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Date: Thu, 17 Feb 2022 23:43:58 +0800
From: Abel Wu <wuyun.abel@...edance.com>
To: Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
Daniel Bristot de Oliveira <bristot@...hat.com>
Cc: linux-kernel@...r.kernel.org
Subject: [RFC PATCH 2/5] sched/fair: introduce sched-idle balance
The goal of the sched-idle balancing is to let the non-idle
tasks make full use of cpu resources. To achieve that, we
mainly do two things:
- pull non-idle tasks for sched-idle or idle rqs
from the overloaded ones, and
- prevent pulling the last non-idle task in an rq
We do sched-idle balance at normal load balancing and newly
idle if necessary. The idle balancing is ignored due to high
wakeup latency.
Signed-off-by: Abel Wu <wuyun.abel@...edance.com>
---
include/linux/sched/idle.h | 1 +
kernel/sched/fair.c | 128 +++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 129 insertions(+)
diff --git a/include/linux/sched/idle.h b/include/linux/sched/idle.h
index d73d314d59c6..50ec5c770f85 100644
--- a/include/linux/sched/idle.h
+++ b/include/linux/sched/idle.h
@@ -8,6 +8,7 @@ enum cpu_idle_type {
CPU_IDLE,
CPU_NOT_IDLE,
CPU_NEWLY_IDLE,
+ CPU_SCHED_IDLE,
CPU_MAX_IDLE_TYPES
};
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0a0438c3319b..070a6fb1d2bf 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -456,6 +456,21 @@ static int se_is_idle(struct sched_entity *se)
return cfs_rq_is_idle(group_cfs_rq(se));
}
+/* Is task idle from the top hierarchy POV */
+static int task_h_idle(struct task_struct *p)
+{
+ struct sched_entity *se = &p->se;
+
+ if (task_has_idle_policy(p))
+ return 1;
+
+ for_each_sched_entity(se)
+ if (cfs_rq_is_idle(cfs_rq_of(se)))
+ return 1;
+
+ return 0;
+}
+
#else /* !CONFIG_FAIR_GROUP_SCHED */
#define for_each_sched_entity(se) \
@@ -508,6 +523,11 @@ static int se_is_idle(struct sched_entity *se)
return 0;
}
+static inline int task_h_idle(struct task_struct *p)
+{
+ return task_has_idle_policy(p);
+}
+
#endif /* CONFIG_FAIR_GROUP_SCHED */
static __always_inline
@@ -6974,6 +6994,11 @@ static inline int cfs_rq_overloaded(struct rq *rq)
return rq->cfs.h_nr_running - rq->cfs.idle_h_nr_running > 1;
}
+static inline bool need_pull_cfs_task(struct rq *rq)
+{
+ return rq->cfs.h_nr_running == rq->cfs.idle_h_nr_running;
+}
+
/* Must be called with rq locked */
static void update_overload_status(struct rq *rq)
{
@@ -7767,6 +7792,22 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
if (kthread_is_per_cpu(p))
return 0;
+ /*
+ * Disregard hierarchically idle tasks during sched-idle
+ * load balancing.
+ */
+ if (env->idle == CPU_SCHED_IDLE && task_h_idle(p))
+ return 0;
+
+ /*
+ * Skip p if it is the last non-idle task in src_rq. This
+ * protects latency and throughput for non-idle tasks at
+ * the cost of temporary load imbalance (which will probably
+ * be fixed soon).
+ */
+ if (!cfs_rq_overloaded(env->src_rq) && !task_h_idle(p))
+ return 0;
+
if (!cpumask_test_cpu(env->dst_cpu, p->cpus_ptr)) {
int cpu;
@@ -10265,6 +10306,83 @@ static inline bool update_newidle_cost(struct sched_domain *sd, u64 cost)
}
/*
+ * The sched-idle balancing tries to eliminate overloaded cfs rqs
+ * by spreading out non-idle tasks prior to normal load balancing.
+ */
+static void sched_idle_balance(struct rq *dst_rq)
+{
+ struct sched_domain *sd;
+ struct task_struct *p;
+ int dst_cpu = cpu_of(dst_rq), cpu;
+
+ sd = rcu_dereference(per_cpu(sd_llc, dst_cpu));
+ if (unlikely(!sd))
+ return;
+
+ if (!atomic_read(&sd->shared->nr_overloaded))
+ return;
+
+ for_each_cpu_wrap(cpu, sdo_mask(sd->shared), dst_cpu + 1) {
+ struct rq *rq = cpu_rq(cpu);
+ struct rq_flags rf;
+ struct lb_env env;
+
+ if (cpu == dst_cpu)
+ continue;
+
+ if (!cfs_rq_overloaded(rq))
+ continue;
+
+ rq_lock_irqsave(rq, &rf);
+
+ /*
+ * Check again to ensure there are pullable tasks.
+ * This is necessary because multiple rqs can pull
+ * tasks at the same time. IOW contention on this
+ * rq is heavy, so it would be better clear this
+ * cpu from overloaded mask.
+ */
+ if (unlikely(!cfs_rq_overloaded(rq))) {
+ update_overload_status(rq);
+ rq_unlock_irqrestore(rq, &rf);
+ continue;
+ }
+
+ env = (struct lb_env) {
+ .sd = sd,
+ .dst_cpu = dst_cpu,
+ .dst_rq = dst_rq,
+ .src_cpu = cpu,
+ .src_rq = rq,
+ .idle = CPU_SCHED_IDLE, /* non-idle only */
+ .flags = LBF_DST_PINNED, /* pin dst_cpu */
+ };
+
+ update_rq_clock(rq);
+ p = detach_one_task(&env);
+
+ /*
+ * Lazy updating overloaded mask here. If the rq is
+ * still overloaded then we are just wasting cycles.
+ * And it's OK even if the rq becomes un-overloaded
+ * since the cost of peeking rq's data without lock
+ * won't be much in next loops (during which the rq
+ * can even be overloaded again).
+ */
+
+ rq_unlock(rq, &rf);
+
+ if (p) {
+ attach_one_task(dst_rq, p);
+ local_irq_restore(rf.flags);
+ return;
+ }
+
+ local_irq_restore(rf.flags);
+ }
+}
+
+/*
* It checks each scheduling domain to see if it is due to be balanced,
* and initiates a balancing operation if so.
*
@@ -10284,6 +10402,10 @@ static void rebalance_domains(struct rq *rq, enum cpu_idle_type idle)
u64 max_cost = 0;
rcu_read_lock();
+
+ if (need_pull_cfs_task(rq))
+ sched_idle_balance(rq);
+
for_each_domain(cpu, sd) {
/*
* Decay the newidle max times here because this is a regular
@@ -10913,6 +11035,12 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
update_blocked_averages(this_cpu);
rcu_read_lock();
+
+ sched_idle_balance(this_rq);
+ t1 = sched_clock_cpu(this_cpu);
+ curr_cost += t1 - t0;
+ t0 = t1;
+
for_each_domain(this_cpu, sd) {
int continue_balancing = 1;
u64 domain_cost;
--
2.11.0
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