[RFC PATCH v2] sched/fair: select idle cpu from idle cpumask in sched domain

Aubrey Li
2020-09-16T04:32:41Z

Added idle cpumask to track idle cpus in sched domain. When a CPU
enters idle, its corresponding bit in the idle cpumask will be set,
and when the CPU exits idle, its bit will be cleared.

When a task wakes up to select an idle cpu, scanning idle cpumask
has low cost than scanning all the cpus in last level cache domain,
especially when the system is heavily loaded.

The following benchmarks were tested on a x86 4 socket system with
24 cores per socket and 2 hyperthreads per core, total 192 CPUs:

uperf throughput: netperf workload, tcp_nodelay, r/w size = 90

threads baseline-avg %std patch-avg %std
96 1 1.24 0.98 2.76
144 1 1.13 1.35 4.01
192 1 0.58 1.67 3.25
240 1 2.49 1.68 3.55

hackbench: process mode, 100000 loops, 40 file descriptors per group

group baseline-avg %std patch-avg %std
2(80) 1 12.05 0.97 9.88
3(120) 1 12.48 0.95 11.62
4(160) 1 13.83 0.97 13.22
5(200) 1 2.76 1.01 2.94

schbench: 99th percentile latency, 16 workers per message thread

mthread baseline-avg %std patch-avg %std
6(96) 1 1.24 0.993 1.73
9(144) 1 0.38 0.998 0.39
12(192) 1 1.58 0.995 1.64
15(240) 1 51.71 0.606 37.41

sysbench mysql throughput: read/write, table size = 10,000,000

thread baseline-avg %std patch-avg %std
96 1 1.77 1.015 1.71
144 1 3.39 0.998 4.05
192 1 2.88 1.002 2.81
240 1 2.07 1.011 2.09

kbuild: kexec reboot every time

baseline-avg patch-avg
1 1

v1->v2:
- idle cpumask is updated in the nohz routines, by initializing idle
cpumask with sched_domain_span(sd), nohz=off case remains the original
behavior.

Cc: Qais Yousef <qais.yousef@arm.com>
Cc: Valentin Schneider <valentin.schneider@arm.com>
Cc: Jiang Biao <benbjiang@gmail.com>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Aubrey Li <aubrey.li@linux.intel.com>
---
include/linux/sched/topology.h | 13 +++++++++++++
kernel/sched/fair.c | 9 ++++++++-
kernel/sched/topology.c | 3 ++-
3 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
index fb11091129b3..43a641d26154 100644
--- a/include/linux/sched/topology.h
+++ b/include/linux/sched/topology.h
@@ -65,8 +65,21 @@ struct sched_domain_shared {
atomic_t ref;
atomic_t nr_busy_cpus;
int has_idle_cores;
+ /*
+ * Span of all idle CPUs in this domain.
+ *
+ * NOTE: this field is variable length. (Allocated dynamically
+ * by attaching extra space to the end of the structure,
+ * depending on how many CPUs the kernel has booted up with)
+ */
+ unsigned long idle_cpus_span[];
};

+static inline struct cpumask *sds_idle_cpus(struct sched_domain_shared *sds)
+{
+ return to_cpumask(sds->idle_cpus_span);
+}
+
struct sched_domain {
/* These fields must be setup */
struct sched_domain __rcu *parent; /* top domain must be null terminated */
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6b3b59cc51d6..cfe78fcf69da 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6136,7 +6136,12 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t

time = cpu_clock(this);

- cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
+ /*
+ * sched_domain_shared is set only at shared cache level,
+ * this works only because select_idle_cpu is called with
+ * sd_llc.
+ */
+ cpumask_and(cpus, sds_idle_cpus(sd->shared), p->cpus_ptr);

for_each_cpu_wrap(cpu, cpus, target) {
if (!--nr)
@@ -10182,6 +10187,7 @@ static void set_cpu_sd_state_busy(int cpu)
sd->nohz_idle = 0;

atomic_inc(&sd->shared->nr_busy_cpus);
+ cpumask_clear_cpu(cpu, sds_idle_cpus(sd->shared));
unlock:
rcu_read_unlock();
}
@@ -10212,6 +10218,7 @@ static void set_cpu_sd_state_idle(int cpu)
sd->nohz_idle = 1;

atomic_dec(&sd->shared->nr_busy_cpus);
+ cpumask_set_cpu(cpu, sds_idle_cpus(sd->shared));
unlock:
rcu_read_unlock();
}
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 9079d865a935..f14a6ef4de57 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -1407,6 +1407,7 @@ sd_init(struct sched_domain_topology_level *tl,
sd->shared = *per_cpu_ptr(sdd->sds, sd_id);
atomic_inc(&sd->shared->ref);
atomic_set(&sd->shared->nr_busy_cpus, sd_weight);
+ cpumask_copy(sds_idle_cpus(sd->shared), sched_domain_span(sd));
}

sd->private = sdd;
@@ -1769,7 +1770,7 @@ static int __sdt_alloc(const struct cpumask *cpu_map)

*per_cpu_ptr(sdd->sd, j) = sd;

- sds = kzalloc_node(sizeof(struct sched_domain_shared),
+ sds = kzalloc_node(sizeof(struct sched_domain_shared) + cpumask_size(),
GFP_KERNEL, cpu_to_node(j));
if (!sds)
return -ENOMEM;
--
2.25.1