[RFC PATCH v4] sched/fair: select idle cpu from idle cpumask for task wakeup

Aubrey Li
2020-11-18T03:48:37Z

From: Aubrey Li <aubrey.li@intel.com>

Add idle cpumask to track idle cpus in sched domain. When a CPU
enters idle, if the idle driver indicates to stop tick, this CPU
is set in the idle cpumask to be a wakeup target. And if the CPU
is not in idle, the CPU is cleared in idle cpumask during scheduler
tick to ratelimit idle cpumask update.

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.

Benchmarks were tested on a x86 4 socket system with 24 cores per
socket and 2 hyperthreads per core, total 192 CPUs. Hackbench and
schbench have no notable change, uperf has:

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

threads baseline-avg %std patch-avg %std
96 1 0.83 1.23 3.27
144 1 1.03 1.67 2.67
192 1 0.69 1.81 3.59
240 1 2.84 1.51 2.67

Cc: Mel Gorman <mgorman@suse.de>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
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/core.c | 2 ++
kernel/sched/fair.c | 52 +++++++++++++++++++++++++++++++++-
kernel/sched/idle.c | 7 +++--
kernel/sched/sched.h | 2 ++
kernel/sched/topology.c | 3 +-
6 files changed, 74 insertions(+), 5 deletions(-)

diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
index 820511289857..b47b85163607 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/core.c b/kernel/sched/core.c
index b1e0da56abca..c86ae0495163 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3994,6 +3994,7 @@ void scheduler_tick(void)
rq_lock(rq, &rf);

update_rq_clock(rq);
+ update_idle_cpumask(rq, false);
thermal_pressure = arch_scale_thermal_pressure(cpu_of(rq));
update_thermal_load_avg(rq_clock_thermal(rq), rq, thermal_pressure);
curr->sched_class->task_tick(rq, curr, 0);
@@ -7192,6 +7193,7 @@ void __init sched_init(void)
rq_csd_init(rq, &rq->nohz_csd, nohz_csd_func);
#endif
#endif /* CONFIG_SMP */
+ rq->last_idle_state = 1;
hrtick_rq_init(rq);
atomic_set(&rq->nr_iowait, 0);
}
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 48a6d442b444..d67fba5e406b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6145,7 +6145,12 @@ 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)
@@ -6807,6 +6812,51 @@ balance_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
}
#endif /* CONFIG_SMP */

+/*
+ * Update cpu idle state and record this information
+ * in sd_llc_shared->idle_cpus_span.
+ */
+void update_idle_cpumask(struct rq *rq, bool set_idle)
+{
+ struct sched_domain *sd;
+ int cpu = cpu_of(rq);
+ int idle_state;
+
+ /*
+ * If called from scheduler tick, only update
+ * idle cpumask if the CPU is busy, as idle
+ * cpumask is also updated on idle entry.
+ *
+ */
+ if (!set_idle && idle_cpu(cpu))
+ return;
+ /*
+ * Also set SCHED_IDLE cpu in idle cpumask to
+ * allow SCHED_IDLE cpu as a wakeup target
+ */
+ idle_state = set_idle || sched_idle_cpu(cpu);
+ /*
+ * No need to update idle cpumask if the state
+ * does not change.
+ */
+ if (rq->last_idle_state == idle_state)
+ return;
+
+ rcu_read_lock();
+ sd = rcu_dereference(per_cpu(sd_llc, cpu));
+ if (!sd || !sd->shared)
+ goto unlock;
+
+ if (idle_state)
+ cpumask_set_cpu(cpu, sds_idle_cpus(sd->shared));
+ else
+ cpumask_clear_cpu(cpu, sds_idle_cpus(sd->shared));
+
+ rq->last_idle_state = idle_state;
+unlock:
+ rcu_read_unlock();
+}
+
static unsigned long wakeup_gran(struct sched_entity *se)
{
unsigned long gran = sysctl_sched_wakeup_granularity;
diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
index f324dc36fc43..0bd83c00c22a 100644
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -164,7 +164,7 @@ static void cpuidle_idle_call(void)

if (cpuidle_not_available(drv, dev)) {
tick_nohz_idle_stop_tick();
-
+ update_idle_cpumask(this_rq(), true);
default_idle_call();
goto exit_idle;
}
@@ -205,9 +205,10 @@ static void cpuidle_idle_call(void)
*/
next_state = cpuidle_select(drv, dev, &stop_tick);

- if (stop_tick || tick_nohz_tick_stopped())
+ if (stop_tick || tick_nohz_tick_stopped()) {
+ update_idle_cpumask(this_rq(), true);
tick_nohz_idle_stop_tick();
- else
+ } else
tick_nohz_idle_retain_tick();

entered_state = call_cpuidle(drv, dev, next_state);
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 8d1ca65db3b0..db460b20217a 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1004,6 +1004,7 @@ struct rq {
/* This is used to determine avg_idle's max value */
u64 max_idle_balance_cost;
#endif /* CONFIG_SMP */
+ unsigned char last_idle_state;

#ifdef CONFIG_IRQ_TIME_ACCOUNTING
u64 prev_irq_time;
@@ -1088,6 +1089,7 @@ static inline void update_idle_core(struct rq *rq)
#else
static inline void update_idle_core(struct rq *rq) { }
#endif
+void update_idle_cpumask(struct rq *rq, bool set_idle);

DECLARE_PER_CPU_SHARED_ALIGNED(struct rq, runqueues);

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 1bd7e3af904f..541bd3a7de4d 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