[PATCH v10 2/3] sched/fair: Scan cluster before scanning LLC in wake-up path

From: Yicong Yang
Date: Thu Oct 12 2023 - 08:20:12 EST

From: Barry Song <song.bao.hua@xxxxxxxxxxxxx>

For platforms having clusters like Kunpeng920, CPUs within the same cluster
have lower latency when synchronizing and accessing shared resources like
cache. Thus, this patch tries to find an idle cpu within the cluster of the
target CPU before scanning the whole LLC to gain lower latency. This
will be implemented in 2 steps in select_idle_sibling():
1. When the prev_cpu/recent_used_cpu are good wakeup candidates, use them
if they're sharing cluster with the target CPU. Otherwise trying to
scan for an idle CPU in the target's cluster.
2. Scanning the cluster prior to the LLC of the target CPU for an
idle CPU to wakeup.

Testing has been done on Kunpeng920 by pinning tasks to one numa and two
numa. On Kunpeng920, Each numa has 8 clusters and each cluster has 4 CPUs.

With this patch, We noticed enhancement on tbench and netperf within one
numa or cross two numa on top of tip-sched-core commit
9b46f1abc6d4 ("sched/debug: Print 'tgid' in sched_show_task()")

tbench results (node 0):
baseline patched
1: 327.2833 372.4623 ( 13.80%)
4: 1320.5933 1479.8833 ( 12.06%)
8: 2638.4867 2921.5267 ( 10.73%)
16: 5282.7133 5891.5633 ( 11.53%)
32: 9810.6733 9877.3400 ( 0.68%)
64: 7408.9367 7447.9900 ( 0.53%)
128: 6203.2600 6191.6500 ( -0.19%)
tbench results (node 0-1):
baseline patched
1: 332.0433 372.7223 ( 12.25%)
4: 1325.4667 1477.6733 ( 11.48%)
8: 2622.9433 2897.9967 ( 10.49%)
16: 5218.6100 5878.2967 ( 12.64%)
32: 10211.7000 11494.4000 ( 12.56%)
64: 13313.7333 16740.0333 ( 25.74%)
128: 13959.1000 14533.9000 ( 4.12%)

netperf results TCP_RR (node 0):
baseline patched
1: 76546.5033 90649.9867 ( 18.42%)
4: 77292.4450 90932.7175 ( 17.65%)
8: 77367.7254 90882.3467 ( 17.47%)
16: 78519.9048 90938.8344 ( 15.82%)
32: 72169.5035 72851.6730 ( 0.95%)
64: 25911.2457 25882.2315 ( -0.11%)
128: 10752.6572 10768.6038 ( 0.15%)

netperf results TCP_RR (node 0-1):
baseline patched
1: 76857.6667 90892.2767 ( 18.26%)
4: 78236.6475 90767.3017 ( 16.02%)
8: 77929.6096 90684.1633 ( 16.37%)
16: 77438.5873 90502.5787 ( 16.87%)
32: 74205.6635 88301.5612 ( 19.00%)
64: 69827.8535 71787.6706 ( 2.81%)
128: 25281.4366 25771.3023 ( 1.94%)

netperf results UDP_RR (node 0):
baseline patched
1: 96869.8400 110800.8467 ( 14.38%)
4: 97744.9750 109680.5425 ( 12.21%)
8: 98783.9863 110409.9637 ( 11.77%)
16: 99575.0235 110636.2435 ( 11.11%)
32: 95044.7250 97622.8887 ( 2.71%)
64: 32925.2146 32644.4991 ( -0.85%)
128: 12859.2343 12824.0051 ( -0.27%)

netperf results UDP_RR (node 0-1):
baseline patched
1: 97202.4733 110190.1200 ( 13.36%)
4: 95954.0558 106245.7258 ( 10.73%)
8: 96277.1958 105206.5304 ( 9.27%)
16: 97692.7810 107927.2125 ( 10.48%)
32: 79999.6702 103550.2999 ( 29.44%)
64: 80592.7413 87284.0856 ( 8.30%)
128: 27701.5770 29914.5820 ( 7.99%)

Note neither Kunpeng920 nor x86 Jacobsville supports SMT, so the SMT branch
in the code has not been tested but it supposed to work.

Chen Yu also noticed this will improve the performance of tbench and
netperf on a 24 CPUs Jacobsville machine, there are 4 CPUs in one
cluster sharing L2 Cache.

Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
[https://lore.kernel.org/lkml/Ytfjs+m1kUs0ScSn@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx]
Tested-by: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
Signed-off-by: Barry Song <song.bao.hua@xxxxxxxxxxxxx>
Signed-off-by: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
Reviewed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Reviewed-by: Chen Yu <yu.c.chen@xxxxxxxxx>
---
kernel/sched/fair.c | 41 +++++++++++++++++++++++++++++++++++++----
kernel/sched/sched.h | 1 +
kernel/sched/topology.c | 12 ++++++++++++
3 files changed, 50 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 666ece65987f..4039f9b348ec 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7259,6 +7259,30 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
}
}

+ if (static_branch_unlikely(&sched_cluster_active)) {
+ struct sched_group *sg = sd->groups;
+
+ if (sg->flags & SD_CLUSTER) {
+ for_each_cpu_wrap(cpu, sched_group_span(sg), target + 1) {
+ if (!cpumask_test_cpu(cpu, cpus))
+ continue;
+
+ if (has_idle_core) {
+ i = select_idle_core(p, cpu, cpus, &idle_cpu);
+ if ((unsigned int)i < nr_cpumask_bits)
+ return i;
+ } else {
+ if (--nr <= 0)
+ return -1;
+ idle_cpu = __select_idle_cpu(cpu, p);
+ if ((unsigned int)idle_cpu < nr_cpumask_bits)
+ return idle_cpu;
+ }
+ }
+ cpumask_andnot(cpus, cpus, sched_group_span(sg));
+ }
+ }
+
for_each_cpu_wrap(cpu, cpus, target + 1) {
if (has_idle_core) {
i = select_idle_core(p, cpu, cpus, &idle_cpu);
@@ -7266,7 +7290,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
return i;

} else {
- if (!--nr)
+ if (--nr <= 0)
return -1;
idle_cpu = __select_idle_cpu(cpu, p);
if ((unsigned int)idle_cpu < nr_cpumask_bits)
@@ -7395,8 +7419,13 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
*/
if (prev != target && cpus_share_cache(prev, target) &&
(available_idle_cpu(prev) || sched_idle_cpu(prev)) &&
- asym_fits_cpu(task_util, util_min, util_max, prev))
- return prev;
+ asym_fits_cpu(task_util, util_min, util_max, prev)) {
+ if (!static_branch_unlikely(&sched_cluster_active))
+ return prev;
+
+ if (cpus_share_resources(prev, target))
+ return prev;
+ }

/*
* Allow a per-cpu kthread to stack with the wakee if the
@@ -7423,7 +7452,11 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
(available_idle_cpu(recent_used_cpu) || sched_idle_cpu(recent_used_cpu)) &&
cpumask_test_cpu(recent_used_cpu, p->cpus_ptr) &&
asym_fits_cpu(task_util, util_min, util_max, recent_used_cpu)) {
- return recent_used_cpu;
+ if (!static_branch_unlikely(&sched_cluster_active))
+ return recent_used_cpu;
+
+ if (cpus_share_resources(recent_used_cpu, target))
+ return recent_used_cpu;
}

/*
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 998f03d02de0..ef4fe7bcf740 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1859,6 +1859,7 @@ DECLARE_PER_CPU(struct sched_domain __rcu *, sd_numa);
DECLARE_PER_CPU(struct sched_domain __rcu *, sd_asym_packing);
DECLARE_PER_CPU(struct sched_domain __rcu *, sd_asym_cpucapacity);
extern struct static_key_false sched_asym_cpucapacity;
+extern struct static_key_false sched_cluster_active;

static __always_inline bool sched_asym_cpucap_active(void)
{
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 48cd88350d18..925cd68abc8b 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -673,7 +673,9 @@ DEFINE_PER_CPU(struct sched_domain_shared __rcu *, sd_llc_shared);
DEFINE_PER_CPU(struct sched_domain __rcu *, sd_numa);
DEFINE_PER_CPU(struct sched_domain __rcu *, sd_asym_packing);
DEFINE_PER_CPU(struct sched_domain __rcu *, sd_asym_cpucapacity);
+
DEFINE_STATIC_KEY_FALSE(sched_asym_cpucapacity);
+DEFINE_STATIC_KEY_FALSE(sched_cluster_active);

static void update_top_cache_domain(int cpu)
{
@@ -2386,6 +2388,7 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
struct rq *rq = NULL;
int i, ret = -ENOMEM;
bool has_asym = false;
+ bool has_cluster = false;

if (WARN_ON(cpumask_empty(cpu_map)))
goto error;
@@ -2514,12 +2517,18 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
WRITE_ONCE(d.rd->max_cpu_capacity, capacity);

cpu_attach_domain(sd, d.rd, i);
+
+ if (lowest_flag_domain(i, SD_CLUSTER))
+ has_cluster = true;
}
rcu_read_unlock();

if (has_asym)
static_branch_inc_cpuslocked(&sched_asym_cpucapacity);

+ if (has_cluster)
+ static_branch_inc_cpuslocked(&sched_cluster_active);
+
if (rq && sched_debug_verbose) {
pr_info("root domain span: %*pbl (max cpu_capacity = %lu)\n",
cpumask_pr_args(cpu_map), rq->rd->max_cpu_capacity);
@@ -2619,6 +2628,9 @@ static void detach_destroy_domains(const struct cpumask *cpu_map)
if (rcu_access_pointer(per_cpu(sd_asym_cpucapacity, cpu)))
static_branch_dec_cpuslocked(&sched_asym_cpucapacity);

+ if (static_branch_unlikely(&sched_cluster_active))
+ static_branch_dec_cpuslocked(&sched_cluster_active);
+
rcu_read_lock();
for_each_cpu(i, cpu_map)
cpu_attach_domain(NULL, &def_root_domain, i);
--
2.24.0