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Message-ID: <1408528081.23412.92.camel@tkhai>
Date: Wed, 20 Aug 2014 13:48:01 +0400
From: Kirill Tkhai <ktkhai@...allels.com>
To: <linux-kernel@...r.kernel.org>
CC: Peter Zijlstra <peterz@...radead.org>,
Paul Turner <pjt@...gle.com>, Oleg Nesterov <oleg@...hat.com>,
Steven Rostedt <rostedt@...dmis.org>,
"Mike Galbraith" <umgwanakikbuti@...il.com>,
Kirill Tkhai <tkhai@...dex.ru>,
"Tim Chen" <tim.c.chen@...ux.intel.com>,
Ingo Molnar <mingo@...nel.org>,
"Nicolas Pitre" <nicolas.pitre@...aro.org>
Subject: [PATCH v5 4/5] sched/fair: Remove double_lock_balance() from
active_load_balance_cpu_stop()
Avoid double_rq_lock() and use TASK_ON_RQ_MIGRATING for
active_load_balance_cpu_stop(). The advantage is (obviously) not
holding two 'rq->lock's at the same time and thereby increasing
parallelism.
Further note that if there was no task to migrate we will not have
acquired the second rq->lock at all.
The important point to note is that because we acquire dst->lock
immediately after releasing src->lock the potential wait time of
task_rq_lock() callers on TASK_ON_RQ_MIGRATING is not longer than
it would have been in the double rq lock scenario.
Signed-off-by: Kirill Tkhai <ktkhai@...allels.com>
---
kernel/sched/fair.c | 60 +++++++++++++++++++++++++++++++++++++--------------
1 file changed, 44 insertions(+), 16 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 9e6ca0d..7e5cf05 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5138,6 +5138,8 @@ static int task_hot(struct task_struct *p, struct lb_env *env)
{
s64 delta;
+ lockdep_assert_held(&env->src_rq->lock);
+
if (p->sched_class != &fair_sched_class)
return 0;
@@ -5257,6 +5259,9 @@ static
int can_migrate_task(struct task_struct *p, struct lb_env *env)
{
int tsk_cache_hot = 0;
+
+ lockdep_assert_held(&env->src_rq->lock);
+
/*
* We do not migrate tasks that are:
* 1) throttled_lb_pair, or
@@ -5341,30 +5346,49 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
}
/*
- * move_one_task tries to move exactly one task from busiest to this_rq, as
+ * detach_one_task() -- tries to dequeue exactly one task from env->src_rq, as
* part of active balancing operations within "domain".
- * Returns 1 if successful and 0 otherwise.
*
- * Called with both runqueues locked.
+ * Returns a task if successful and NULL otherwise.
*/
-static int move_one_task(struct lb_env *env)
+static struct task_struct *detach_one_task(struct lb_env *env)
{
struct task_struct *p, *n;
+ lockdep_assert_held(&env->src_rq->lock);
+
list_for_each_entry_safe(p, n, &env->src_rq->cfs_tasks, se.group_node) {
if (!can_migrate_task(p, env))
continue;
- move_task(p, env);
+ deactivate_task(env->src_rq, p, 0);
+ p->on_rq = TASK_ON_RQ_MIGRATING;
+ set_task_cpu(p, env->dst_cpu);
+
/*
- * Right now, this is only the second place move_task()
- * is called, so we can safely collect move_task()
- * stats here rather than inside move_task().
+ * Right now, this is only the second place where
+ * lb_gained[env->idle] is updated (other is move_tasks)
+ * so we can safely collect stats here rather than
+ * inside move_tasks().
*/
schedstat_inc(env->sd, lb_gained[env->idle]);
- return 1;
+ return p;
}
- return 0;
+ return NULL;
+}
+
+/*
+ * attach_one_task() -- attaches the task returned from detach_one_task() to
+ * its new rq.
+ */
+static void attach_one_task(struct rq *rq, struct task_struct *p)
+{
+ raw_spin_lock(&rq->lock);
+ BUG_ON(task_rq(p) != rq);
+ p->on_rq = TASK_ON_RQ_QUEUED;
+ activate_task(rq, p, 0);
+ check_preempt_curr(rq, p, 0);
+ raw_spin_unlock(&rq->lock);
}
static const unsigned int sched_nr_migrate_break = 32;
@@ -6943,6 +6967,7 @@ static int active_load_balance_cpu_stop(void *data)
int target_cpu = busiest_rq->push_cpu;
struct rq *target_rq = cpu_rq(target_cpu);
struct sched_domain *sd;
+ struct task_struct *p = NULL;
raw_spin_lock_irq(&busiest_rq->lock);
@@ -6962,9 +6987,6 @@ static int active_load_balance_cpu_stop(void *data)
*/
BUG_ON(busiest_rq == target_rq);
- /* move a task from busiest_rq to target_rq */
- double_lock_balance(busiest_rq, target_rq);
-
/* Search for an sd spanning us and the target CPU. */
rcu_read_lock();
for_each_domain(target_cpu, sd) {
@@ -6985,16 +7007,22 @@ static int active_load_balance_cpu_stop(void *data)
schedstat_inc(sd, alb_count);
- if (move_one_task(&env))
+ p = detach_one_task(&env);
+ if (p)
schedstat_inc(sd, alb_pushed);
else
schedstat_inc(sd, alb_failed);
}
rcu_read_unlock();
- double_unlock_balance(busiest_rq, target_rq);
out_unlock:
busiest_rq->active_balance = 0;
- raw_spin_unlock_irq(&busiest_rq->lock);
+ raw_spin_unlock(&busiest_rq->lock);
+
+ if (p)
+ attach_one_task(target_rq, p);
+
+ local_irq_enable();
+
return 0;
}
--
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