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Date: Wed, 6 Aug 2014 12:06:56 +0400
From: Kirill Tkhai <ktkhai@...allels.com>
To: <linux-kernel@...r.kernel.org>
CC: <peterz@...radead.org>, <pjt@...gle.com>, <oleg@...hat.com>,
<rostedt@...dmis.org>, <umgwanakikbuti@...il.com>,
<tkhai@...dex.ru>, <tim.c.chen@...ux.intel.com>,
<mingo@...nel.org>, <nicolas.pitre@...aro.org>
Subject: [PATCH v4 5/6] sched/fair: Remove double_lock_balance() from
active_load_balance_cpu_stop()
Bad situation:
double_lock_balance() drops busiest_rq lock. The busiest_rq is *busiest*,
and a lot of tasks and context switches there. We are dropping the lock
and waiting for it again.
Let's just detach the task and once finally unlock it!
Warning: this admits unlocked using of can_migrate_task(), throttled_lb_pair(),
and task_hot(). I've added lockdep asserts to point on this.
Signed-off-by: Kirill Tkhai <ktkhai@...allels.com>
---
kernel/sched/fair.c | 55 +++++++++++++++++++++++++++++++++++----------------
1 file changed, 38 insertions(+), 17 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d54b72c..cfeafb1 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3297,6 +3297,8 @@ static inline int throttled_hierarchy(struct cfs_rq *cfs_rq)
* Ensure that neither of the group entities corresponding to src_cpu or
* dest_cpu are members of a throttled hierarchy when performing group
* load-balance operations.
+ *
+ * Note: RQs may be unlocked.
*/
static inline int throttled_lb_pair(struct task_group *tg,
int src_cpu, int dest_cpu)
@@ -5133,6 +5135,8 @@ 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;
@@ -5252,6 +5256,9 @@ 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
@@ -5336,30 +5343,34 @@ 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 = ONRQ_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;
}
static const unsigned int sched_nr_migrate_break = 32;
@@ -6914,6 +6925,7 @@ 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);
@@ -6933,9 +6945,6 @@ 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) {
@@ -6956,16 +6965,28 @@ 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) {
+ raw_spin_lock(&target_rq->lock);
+ BUG_ON(task_rq(p) != target_rq);
+ p->on_rq = ONRQ_QUEUED;
+ activate_task(target_rq, p, 0);
+ check_preempt_curr(target_rq, p, 0);
+ raw_spin_unlock(&target_rq->lock);
+ }
+
+ local_irq_enable();
+
return 0;
}
--
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