[PATCH v5 0/2] Adjust NUMA imbalance for multiple LLCs

[PATCH v5 0/2] Adjust NUMA imbalance for multiple LLCs

[Mel Gorman]

Changelog since V4
o Scale imbalance based on the top domain that prefers siblings
o Keep allowed imbalance as 2 up to the point where LLCs can be overloaded

Changelog since V3
o Calculate imb_numa_nr for multiple SD_NUMA domains
o Restore behaviour where communicating pairs remain on the same node

Commit 7d2b5dd0bcc4 ("sched/numa: Allow a floating imbalance between NUMA
nodes") allowed an imbalance between NUMA nodes such that communicating
tasks would not be pulled apart by the load balancer. This works fine when
there is a 1:1 relationship between LLC and node but can be suboptimal
for multiple LLCs if independent tasks prematurely use CPUs sharing cache.

The series addresses two problems -- inconsistent logic when allowing a
NUMA imbalance and sub-optimal performance when there are many LLCs per
NUMA node.

 include/linux/sched/topology.h |  1 +
 kernel/sched/fair.c            | 30 ++++++++++---------
 kernel/sched/topology.c        | 53 ++++++++++++++++++++++++++++++++++
 3 files changed, 71 insertions(+), 13 deletions(-)

-- 
2.31.1

Mel Gorman (2):
  sched/fair: Improve consistency of allowed NUMA balance calculations
  sched/fair: Adjust the allowed NUMA imbalance when SD_NUMA spans
    multiple LLCs

 include/linux/sched/topology.h |  1 +
 kernel/sched/fair.c            | 30 ++++++++++---------
 kernel/sched/topology.c        | 53 ++++++++++++++++++++++++++++++++++
 3 files changed, 71 insertions(+), 13 deletions(-)

-- 
2.31.1

[PATCH 1/2] sched/fair: Improve consistency of allowed NUMA balance calculations

[Mel Gorman]

There are inconsistencies when determining if a NUMA imbalance is allowed
that should be corrected.

o allow_numa_imbalance changes types and is not always examining
  the destination group so both the type should be corrected as
  well as the naming.
o find_idlest_group uses the sched_domain's weight instead of the
  group weight which is different to find_busiest_group
o find_busiest_group uses the source group instead of the destination
  which is different to task_numa_find_cpu
o Both find_idlest_group and find_busiest_group should account
  for the number of running tasks if a move was allowed to be
  consistent with task_numa_find_cpu

Fixes: 7d2b5dd0bcc4 ("sched/numa: Allow a floating imbalance between NUMA nodes")
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
---
 kernel/sched/fair.c | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 095b0aa378df..4592ccf82c34 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9003,9 +9003,10 @@ static bool update_pick_idlest(struct sched_group *idlest,
  * This is an approximation as the number of running tasks may not be
  * related to the number of busy CPUs due to sched_setaffinity.
  */
-static inline bool allow_numa_imbalance(int dst_running, int dst_weight)
+static inline bool
+allow_numa_imbalance(unsigned int running, unsigned int weight)
 {
-	return (dst_running < (dst_weight >> 2));
+	return (running < (weight >> 2));
 }
 
 /*
@@ -9139,12 +9140,13 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu)
 				return idlest;
 #endif
 			/*
-			 * Otherwise, keep the task on this node to stay close
-			 * its wakeup source and improve locality. If there is
-			 * a real need of migration, periodic load balance will
-			 * take care of it.
+			 * Otherwise, keep the task close to the wakeup source
+			 * and improve locality if the number of running tasks
+			 * would remain below threshold where an imbalance is
+			 * allowed. If there is a real need of migration,
+			 * periodic load balance will take care of it.
 			 */
-			if (allow_numa_imbalance(local_sgs.sum_nr_running, sd->span_weight))
+			if (allow_numa_imbalance(local_sgs.sum_nr_running + 1, local_sgs.group_weight))
 				return NULL;
 		}
 
@@ -9350,7 +9352,7 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
 		/* Consider allowing a small imbalance between NUMA groups */
 		if (env->sd->flags & SD_NUMA) {
 			env->imbalance = adjust_numa_imbalance(env->imbalance,
-				busiest->sum_nr_running, busiest->group_weight);
+				local->sum_nr_running + 1, local->group_weight);
 		}
 
 		return;
-- 
2.31.1

[PATCH 2/2] sched/fair: Adjust the allowed NUMA imbalance when SD_NUMA spans multiple LLCs

[Mel Gorman]

Commit 7d2b5dd0bcc4 ("sched/numa: Allow a floating imbalance between NUMA
nodes") allowed an imbalance between NUMA nodes such that communicating
tasks would not be pulled apart by the load balancer. This works fine when
there is a 1:1 relationship between LLC and node but can be suboptimal
for multiple LLCs if independent tasks prematurely use CPUs sharing cache.

Zen* has multiple LLCs per node with local memory channels and due to
the allowed imbalance, it's far harder to tune some workloads to run
optimally than it is on hardware that has 1 LLC per node. This patch
allows an imbalance to exist up to the point where LLCs should be balanced
between nodes.

On a Zen3 machine running STREAM parallelised with OMP to have on instance
per LLC the results and without binding, the results are

                            5.17.0-rc0             5.17.0-rc0
                               vanilla       sched-numaimb-v5
MB/sec copy-16    162596.94 (   0.00%)   501967.12 ( 208.72%)
MB/sec scale-16   136901.28 (   0.00%)   376531.50 ( 175.04%)
MB/sec add-16     157300.70 (   0.00%)   569997.42 ( 262.36%)
MB/sec triad-16   151446.88 (   0.00%)   553204.54 ( 265.28%)

STREAM can use directives to force the spread if the OpenMP is new
enough but that doesn't help if an application uses threads and
it's not known in advance how many threads will be created.

Coremark is a CPU and cache intensive benchmark parallelised with
threads. When running with 1 thread per core, the vanilla kernel
allows threads to contend on cache. With the patch;

                               5.17.0-rc0             5.17.0-rc0
                                  vanilla       sched-numaimb-v5
Min       Score-16   368239.36 (   0.00%)   400876.92 (   8.86%)
Hmean     Score-16   388607.33 (   0.00%)   441447.30 *  13.60%*
Max       Score-16   408945.69 (   0.00%)   478826.87 (  17.09%)
Stddev    Score-16    15247.04 (   0.00%)    34061.76 (-123.40%)
CoeffVar  Score-16        3.92 (   0.00%)        7.67 ( -95.82%)

It can also make a big difference for semi-realistic workloads
like specjbb which can execute arbitrary numbers of threads without
advance knowledge of how they should be placed

                               5.17.0-rc0             5.17.0-rc0
                                  vanilla       sched-numaimb-v5
Hmean     tput-1      71631.55 (   0.00%)    70383.46 (  -1.74%)
Hmean     tput-8     582758.78 (   0.00%)   607290.89 *   4.21%*
Hmean     tput-16   1020372.75 (   0.00%)  1031257.25 (   1.07%)
Hmean     tput-24   1416430.67 (   0.00%)  1587576.33 *  12.08%*
Hmean     tput-32   1687702.72 (   0.00%)  1724207.51 (   2.16%)
Hmean     tput-40   1798094.90 (   0.00%)  1983053.56 *  10.29%*
Hmean     tput-48   1972731.77 (   0.00%)  2157461.70 (   9.36%)
Hmean     tput-56   2386872.38 (   0.00%)  2193237.42 (  -8.11%)
Hmean     tput-64   2536954.17 (   0.00%)  2588741.08 (   2.04%)
Hmean     tput-72   2585071.36 (   0.00%)  2654776.36 (   2.70%)
Hmean     tput-80   2960523.94 (   0.00%)  2894657.12 (  -2.22%)
Hmean     tput-88   3061408.57 (   0.00%)  2903167.72 (  -5.17%)
Hmean     tput-96   3052394.82 (   0.00%)  2872605.46 (  -5.89%)
Hmean     tput-104  2997814.76 (   0.00%)  3013660.26 (   0.53%)
Hmean     tput-112  2955353.29 (   0.00%)  3029122.16 (   2.50%)
Hmean     tput-120  2889770.71 (   0.00%)  2957739.88 (   2.35%)
Hmean     tput-128  2871713.84 (   0.00%)  2912410.18 (   1.42%)

In general, the standard deviation figures also are a lot more
stable.

Similarly, for embarassingly parallel problems like NPB-ep, there are
improvements due to better spreading across LLC when the machine is not
fully utilised.

                              vanilla    sched-numaimb-v5r12
Min       ep.D       31.79 (   0.00%)       26.11 (  17.87%)
Amean     ep.D       31.86 (   0.00%)       26.26 *  17.58%*
Stddev    ep.D        0.07 (   0.00%)        0.18 (-157.54%)
CoeffVar  ep.D        0.22 (   0.00%)        0.69 (-212.46%)
Max       ep.D       31.93 (   0.00%)       26.46 (  17.13%)

Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
---
 include/linux/sched/topology.h |  1 +
 kernel/sched/fair.c            | 22 +++++++-------
 kernel/sched/topology.c        | 53 ++++++++++++++++++++++++++++++++++
 3 files changed, 66 insertions(+), 10 deletions(-)

diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
index 8054641c0a7b..56cffe42abbc 100644
--- a/include/linux/sched/topology.h
+++ b/include/linux/sched/topology.h
@@ -93,6 +93,7 @@ struct sched_domain {
 	unsigned int busy_factor;	/* less balancing by factor if busy */
 	unsigned int imbalance_pct;	/* No balance until over watermark */
 	unsigned int cache_nice_tries;	/* Leave cache hot tasks for # tries */
+	unsigned int imb_numa_nr;	/* Nr running tasks that allows a NUMA imbalance */
 
 	int nohz_idle;			/* NOHZ IDLE status */
 	int flags;			/* See SD_* */
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 4592ccf82c34..86abf97a8df6 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1489,6 +1489,7 @@ struct task_numa_env {
 
 	int src_cpu, src_nid;
 	int dst_cpu, dst_nid;
+	int imb_numa_nr;
 
 	struct numa_stats src_stats, dst_stats;
 
@@ -1503,7 +1504,7 @@ struct task_numa_env {
 static unsigned long cpu_load(struct rq *rq);
 static unsigned long cpu_runnable(struct rq *rq);
 static inline long adjust_numa_imbalance(int imbalance,
-					int dst_running, int dst_weight);
+					int dst_running, int imb_numa_nr);
 
 static inline enum
 numa_type numa_classify(unsigned int imbalance_pct,
@@ -1884,7 +1885,7 @@ static void task_numa_find_cpu(struct task_numa_env *env,
 		dst_running = env->dst_stats.nr_running + 1;
 		imbalance = max(0, dst_running - src_running);
 		imbalance = adjust_numa_imbalance(imbalance, dst_running,
-							env->dst_stats.weight);
+						  env->imb_numa_nr);
 
 		/* Use idle CPU if there is no imbalance */
 		if (!imbalance) {
@@ -1949,8 +1950,10 @@ static int task_numa_migrate(struct task_struct *p)
 	 */
 	rcu_read_lock();
 	sd = rcu_dereference(per_cpu(sd_numa, env.src_cpu));
-	if (sd)
+	if (sd) {
 		env.imbalance_pct = 100 + (sd->imbalance_pct - 100) / 2;
+		env.imb_numa_nr = sd->imb_numa_nr;
+	}
 	rcu_read_unlock();
 
 	/*
@@ -9003,10 +9006,9 @@ static bool update_pick_idlest(struct sched_group *idlest,
  * This is an approximation as the number of running tasks may not be
  * related to the number of busy CPUs due to sched_setaffinity.
  */
-static inline bool
-allow_numa_imbalance(unsigned int running, unsigned int weight)
+static inline bool allow_numa_imbalance(int running, int imb_numa_nr)
 {
-	return (running < (weight >> 2));
+	return running < imb_numa_nr;
 }
 
 /*
@@ -9146,7 +9148,7 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu)
 			 * allowed. If there is a real need of migration,
 			 * periodic load balance will take care of it.
 			 */
-			if (allow_numa_imbalance(local_sgs.sum_nr_running + 1, local_sgs.group_weight))
+			if (allow_numa_imbalance(local_sgs.sum_nr_running + 1, sd->imb_numa_nr))
 				return NULL;
 		}
 
@@ -9238,9 +9240,9 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
 #define NUMA_IMBALANCE_MIN 2
 
 static inline long adjust_numa_imbalance(int imbalance,
-				int dst_running, int dst_weight)
+				int dst_running, int imb_numa_nr)
 {
-	if (!allow_numa_imbalance(dst_running, dst_weight))
+	if (!allow_numa_imbalance(dst_running, imb_numa_nr))
 		return imbalance;
 
 	/*
@@ -9352,7 +9354,7 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
 		/* Consider allowing a small imbalance between NUMA groups */
 		if (env->sd->flags & SD_NUMA) {
 			env->imbalance = adjust_numa_imbalance(env->imbalance,
-				local->sum_nr_running + 1, local->group_weight);
+				local->sum_nr_running + 1, env->sd->imb_numa_nr);
 		}
 
 		return;
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index d201a7052a29..e6cd55951304 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -2242,6 +2242,59 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
 		}
 	}
 
+	/*
+	 * Calculate an allowed NUMA imbalance such that LLCs do not get
+	 * imbalanced.
+	 */
+	for_each_cpu(i, cpu_map) {
+		unsigned int imb = 0;
+		unsigned int imb_span = 1;
+
+		for (sd = *per_cpu_ptr(d.sd, i); sd; sd = sd->parent) {
+			struct sched_domain *child = sd->child;
+
+			if (!(sd->flags & SD_SHARE_PKG_RESOURCES) && child &&
+			    (child->flags & SD_SHARE_PKG_RESOURCES)) {
+				struct sched_domain *top, *top_p;
+				unsigned int nr_llcs;
+
+				/*
+				 * For a single LLC per node, allow an
+				 * imbalance up to 25% of the node. This is an
+				 * arbitrary cutoff based on SMT-2 to balance
+				 * between memory bandwidth and avoiding
+				 * premature sharing of HT resources and SMT-4
+				 * or SMT-8 *may* benefit from a different
+				 * cutoff.
+				 *
+				 * For multiple LLCs, allow an imbalance
+				 * until multiple tasks would share an LLC
+				 * on one node while LLCs on another node
+				 * remain idle.
+				 */
+				nr_llcs = sd->span_weight / child->span_weight;
+				if (nr_llcs == 1)
+					imb = sd->span_weight >> 2;
+				else
+					imb = nr_llcs;
+				sd->imb_numa_nr = imb;
+
+				/* Set span based on the first NUMA domain. */
+				top = sd;
+				top_p = top->parent;
+				while (top_p && !(top_p->flags & SD_NUMA)) {
+					top = top->parent;
+					top_p = top->parent;
+				}
+				imb_span = top_p ? top_p->span_weight : sd->span_weight;
+			} else {
+				int factor = max(1U, (sd->span_weight / imb_span));
+
+				sd->imb_numa_nr = imb * factor;
+			}
+		}
+	}
+
 	/* Calculate CPU capacity for physical packages and nodes */
 	for (i = nr_cpumask_bits-1; i >= 0; i--) {
 		if (!cpumask_test_cpu(i, cpu_map))
-- 
2.31.1

Re: [PATCH 2/2] sched/fair: Adjust the allowed NUMA imbalance when SD_NUMA spans multiple LLCs

[Srikar Dronamraju]

* Mel Gorman <mgorman@techsingularity.net> [2022-02-03 14:46:52]:

> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index d201a7052a29..e6cd55951304 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -2242,6 +2242,59 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
>  		}
>  	}
> 
> +	/*
> +	 * Calculate an allowed NUMA imbalance such that LLCs do not get
> +	 * imbalanced.
> +	 */

We seem to adding this hunk before the sched_domains may be degenerated.
Wondering if we really want to do it before degeneration.

Let say we have 3 sched domains and we calculated the sd->imb_numa_nr for
all the 3 domains, then lets say the middle sched_domain gets degenerated. 
Would the sd->imb_numa_nr's still be relevant?


> +	for_each_cpu(i, cpu_map) {
> +		unsigned int imb = 0;
> +		unsigned int imb_span = 1;
> +
> +		for (sd = *per_cpu_ptr(d.sd, i); sd; sd = sd->parent) {
> +			struct sched_domain *child = sd->child;
> +
> +			if (!(sd->flags & SD_SHARE_PKG_RESOURCES) && child &&
> +			    (child->flags & SD_SHARE_PKG_RESOURCES)) {
> +				struct sched_domain *top, *top_p;
> +				unsigned int nr_llcs;
> +
> +				/*
> +				 * For a single LLC per node, allow an
> +				 * imbalance up to 25% of the node. This is an
> +				 * arbitrary cutoff based on SMT-2 to balance
> +				 * between memory bandwidth and avoiding
> +				 * premature sharing of HT resources and SMT-4
> +				 * or SMT-8 *may* benefit from a different
> +				 * cutoff.
> +				 *
> +				 * For multiple LLCs, allow an imbalance
> +				 * until multiple tasks would share an LLC
> +				 * on one node while LLCs on another node
> +				 * remain idle.
> +				 */
> +				nr_llcs = sd->span_weight / child->span_weight;
> +				if (nr_llcs == 1)
> +					imb = sd->span_weight >> 2;
> +				else
> +					imb = nr_llcs;
> +				sd->imb_numa_nr = imb;
> +
> +				/* Set span based on the first NUMA domain. */
> +				top = sd;
> +				top_p = top->parent;
> +				while (top_p && !(top_p->flags & SD_NUMA)) {
> +					top = top->parent;
> +					top_p = top->parent;
> +				}
> +				imb_span = top_p ? top_p->span_weight : sd->span_weight;

I am getting confused by imb_span.
Let say we have a topology of SMT -> MC -> DIE -> NUMA -> NUMA, with SMT and
MC domains having SD_SHARE_PKG_RESOURCES flag set.
We come here only for DIE domain.

imb_span set here is being used for both the subsequent sched domains
most likely they will be NUMA domains. Right?

> +			} else {
> +				int factor = max(1U, (sd->span_weight / imb_span));
> +
> +				sd->imb_numa_nr = imb * factor;

For SMT, (or any sched domains below the llcs) factor would be
sd->span_weight but imb_numa_nr and imb would be 0.
For NUMA (or any sched domain just above DIE), factor would be
sd->imb_numa_nr would be nr_llcs.
For subsequent sched_domains, the sd->imb_numa_nr would be some multiple of
nr_llcs. Right?


> +			}
> +		}
> +	}
> +
>  	/* Calculate CPU capacity for physical packages and nodes */
>  	for (i = nr_cpumask_bits-1; i >= 0; i--) {
>  		if (!cpumask_test_cpu(i, cpu_map))
> -- 
> 2.31.1
> 

-- 
Thanks and Regards
Srikar Dronamraju

Re: [PATCH 2/2] sched/fair: Adjust the allowed NUMA imbalance when SD_NUMA spans multiple LLCs

[Mel Gorman]

On Fri, Feb 04, 2022 at 12:36:54PM +0530, Srikar Dronamraju wrote:
> * Mel Gorman <mgorman@techsingularity.net> [2022-02-03 14:46:52]:
> 
> > diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> > index d201a7052a29..e6cd55951304 100644
> > --- a/kernel/sched/topology.c
> > +++ b/kernel/sched/topology.c
> > @@ -2242,6 +2242,59 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
> >  		}
> >  	}
> > 
> > +	/*
> > +	 * Calculate an allowed NUMA imbalance such that LLCs do not get
> > +	 * imbalanced.
> > +	 */
> 
> We seem to adding this hunk before the sched_domains may be degenerated.
> Wondering if we really want to do it before degeneration.
> 

There was no obvious advantage versus doing it at the same time
characteristics like groups were being determined.

> Let say we have 3 sched domains and we calculated the sd->imb_numa_nr for
> all the 3 domains, then lets say the middle sched_domain gets degenerated. 
> Would the sd->imb_numa_nr's still be relevant?
> 

It's expected that it is still relevant as the ratios with respect to
SD_SHARE_PKG_RESOURCES should still be consistent.

> 
> > +	for_each_cpu(i, cpu_map) {
> > +		unsigned int imb = 0;
> > +		unsigned int imb_span = 1;
> > +
> > +		for (sd = *per_cpu_ptr(d.sd, i); sd; sd = sd->parent) {
> > +			struct sched_domain *child = sd->child;
> > +
> > +			if (!(sd->flags & SD_SHARE_PKG_RESOURCES) && child &&
> > +			    (child->flags & SD_SHARE_PKG_RESOURCES)) {
> > +				struct sched_domain *top, *top_p;
> > +				unsigned int nr_llcs;
> > +
> > +				/*
> > +				 * For a single LLC per node, allow an
> > +				 * imbalance up to 25% of the node. This is an
> > +				 * arbitrary cutoff based on SMT-2 to balance
> > +				 * between memory bandwidth and avoiding
> > +				 * premature sharing of HT resources and SMT-4
> > +				 * or SMT-8 *may* benefit from a different
> > +				 * cutoff.
> > +				 *
> > +				 * For multiple LLCs, allow an imbalance
> > +				 * until multiple tasks would share an LLC
> > +				 * on one node while LLCs on another node
> > +				 * remain idle.
> > +				 */
> > +				nr_llcs = sd->span_weight / child->span_weight;
> > +				if (nr_llcs == 1)
> > +					imb = sd->span_weight >> 2;
> > +				else
> > +					imb = nr_llcs;
> > +				sd->imb_numa_nr = imb;
> > +
> > +				/* Set span based on the first NUMA domain. */
> > +				top = sd;
> > +				top_p = top->parent;
> > +				while (top_p && !(top_p->flags & SD_NUMA)) {
> > +					top = top->parent;
> > +					top_p = top->parent;
> > +				}
> > +				imb_span = top_p ? top_p->span_weight : sd->span_weight;
> 
> I am getting confused by imb_span.
> Let say we have a topology of SMT -> MC -> DIE -> NUMA -> NUMA, with SMT and
> MC domains having SD_SHARE_PKG_RESOURCES flag set.
> We come here only for DIE domain.
> 
> imb_span set here is being used for both the subsequent sched domains
> most likely they will be NUMA domains. Right?
> 

Right.

> > +			} else {
> > +				int factor = max(1U, (sd->span_weight / imb_span));
> > +
> > +				sd->imb_numa_nr = imb * factor;
> 
> For SMT, (or any sched domains below the llcs) factor would be
> sd->span_weight but imb_numa_nr and imb would be 0.

Yes.

> For NUMA (or any sched domain just above DIE), factor would be
> sd->imb_numa_nr would be nr_llcs.
> For subsequent sched_domains, the sd->imb_numa_nr would be some multiple of
> nr_llcs. Right?
> 

Right.

-- 
Mel Gorman
SUSE Labs

Re: [PATCH 2/2] sched/fair: Adjust the allowed NUMA imbalance when SD_NUMA spans multiple LLCs

[Nayak, KPrateek (K Prateek)]

Hello Mel,

On 2/3/2022 8:16 PM, Mel Gorman wrote:
> Commit 7d2b5dd0bcc4 ("sched/numa: Allow a floating imbalance between NUMA
> nodes") allowed an imbalance between NUMA nodes such that communicating
> tasks would not be pulled apart by the load balancer. This works fine when
> there is a 1:1 relationship between LLC and node but can be suboptimal
> for multiple LLCs if independent tasks prematurely use CPUs sharing cache.
> 
> Zen* has multiple LLCs per node with local memory channels and due to
> the allowed imbalance, it's far harder to tune some workloads to run
> optimally than it is on hardware that has 1 LLC per node. This patch
> allows an imbalance to exist up to the point where LLCs should be balanced
> between nodes.
> 
> On a Zen3 machine running STREAM parallelised with OMP to have on instance
> per LLC the results and without binding, the results are
> 
>                             5.17.0-rc0             5.17.0-rc0
>                                vanilla       sched-numaimb-v5
> MB/sec copy-16    162596.94 (   0.00%)   501967.12 ( 208.72%)
> MB/sec scale-16   136901.28 (   0.00%)   376531.50 ( 175.04%)
> MB/sec add-16     157300.70 (   0.00%)   569997.42 ( 262.36%)
> MB/sec triad-16   151446.88 (   0.00%)   553204.54 ( 265.28%)
> 
> STREAM can use directives to force the spread if the OpenMP is new
> enough but that doesn't help if an application uses threads and
> it's not known in advance how many threads will be created.
> 
> Coremark is a CPU and cache intensive benchmark parallelised with
> threads. When running with 1 thread per core, the vanilla kernel
> allows threads to contend on cache. With the patch;
> 
>                                5.17.0-rc0             5.17.0-rc0
>                                   vanilla       sched-numaimb-v5
> Min       Score-16   368239.36 (   0.00%)   400876.92 (   8.86%)
> Hmean     Score-16   388607.33 (   0.00%)   441447.30 *  13.60%*
> Max       Score-16   408945.69 (   0.00%)   478826.87 (  17.09%)
> Stddev    Score-16    15247.04 (   0.00%)    34061.76 (-123.40%)
> CoeffVar  Score-16        3.92 (   0.00%)        7.67 ( -95.82%)
> 
> It can also make a big difference for semi-realistic workloads
> like specjbb which can execute arbitrary numbers of threads without
> advance knowledge of how they should be placed
> 
>                                5.17.0-rc0             5.17.0-rc0
>                                   vanilla       sched-numaimb-v5
> Hmean     tput-1      71631.55 (   0.00%)    70383.46 (  -1.74%)
> Hmean     tput-8     582758.78 (   0.00%)   607290.89 *   4.21%*
> Hmean     tput-16   1020372.75 (   0.00%)  1031257.25 (   1.07%)
> Hmean     tput-24   1416430.67 (   0.00%)  1587576.33 *  12.08%*
> Hmean     tput-32   1687702.72 (   0.00%)  1724207.51 (   2.16%)
> Hmean     tput-40   1798094.90 (   0.00%)  1983053.56 *  10.29%*
> Hmean     tput-48   1972731.77 (   0.00%)  2157461.70 (   9.36%)
> Hmean     tput-56   2386872.38 (   0.00%)  2193237.42 (  -8.11%)
> Hmean     tput-64   2536954.17 (   0.00%)  2588741.08 (   2.04%)
> Hmean     tput-72   2585071.36 (   0.00%)  2654776.36 (   2.70%)
> Hmean     tput-80   2960523.94 (   0.00%)  2894657.12 (  -2.22%)
> Hmean     tput-88   3061408.57 (   0.00%)  2903167.72 (  -5.17%)
> Hmean     tput-96   3052394.82 (   0.00%)  2872605.46 (  -5.89%)
> Hmean     tput-104  2997814.76 (   0.00%)  3013660.26 (   0.53%)
> Hmean     tput-112  2955353.29 (   0.00%)  3029122.16 (   2.50%)
> Hmean     tput-120  2889770.71 (   0.00%)  2957739.88 (   2.35%)
> Hmean     tput-128  2871713.84 (   0.00%)  2912410.18 (   1.42%)
> 
> In general, the standard deviation figures also are a lot more
> stable.
> 
> Similarly, for embarassingly parallel problems like NPB-ep, there are
> improvements due to better spreading across LLC when the machine is not
> fully utilised.
> 
>                               vanilla    sched-numaimb-v5r12
> Min       ep.D       31.79 (   0.00%)       26.11 (  17.87%)
> Amean     ep.D       31.86 (   0.00%)       26.26 *  17.58%*
> Stddev    ep.D        0.07 (   0.00%)        0.18 (-157.54%)
> CoeffVar  ep.D        0.22 (   0.00%)        0.69 (-212.46%)
> Max       ep.D       31.93 (   0.00%)       26.46 (  17.13%)
> 
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
> ---
>  include/linux/sched/topology.h |  1 +
>  kernel/sched/fair.c            | 22 +++++++-------
>  kernel/sched/topology.c        | 53 ++++++++++++++++++++++++++++++++++
>  3 files changed, 66 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
> index 8054641c0a7b..56cffe42abbc 100644
> --- a/include/linux/sched/topology.h
> +++ b/include/linux/sched/topology.h
> @@ -93,6 +93,7 @@ struct sched_domain {
>  	unsigned int busy_factor;	/* less balancing by factor if busy */
>  	unsigned int imbalance_pct;	/* No balance until over watermark */
>  	unsigned int cache_nice_tries;	/* Leave cache hot tasks for # tries */
> +	unsigned int imb_numa_nr;	/* Nr running tasks that allows a NUMA imbalance */
>  
>  	int nohz_idle;			/* NOHZ IDLE status */
>  	int flags;			/* See SD_* */
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 4592ccf82c34..86abf97a8df6 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1489,6 +1489,7 @@ struct task_numa_env {
>  
>  	int src_cpu, src_nid;
>  	int dst_cpu, dst_nid;
> +	int imb_numa_nr;
>  
>  	struct numa_stats src_stats, dst_stats;
>  
> @@ -1503,7 +1504,7 @@ struct task_numa_env {
>  static unsigned long cpu_load(struct rq *rq);
>  static unsigned long cpu_runnable(struct rq *rq);
>  static inline long adjust_numa_imbalance(int imbalance,
> -					int dst_running, int dst_weight);
> +					int dst_running, int imb_numa_nr);
>  
>  static inline enum
>  numa_type numa_classify(unsigned int imbalance_pct,
> @@ -1884,7 +1885,7 @@ static void task_numa_find_cpu(struct task_numa_env *env,
>  		dst_running = env->dst_stats.nr_running + 1;
>  		imbalance = max(0, dst_running - src_running);
>  		imbalance = adjust_numa_imbalance(imbalance, dst_running,
> -							env->dst_stats.weight);
> +						  env->imb_numa_nr);
>  
>  		/* Use idle CPU if there is no imbalance */
>  		if (!imbalance) {
> @@ -1949,8 +1950,10 @@ static int task_numa_migrate(struct task_struct *p)
>  	 */
>  	rcu_read_lock();
>  	sd = rcu_dereference(per_cpu(sd_numa, env.src_cpu));
> -	if (sd)
> +	if (sd) {
>  		env.imbalance_pct = 100 + (sd->imbalance_pct - 100) / 2;
> +		env.imb_numa_nr = sd->imb_numa_nr;
> +	}
>  	rcu_read_unlock();
>  
>  	/*
> @@ -9003,10 +9006,9 @@ static bool update_pick_idlest(struct sched_group *idlest,
>   * This is an approximation as the number of running tasks may not be
>   * related to the number of busy CPUs due to sched_setaffinity.
>   */
> -static inline bool
> -allow_numa_imbalance(unsigned int running, unsigned int weight)
> +static inline bool allow_numa_imbalance(int running, int imb_numa_nr)
>  {
> -	return (running < (weight >> 2));
> +	return running < imb_numa_nr;
>  }
>  
>  /*
> @@ -9146,7 +9148,7 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu)
>  			 * allowed. If there is a real need of migration,
>  			 * periodic load balance will take care of it.
>  			 */
> -			if (allow_numa_imbalance(local_sgs.sum_nr_running + 1, local_sgs.group_weight))
> +			if (allow_numa_imbalance(local_sgs.sum_nr_running + 1, sd->imb_numa_nr))

Could you please clarify why are we adding 1 to local_sgs.sum_nr_running while allowing imbalance?
allow_numa_imbalance allows the imbalance based on the following inequality:

	running < imb_numa_nr

Consider on a Zen3 CPU with 8 LLCs in the sched group of the NUMA domain.
Assume the group is running 7 task and we are finding the idlest group for the 8th task:

 	sd->imb_numa_nr = 8
	local_sgs.sum_nr_running = 7

In this case, local_sgs.sum_nr_running + 1 is equal to sd->imb_numa_nr and if we allow NUMA imbalance
and place the task in the same group, each task can be given one LLC.
However, allow_numa_imbalance returns 0 for the above case and can lead to task being placed on a different
NUMA group.

In case of Gautham's suggested fix (https://lore.kernel.org/lkml/YcHs37STv71n4erJ@BLR-5CG11610CF.amd.com/),
the v4 patch in question (https://lore.kernel.org/lkml/20211210093307.31701-3-mgorman@techsingularity.net/)
used the inequality "<=" to allow NUMA imbalance where we needed to consider the additional load CPU had
to bear. However that doesn't seem to be the case here.

>  				return NULL;
>  		}
>  
> @@ -9238,9 +9240,9 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
>  #define NUMA_IMBALANCE_MIN 2
>  
>  static inline long adjust_numa_imbalance(int imbalance,
> -				int dst_running, int dst_weight)
> +				int dst_running, int imb_numa_nr)
>  {
> -	if (!allow_numa_imbalance(dst_running, dst_weight))
> +	if (!allow_numa_imbalance(dst_running, imb_numa_nr))
>  		return imbalance;
>  
>  	/*
> @@ -9352,7 +9354,7 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
>  		/* Consider allowing a small imbalance between NUMA groups */
>  		if (env->sd->flags & SD_NUMA) {
>  			env->imbalance = adjust_numa_imbalance(env->imbalance,
> -				local->sum_nr_running + 1, local->group_weight);
> +				local->sum_nr_running + 1, env->sd->imb_numa_nr);
>  		}
>  
>  		return;
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index d201a7052a29..e6cd55951304 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -2242,6 +2242,59 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
>  		}
>  	}
>  
> +	/*
> +	 * Calculate an allowed NUMA imbalance such that LLCs do not get
> +	 * imbalanced.
> +	 */
> +	for_each_cpu(i, cpu_map) {
> +		unsigned int imb = 0;
> +		unsigned int imb_span = 1;
> +
> +		for (sd = *per_cpu_ptr(d.sd, i); sd; sd = sd->parent) {
> +			struct sched_domain *child = sd->child;
> +
> +			if (!(sd->flags & SD_SHARE_PKG_RESOURCES) && child &&
> +			    (child->flags & SD_SHARE_PKG_RESOURCES)) {
> +				struct sched_domain *top, *top_p;
> +				unsigned int nr_llcs;
> +
> +				/*
> +				 * For a single LLC per node, allow an
> +				 * imbalance up to 25% of the node. This is an
> +				 * arbitrary cutoff based on SMT-2 to balance
> +				 * between memory bandwidth and avoiding
> +				 * premature sharing of HT resources and SMT-4
> +				 * or SMT-8 *may* benefit from a different
> +				 * cutoff.
> +				 *
> +				 * For multiple LLCs, allow an imbalance
> +				 * until multiple tasks would share an LLC
> +				 * on one node while LLCs on another node
> +				 * remain idle.
> +				 */

To add to my point above, the comment here says -
"allow an imbalance until multiple tasks would share an LLC on one node"
Whereas, in the case I highlighted above, we see balancing kick in with possibly
one LLC being unaccounted for. 

> +				nr_llcs = sd->span_weight / child->span_weight;
> +				if (nr_llcs == 1)
> +					imb = sd->span_weight >> 2;
> +				else
> +					imb = nr_llcs;
> +				sd->imb_numa_nr = imb;
> +
> +				/* Set span based on the first NUMA domain. */
> +				top = sd;
> +				top_p = top->parent;
> +				while (top_p && !(top_p->flags & SD_NUMA)) {
> +					top = top->parent;
> +					top_p = top->parent;
> +				}
> +				imb_span = top_p ? top_p->span_weight : sd->span_weight;
> +			} else {
> +				int factor = max(1U, (sd->span_weight / imb_span));
> +
> +				sd->imb_numa_nr = imb * factor;
> +			}
> +		}
> +	}
> +
>  	/* Calculate CPU capacity for physical packages and nodes */
>  	for (i = nr_cpumask_bits-1; i >= 0; i--) {
>  		if (!cpumask_test_cpu(i, cpu_map))

Please correct me if I'm wrong.

Thanks and Regards
Prateek

Re: [PATCH 2/2] sched/fair: Adjust the allowed NUMA imbalance when SD_NUMA spans multiple LLCs

[Mel Gorman]

On Fri, Feb 04, 2022 at 08:37:53PM +0530, Nayak, KPrateek (K Prateek) wrote:
> On 2/3/2022 8:16 PM, Mel Gorman wrote:
> > @@ -9003,10 +9006,9 @@ static bool update_pick_idlest(struct sched_group *idlest,
> >   * This is an approximation as the number of running tasks may not be
> >   * related to the number of busy CPUs due to sched_setaffinity.
> >   */
> > -static inline bool
> > -allow_numa_imbalance(unsigned int running, unsigned int weight)
> > +static inline bool allow_numa_imbalance(int running, int imb_numa_nr)
> >  {
> > -	return (running < (weight >> 2));
> > +	return running < imb_numa_nr;
> >  }
> >  
> >  /*
> > @@ -9146,7 +9148,7 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu)
> >  			 * allowed. If there is a real need of migration,
> >  			 * periodic load balance will take care of it.
> >  			 */
> > -			if (allow_numa_imbalance(local_sgs.sum_nr_running + 1, local_sgs.group_weight))
> > +			if (allow_numa_imbalance(local_sgs.sum_nr_running + 1, sd->imb_numa_nr))
> 
> Could you please clarify why are we adding 1 to local_sgs.sum_nr_running while allowing imbalance?

To account for the new task similar to what task_numa_find_cpu before
calling adjust_numa_imbalance.

> allow_numa_imbalance allows the imbalance based on the following inequality:
> 
> 	running < imb_numa_nr
> 
> Consider on a Zen3 CPU with 8 LLCs in the sched group of the NUMA domain.
> Assume the group is running 7 task and we are finding the idlest group for the 8th task:
> 
>  	sd->imb_numa_nr = 8
> 	local_sgs.sum_nr_running = 7
> 
> In this case, local_sgs.sum_nr_running + 1 is equal to sd->imb_numa_nr and if we allow NUMA imbalance
> and place the task in the same group, each task can be given one LLC.
> However, allow_numa_imbalance returns 0 for the above case and can lead to task being placed on a different
> NUMA group.
> 
> In case of Gautham's suggested fix (https://lore.kernel.org/lkml/YcHs37STv71n4erJ@BLR-5CG11610CF.amd.com/),
> the v4 patch in question (https://lore.kernel.org/lkml/20211210093307.31701-3-mgorman@techsingularity.net/)
> used the inequality "<=" to allow NUMA imbalance where we needed to consider the additional load CPU had
> to bear. However that doesn't seem to be the case here.
> 

I failed to change < to <= in allow_numa_imbalance, I'll fix and retest.

-- 
Mel Gorman
SUSE Labs