.

I like this rework, and I'll take it in bitmap-for-next for testing.

I see Vincent already acked it, so I can also move it upstream, if no
objections from Peter, Juri and Ingo.

Thanks,
Yury

Re: [PATCH v2 2/2] sched/topology: Optimize topology_span_sane()

[Valentin Schneider]

On 10/04/24 16:33, Kyle Meyer wrote:
> Optimize topology_span_sane() by removing duplicate comparisons.
>
> Since topology_span_sane() is called inside of for_each_cpu(), each
> pervious CPU has already been compared against every other CPU. The
> current CPU only needs to be compared against higher-numbered CPUs.
>
> The total number of comparisons is reduced from N * (N - 1) to
> N * (N - 1) / 2 on each 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>

Reviewed-by: Valentin Schneider <vschneid@redhat.com>