[PATCH v3 0/7] Improve scheduler scalability for fast path

[PATCH v3 0/7] Improve scheduler scalability for fast path

[subhra mazumdar]

Current select_idle_sibling first tries to find a fully idle core using
select_idle_core which can potentially search all cores and if it fails it
finds any idle cpu using select_idle_cpu. select_idle_cpu can potentially
search all cpus in the llc domain. This doesn't scale for large llc domains
and will only get worse with more cores in future.

This patch solves the scalability problem by:
 - Setting an upper and lower limit of idle cpu search in select_idle_cpu
   to keep search time low and constant
 - Adding a new sched feature SIS_CORE to disable select_idle_core

Additionally it also introduces a new per-cpu variable next_cpu to track
the limit of search so that every time search starts from where it ended.
This rotating search window over cpus in LLC domain ensures that idle
cpus are eventually found in case of high load.

Following are the performance numbers with various benchmarks with SIS_CORE
true (idle core search enabled).

Hackbench process on 2 socket, 44 core and 88 threads Intel x86 machine
(lower is better):
groups  baseline           %stdev  patch           %stdev
1       0.5816             8.94    0.5903 (-1.5%)  11.28 
2       0.6428             10.64   0.5843 (9.1%)   4.93
4       1.0152             1.99    0.9965 (1.84%)  1.83
8       1.8128             1.4     1.7921 (1.14%)  1.76
16      3.1666             0.8     3.1345 (1.01%)  0.81
32      5.6084             0.83    5.5677 (0.73%)  0.8 

Sysbench MySQL on 2 socket, 44 core and 88 threads Intel x86 machine
(higher is better):
threads baseline     %stdev     patch              %stdev
8       2095.45      1.82       2102.6 (0.34%)     2.11
16      4218.45      0.06       4221.35 (0.07%)    0.38
32      7531.36      0.49       7607.18 (1.01%)    0.25
48      10206.42     0.21       10324.26 (1.15%)   0.13
64      12053.73     0.1        12158.3 (0.87%)    0.24
128     14810.33     0.04       14840.4 (0.2%)     0.38

Oracle DB on 2 socket, 44 core and 88 threads Intel x86 machine
(normalized, higher is better):
users   baseline        %stdev  patch            %stdev
20      1               0.9     1.0068 (0.68%)   0.27
40      1               0.8     1.0103 (1.03%)   1.24
60      1               0.34    1.0178 (1.78%)   0.49
80      1               0.53    1.0092 (0.92%)   1.5
100     1               0.79    1.0090 (0.9%)    0.88
120     1               0.06    1.0048 (0.48%)   0.72
140     1               0.22    1.0116 (1.16%)   0.05
160     1               0.57    1.0264 (2.64%)   0.67
180     1               0.81    1.0194 (1.94%)   0.91
200     1               0.44    1.028 (2.8%)     3.09
220     1               1.74    1.0229 (2.29%)   0.21

Uperf pingpong on 2 socket, 44 core and 88 threads Intel x86 machine with
message size = 8k (higher is better):
threads baseline        %stdev  patch            %stdev
8       45.36           0.43    46.28 (2.01%)    0.29
16      87.81           0.82    89.67 (2.12%)    0.38
32      151.19          0.02    153.5 (1.53%)    0.41
48      190.2           0.21    194.79 (2.41%)   0.07
64      190.42          0.35    202.9 (6.55%)    1.66
128     323.86          0.28    343.56 (6.08%)   1.34 

Dbench on 2 socket, 44 core and 88 threads Intel x86 machine
(higher is better):
clients baseline        patch
1       629.8           603.83 (-4.12%)
2       1159.65         1155.75 (-0.34%)
4       2121.61         2093.99 (-1.3%)
8       2620.52         2641.51 (0.8%)
16      2879.31         2897.6 (0.64%)

Tbench on 2 socket, 44 core and 88 threads Intel x86 machine
(higher is better):
clients baseline        patch
1       256.41          255.8 (-0.24%)
2       509.89          504.52 (-1.05%)
4       999.44          1003.74 (0.43%)
8       1982.7          1976.42 (-0.32%)
16      3891.51         3916.04 (0.63%)
32      6819.24         6845.06 (0.38%)
64      8542.95         8568.28 (0.3%)
128     15277.6         15754.6 (3.12%)

Schbench on 2 socket, 44 core and 88 threads Intel x86 machine with 44
tasks (lower is better):
percentile      baseline      %stdev   patch             %stdev
50              94            2.82     92 (2.13%)        2.17
75              124           2.13     122 (1.61%)       1.42
90              152           1.74     151 (0.66%)       0.66 
95              171           2.11     170 (0.58%)       0
99              512.67        104.96   208.33 (59.36%)   1.2
99.5            2296          82.55    3674.66 (-60.05%) 22.19
99.9            12517.33      2.38     12784 (-2.13%)    0.66

Hackbench process on 2 socket, 16 core and 128 threads SPARC machine
(lower is better):
groups  baseline           %stdev  patch             %stdev
1       1.3085             6.65    1.2213 (6.66%)    10.32 
2       1.4559             8.55    1.5048 (-3.36%)   4.72
4       2.6271             1.74    2.5532 (2.81%)    2.02
8       4.7089             3.01    4.5118 (4.19%)    2.74
16      8.7406             2.25    8.6801 (0.69%)    4.78
32      17.7835            1.01    16.759 (5.76%)    1.38
64      36.1901            0.65    34.6652 (4.21%)   1.24
128     72.6585            0.51    70.9762 (2.32%)   0.9

Following are the performance numbers with various benchmarks with SIS_CORE
false (idle core search disabled). This improves throughput of certain
workloads but increases latency of other workloads.

Hackbench process on 2 socket, 44 core and 88 threads Intel x86 machine
(lower is better):
groups  baseline           %stdev  patch            %stdev
1       0.5816             8.94    0.5835 (-0.33%)  8.21  
2       0.6428             10.64   0.5752 (10.52%)  4.05  
4       1.0152             1.99    0.9946 (2.03%)   2.56  
8       1.8128             1.4     1.7619 (2.81%)   1.88  
16      3.1666             0.8     3.1275 (1.23%)   0.42  
32      5.6084             0.83    5.5856 (0.41%)   0.89   

Sysbench MySQL on 2 socket, 44 core and 88 threads Intel x86 machine
(higher is better):
threads baseline     %stdev     patch              %stdev
8       2095.45      1.82       2084.72 (-0.51%)   1.65
16      4218.45      0.06       4179.69 (-0.92%)   0.18
32      7531.36      0.49       7623.18 (1.22%)    0.39
48      10206.42     0.21       10159.16 (-0.46%)  0.21
64      12053.73     0.1        12087.21 (0.28%)   0.19
128     14810.33     0.04       14894.08 (0.57%)   0.08

Oracle DB on 2 socket, 44 core and 88 threads Intel x86 machine
(normalized, higher is better):
users   baseline        %stdev  patch            %stdev
20      1               0.9     1.0056 (0.56%)   0.34
40      1               0.8     1.0173 (1.73%)   0.13
60      1               0.34    0.9995 (-0.05%)  0.85
80      1               0.53    1.0175 (1.75%)   1.56
100     1               0.79    1.0151 (1.51%)   1.31
120     1               0.06    1.0244 (2.44%)   0.5 
140     1               0.22    1.034 (3.4%)     0.66
160     1               0.57    1.0362 (3.62%)   0.07
180     1               0.81    1.041 (4.1%)     0.8
200     1               0.44    1.0233 (2.33%)   1.4  
220     1               1.74    1.0125 (1.25%)   1.41

Uperf pingpong on 2 socket, 44 core and 88 threads Intel x86 machine with
message size = 8k (higher is better):
threads baseline        %stdev  patch            %stdev
8       45.36           0.43    46.94 (3.48%)    0.2  
16      87.81           0.82    91.75 (4.49%)    0.43 
32      151.19          0.02    167.74 (10.95%)  1.29 
48      190.2           0.21    200.57 (5.45%)   0.89 
64      190.42          0.35    226.74 (19.07%)  1.79 
128     323.86          0.28    348.12 (7.49%)   0.77 

Dbench on 2 socket, 44 core and 88 threads Intel x86 machine
(higher is better):
clients baseline        patch
1       629.8           600.19 (-4.7%)  
2       1159.65         1162.07 (0.21%) 
4       2121.61         2112.27 (-0.44%)
8       2620.52         2645.55 (0.96%) 
16      2879.31         2828.87 (-1.75%)
32      2791.24         2760.97 (-1.08%)
64      1853.07         1747.66 (-5.69%)
128     1484.95         1459.81 (-1.69%)

Tbench on 2 socket, 44 core and 88 threads Intel x86 machine
(higher is better):
clients baseline        patch
1       256.41          258.11 (0.67%)  
2       509.89          509.13 (-0.15%) 
4       999.44          1016.58 (1.72%) 
8       1982.7          2006.53 (1.2%)  
16      3891.51         3964.43 (1.87%) 
32      6819.24         7376.92 (8.18%) 
64      8542.95         9660.45 (13.08%) 
128     15277.6         15438.4 (1.05%) 

Schbench on 2 socket, 44 core and 88 threads Intel x86 machine with 44
tasks (lower is better):
percentile      baseline      %stdev   patch                %stdev
50              94            2.82     94.67 (-0.71%)       2.2  
75              124           2.13     124.67 (-0.54%)      1.67 
90              152           1.74     154.33 (-1.54%)      0.75 
95              171           2.11     176.67 (-3.31%)      0.86 
99              512.67        104.96   4130.33 (-705.65%)   79.41
99.5            2296          82.55    10066.67 (-338.44%)  26.15
99.9            12517.33      2.38     12869.33 (-2.81%)    0.8 

Hackbench process on 2 socket, 16 core and 128 threads SPARC machine
(lower is better):
groups  baseline           %stdev  patch             %stdev
1       1.3085             6.65    1.2514 (4.36%)    11.1
2       1.4559             8.55    1.5433 (-6%)      3.05
4       2.6271             1.74    2.5626 (2.5%)     2.69
8       4.7089             3.01    4.5316 (3.77%)    2.95
16      8.7406             2.25    8.6585 (0.94%)    2.91
32      17.7835            1.01    17.175 (3.42%)    1.38
64      36.1901            0.65    35.5294 (1.83%)   1.02
128     72.6585            0.51    71.8821 (1.07%)   1.05

Following are the schbench performance numbers with SIS_CORE false and
SIS_PROP false. This recovers the latency increase by having SIS_CORE
false.

Schbench on 2 socket, 44 core and 88 threads Intel x86 machine with 44
tasks (lower is better):
percentile      baseline      %stdev   patch               %stdev
50              94            2.82     93.33 (0.71%)       1.24  
75              124           2.13     122.67 (1.08%)      1.7   
90              152           1.74     149.33 (1.75%)      2.35  
95              171           2.11     167 (2.34%)         2.74
99              512.67        104.96   206 (59.82%)        8.86
99.5            2296          82.55    3121.67 (-35.96%)   97.37 
99.9            12517.33      2.38     12592 (-0.6%)       1.67

Changes from v2->v3:
-Use shift operator instead of multiplication to compute limit
-Use per-CPU variable to precompute the number of sibling SMTs for x86

subhra mazumdar (7):
  sched: limit cpu search in select_idle_cpu
  sched: introduce per-cpu var next_cpu to track search limit
  sched: rotate the cpu search window for better spread
  sched: add sched feature to disable idle core search
  sched: SIS_CORE to disable idle core search
  x86/smpboot: introduce per-cpu variable for HT siblings
  sched: use per-cpu variable cpumask_weight_sibling

 arch/x86/include/asm/smp.h      |  1 +
 arch/x86/include/asm/topology.h |  1 +
 arch/x86/kernel/smpboot.c       | 17 ++++++++++++++++-
 include/linux/topology.h        |  4 ++++
 kernel/sched/core.c             |  2 ++
 kernel/sched/fair.c             | 31 +++++++++++++++++++++++--------
 kernel/sched/features.h         |  1 +
 kernel/sched/sched.h            |  1 +
 8 files changed, 49 insertions(+), 9 deletions(-)

-- 
2.9.3

[PATCH v3 1/7] sched: limit cpu search in select_idle_cpu

[subhra mazumdar]

Put upper and lower limit on cpu search of select_idle_cpu. The lower limit
is amount of cpus in a core while upper limit is twice that. This ensures
for any architecture we will usually search beyond a core. The upper limit
also helps in keeping the search cost low and constant.

Signed-off-by: subhra mazumdar <subhra.mazumdar@oracle.com>
---
 kernel/sched/fair.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index f35930f..b58f08f 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6188,7 +6188,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
 	u64 avg_cost, avg_idle;
 	u64 time, cost;
 	s64 delta;
-	int cpu, nr = INT_MAX;
+	int cpu, limit, floor, nr = INT_MAX;
 
 	this_sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
 	if (!this_sd)
@@ -6206,10 +6206,17 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
 
 	if (sched_feat(SIS_PROP)) {
 		u64 span_avg = sd->span_weight * avg_idle;
-		if (span_avg > 4*avg_cost)
+		floor = cpumask_weight(topology_sibling_cpumask(target));
+		if (floor < 2)
+			floor = 2;
+		limit = floor << 1;
+		if (span_avg > floor*avg_cost) {
 			nr = div_u64(span_avg, avg_cost);
-		else
-			nr = 4;
+			if (nr > limit)
+				nr = limit;
+		} else {
+			nr = floor;
+		}
 	}
 
 	time = local_clock();
-- 
2.9.3

[PATCH v3 2/7] sched: introduce per-cpu var next_cpu to track search limit

[subhra mazumdar]

Introduce a per-cpu variable to track the limit upto which idle cpu search
was done in select_idle_cpu(). This will help to start the search next time
from there. This is necessary for rotating the search window over entire
LLC domain.

Signed-off-by: subhra mazumdar <subhra.mazumdar@oracle.com>
---
 kernel/sched/core.c  | 2 ++
 kernel/sched/sched.h | 1 +
 2 files changed, 3 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 874c427..80657fc 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -24,6 +24,7 @@
 #include <trace/events/sched.h>
 
 DEFINE_PER_CPU_SHARED_ALIGNED(struct rq, runqueues);
+DEFINE_PER_CPU_SHARED_ALIGNED(int, next_cpu);
 
 #if defined(CONFIG_SCHED_DEBUG) && defined(CONFIG_JUMP_LABEL)
 /*
@@ -5966,6 +5967,7 @@ void __init sched_init(void)
 	for_each_possible_cpu(i) {
 		struct rq *rq;
 
+		per_cpu(next_cpu, i) = -1;
 		rq = cpu_rq(i);
 		raw_spin_lock_init(&rq->lock);
 		rq->nr_running = 0;
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index b52ed1a..4cecfa2 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -994,6 +994,7 @@ static inline void update_idle_core(struct rq *rq) { }
 #endif
 
 DECLARE_PER_CPU_SHARED_ALIGNED(struct rq, runqueues);
+DECLARE_PER_CPU_SHARED_ALIGNED(int, next_cpu);
 
 #define cpu_rq(cpu)		(&per_cpu(runqueues, (cpu)))
 #define this_rq()		this_cpu_ptr(&runqueues)
-- 
2.9.3

[PATCH v3 3/7] sched: rotate the cpu search window for better spread

[subhra mazumdar]

Rotate the cpu search window for better spread of threads. This will ensure
an idle cpu will quickly be found if one exists.

Signed-off-by: subhra mazumdar <subhra.mazumdar@oracle.com>
---
 kernel/sched/fair.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index b58f08f..c1ca88e 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6188,7 +6188,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
 	u64 avg_cost, avg_idle;
 	u64 time, cost;
 	s64 delta;
-	int cpu, limit, floor, nr = INT_MAX;
+	int cpu, limit, floor, target_tmp, nr = INT_MAX;
 
 	this_sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
 	if (!this_sd)
@@ -6219,9 +6219,15 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
 		}
 	}
 
+	if (per_cpu(next_cpu, target) != -1)
+		target_tmp = per_cpu(next_cpu, target);
+	else
+		target_tmp = target;
+
 	time = local_clock();
 
-	for_each_cpu_wrap(cpu, sched_domain_span(sd), target) {
+	for_each_cpu_wrap(cpu, sched_domain_span(sd), target_tmp) {
+		per_cpu(next_cpu, target) = cpu;
 		if (!--nr)
 			return -1;
 		if (!cpumask_test_cpu(cpu, &p->cpus_allowed))
-- 
2.9.3

[PATCH v3 4/7] sched: add sched feature to disable idle core search

[subhra mazumdar]

Add a new sched feature SIS_CORE to have an option to disable idle core
search (select_idle_core).

Signed-off-by: subhra mazumdar <subhra.mazumdar@oracle.com>
---
 kernel/sched/features.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/sched/features.h b/kernel/sched/features.h
index 858589b..de4d506 100644
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -57,6 +57,7 @@ SCHED_FEAT(TTWU_QUEUE, true)
  */
 SCHED_FEAT(SIS_AVG_CPU, false)
 SCHED_FEAT(SIS_PROP, true)
+SCHED_FEAT(SIS_CORE, true)
 
 /*
  * Issue a WARN when we do multiple update_rq_clock() calls
-- 
2.9.3

[PATCH v3 5/7] sched: SIS_CORE to disable idle core search

[subhra mazumdar]

Use SIS_CORE to disable idle core search. For some workloads
select_idle_core becomes a scalability bottleneck, removing it improves
throughput. Also there are workloads where disabling it can hurt latency,
so need to have an option.

Signed-off-by: subhra mazumdar <subhra.mazumdar@oracle.com>
---
 kernel/sched/fair.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index c1ca88e..6a74808 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6280,9 +6280,11 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
 	if (!sd)
 		return target;
 
-	i = select_idle_core(p, sd, target);
-	if ((unsigned)i < nr_cpumask_bits)
-		return i;
+	if (sched_feat(SIS_CORE)) {
+		i = select_idle_core(p, sd, target);
+		if ((unsigned)i < nr_cpumask_bits)
+			return i;
+	}
 
 	i = select_idle_cpu(p, sd, target);
 	if ((unsigned)i < nr_cpumask_bits)
-- 
2.9.3

[PATCH v3 6/7] x86/smpboot: introduce per-cpu variable for HT siblings

[subhra mazumdar]

Introduce a per-cpu variable to keep the number of HT siblings of a cpu.
This will be used for quick lookup in select_idle_cpu to determine the
limits of search. This patch does it only for x86.

Signed-off-by: subhra mazumdar <subhra.mazumdar@oracle.com>
---
 arch/x86/include/asm/smp.h      |  1 +
 arch/x86/include/asm/topology.h |  1 +
 arch/x86/kernel/smpboot.c       | 17 ++++++++++++++++-
 include/linux/topology.h        |  4 ++++
 4 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/smp.h b/arch/x86/include/asm/smp.h
index da545df..1e90cbd 100644
--- a/arch/x86/include/asm/smp.h
+++ b/arch/x86/include/asm/smp.h
@@ -22,6 +22,7 @@ extern int smp_num_siblings;
 extern unsigned int num_processors;
 
 DECLARE_PER_CPU_READ_MOSTLY(cpumask_var_t, cpu_sibling_map);
+DECLARE_PER_CPU_READ_MOSTLY(unsigned int, cpumask_weight_sibling);
 DECLARE_PER_CPU_READ_MOSTLY(cpumask_var_t, cpu_core_map);
 /* cpus sharing the last level cache: */
 DECLARE_PER_CPU_READ_MOSTLY(cpumask_var_t, cpu_llc_shared_map);
diff --git a/arch/x86/include/asm/topology.h b/arch/x86/include/asm/topology.h
index 453cf38..dd19c71 100644
--- a/arch/x86/include/asm/topology.h
+++ b/arch/x86/include/asm/topology.h
@@ -111,6 +111,7 @@ extern const struct cpumask *cpu_coregroup_mask(int cpu);
 #ifdef CONFIG_SMP
 #define topology_core_cpumask(cpu)		(per_cpu(cpu_core_map, cpu))
 #define topology_sibling_cpumask(cpu)		(per_cpu(cpu_sibling_map, cpu))
+#define topology_sibling_weight(cpu)	(per_cpu(cpumask_weight_sibling, cpu))
 
 extern unsigned int __max_logical_packages;
 #define topology_max_packages()			(__max_logical_packages)
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 362dd89..20bf676 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -85,6 +85,10 @@
 DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t, cpu_sibling_map);
 EXPORT_PER_CPU_SYMBOL(cpu_sibling_map);
 
+/* representing number of HT siblings of each CPU */
+DEFINE_PER_CPU_READ_MOSTLY(unsigned int, cpumask_weight_sibling);
+EXPORT_PER_CPU_SYMBOL(cpumask_weight_sibling);
+
 /* representing HT and core siblings of each logical CPU */
 DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t, cpu_core_map);
 EXPORT_PER_CPU_SYMBOL(cpu_core_map);
@@ -520,6 +524,8 @@ void set_cpu_sibling_map(int cpu)
 
 	if (!has_mp) {
 		cpumask_set_cpu(cpu, topology_sibling_cpumask(cpu));
+		per_cpu(cpumask_weight_sibling, cpu) =
+		    cpumask_weight(topology_sibling_cpumask(cpu));
 		cpumask_set_cpu(cpu, cpu_llc_shared_mask(cpu));
 		cpumask_set_cpu(cpu, topology_core_cpumask(cpu));
 		c->booted_cores = 1;
@@ -529,8 +535,12 @@ void set_cpu_sibling_map(int cpu)
 	for_each_cpu(i, cpu_sibling_setup_mask) {
 		o = &cpu_data(i);
 
-		if ((i == cpu) || (has_smt && match_smt(c, o)))
+		if ((i == cpu) || (has_smt && match_smt(c, o))) {
 			link_mask(topology_sibling_cpumask, cpu, i);
+			threads = cpumask_weight(topology_sibling_cpumask(cpu));
+			per_cpu(cpumask_weight_sibling, cpu) = threads;
+			per_cpu(cpumask_weight_sibling, i) = threads;
+		}
 
 		if ((i == cpu) || (has_mp && match_llc(c, o)))
 			link_mask(cpu_llc_shared_mask, cpu, i);
@@ -1173,6 +1183,8 @@ static __init void disable_smp(void)
 	else
 		physid_set_mask_of_physid(0, &phys_cpu_present_map);
 	cpumask_set_cpu(0, topology_sibling_cpumask(0));
+	per_cpu(cpumask_weight_sibling, 0) =
+	    cpumask_weight(topology_sibling_cpumask(0));
 	cpumask_set_cpu(0, topology_core_cpumask(0));
 }
 
@@ -1482,6 +1494,8 @@ static void remove_siblinginfo(int cpu)
 
 	for_each_cpu(sibling, topology_core_cpumask(cpu)) {
 		cpumask_clear_cpu(cpu, topology_core_cpumask(sibling));
+		per_cpu(cpumask_weight_sibling, sibling) =
+		    cpumask_weight(topology_sibling_cpumask(sibling));
 		/*/
 		 * last thread sibling in this cpu core going down
 		 */
@@ -1495,6 +1509,7 @@ static void remove_siblinginfo(int cpu)
 		cpumask_clear_cpu(cpu, cpu_llc_shared_mask(sibling));
 	cpumask_clear(cpu_llc_shared_mask(cpu));
 	cpumask_clear(topology_sibling_cpumask(cpu));
+	per_cpu(cpumask_weight_sibling, cpu) = 0;
 	cpumask_clear(topology_core_cpumask(cpu));
 	c->cpu_core_id = 0;
 	c->booted_cores = 0;
diff --git a/include/linux/topology.h b/include/linux/topology.h
index cb0775e..a85aea1 100644
--- a/include/linux/topology.h
+++ b/include/linux/topology.h
@@ -190,6 +190,10 @@ static inline int cpu_to_mem(int cpu)
 #ifndef topology_sibling_cpumask
 #define topology_sibling_cpumask(cpu)		cpumask_of(cpu)
 #endif
+#ifndef topology_sibling_weight
+#define topology_sibling_weight(cpu)	\
+		cpumask_weight(topology_sibling_cpumask(cpu))
+#endif
 #ifndef topology_core_cpumask
 #define topology_core_cpumask(cpu)		cpumask_of(cpu)
 #endif
-- 
2.9.3

[PATCH v3 7/7] sched: use per-cpu variable cpumask_weight_sibling

[subhra mazumdar]

Use per-cpu var cpumask_weight_sibling for quick lookup in select_idle_cpu.
This is the fast path of scheduler and every cycle is worth saving. Usage
of cpumask_weight can result in iterations.

Signed-off-by: subhra mazumdar <subhra.mazumdar@oracle.com>
---
 kernel/sched/fair.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6a74808..878f11c 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6206,7 +6206,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
 
 	if (sched_feat(SIS_PROP)) {
 		u64 span_avg = sd->span_weight * avg_idle;
-		floor = cpumask_weight(topology_sibling_cpumask(target));
+		floor = topology_sibling_weight(target);
 		if (floor < 2)
 			floor = 2;
 		limit = floor << 1;
-- 
2.9.3