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Date: Thu, 31 Jul 2014 01:43:08 +0400
From: Kirill Tkhai <tkhai@...dex.ru>
To: linux-kernel@...r.kernel.org
Cc: nicolas.pitre@...aro.org, peterz@...radead.org, pjt@...gle.com,
oleg@...hat.com, rostedt@...dmis.org, umgwanakikbuti@...il.com,
ktkhai@...allels.com, tim.c.chen@...ux.intel.com, mingo@...nel.org
Subject: [PATCH v3 5/5] sched/fair: Remove double_lock_balance() from
load_balance()
Keep on_rq = ONRQ_MIGRATING, while task is migrating, instead.
Signed-off-by: Kirill Tkhai <ktkhai@...allels.com>
---
kernel/sched/fair.c | 99 ++++++++++++++++++++++++++++++++++++---------------
1 file changed, 70 insertions(+), 29 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 8e2ee4b..475d05e 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4706,7 +4706,7 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int wake_
return;
/*
- * This is possible from callers such as move_task(), in which we
+ * This is possible from callers such as attach_tasks(), in which we
* unconditionally check_prempt_curr() after an enqueue (which may have
* lead to a throttle). This both saves work and prevents false
* next-buddy nomination below.
@@ -5114,18 +5114,20 @@ struct lb_env {
unsigned int loop_max;
enum fbq_type fbq_type;
+ struct list_head tasks;
};
/*
- * move_task - move a task from one runqueue to another runqueue.
- * Both runqueues must be locked.
+ * detach_task - detach a task for migration specified in env
*/
-static void move_task(struct task_struct *p, struct lb_env *env)
+static void detach_task(struct task_struct *p, struct lb_env *env)
{
+ lockdep_assert_held(&env->src_rq->lock);
+
deactivate_task(env->src_rq, p, 0);
+ list_add(&p->se.group_node, &env->tasks);
+ p->on_rq = ONRQ_MIGRATING;
set_task_cpu(p, env->dst_cpu);
- activate_task(env->dst_rq, p, 0);
- check_preempt_curr(env->dst_rq, p, 0);
}
/*
@@ -5363,9 +5365,9 @@ static struct task_struct *detach_one_task(struct lb_env *env)
/*
* Right now, this is only the second place where
- * lb_gained[env->idle] is updated (other is move_tasks)
+ * lb_gained[env->idle] is updated (other is detach_tasks)
* so we can safely collect stats here rather than
- * inside move_tasks().
+ * inside detach_tasks().
*/
schedstat_inc(env->sd, lb_gained[env->idle]);
return p;
@@ -5376,18 +5378,18 @@ static struct task_struct *detach_one_task(struct lb_env *env)
static const unsigned int sched_nr_migrate_break = 32;
/*
- * move_tasks tries to move up to imbalance weighted load from busiest to
- * this_rq, as part of a balancing operation within domain "sd".
- * Returns 1 if successful and 0 otherwise.
- *
- * Called with both runqueues locked.
+ * detach_tasks tries to detach up to imbalance weighted load from busiest_rq,
+ * as part of a balancing operation within domain "sd".
+ * Returns number of detached tasks if successful and 0 otherwise.
*/
-static int move_tasks(struct lb_env *env)
+static int detach_tasks(struct lb_env *env)
{
struct list_head *tasks = &env->src_rq->cfs_tasks;
struct task_struct *p;
unsigned long load;
- int pulled = 0;
+ int detached = 0;
+
+ lockdep_assert_held(&env->src_rq->lock);
if (env->imbalance <= 0)
return 0;
@@ -5418,14 +5420,15 @@ static int move_tasks(struct lb_env *env)
if ((load / 2) > env->imbalance)
goto next;
- move_task(p, env);
- pulled++;
+ detach_task(p, env);
+
+ detached++;
env->imbalance -= load;
#ifdef CONFIG_PREEMPT
/*
* NEWIDLE balancing is a source of latency, so preemptible
- * kernels will stop after the first task is pulled to minimize
+ * kernels will stop after the first task is detached to minimize
* the critical section.
*/
if (env->idle == CPU_NEWLY_IDLE)
@@ -5445,13 +5448,31 @@ static int move_tasks(struct lb_env *env)
}
/*
- * Right now, this is one of only two places move_task() is called,
- * so we can safely collect move_task() stats here rather than
- * inside move_task().
+ * Right now, this is one of only two places we collect this stat
+ * so we can safely collect detach_one_task() stats here rather
+ * than inside detach_one_task().
*/
- schedstat_add(env->sd, lb_gained[env->idle], pulled);
+ schedstat_add(env->sd, lb_gained[env->idle], detached);
- return pulled;
+ return detached;
+}
+
+/* Attach tasks previously detached in detach_tasks() */
+static void attach_tasks(struct lb_env *env)
+{
+ struct list_head *tasks = &env->tasks;
+ struct task_struct *p;
+
+ lockdep_assert_held(&env->dst_rq->lock);
+
+ while (!list_empty(tasks)) {
+ p = list_first_entry(tasks, struct task_struct, se.group_node);
+ BUG_ON(task_rq(p) != env->dst_rq);
+ list_del_init(&p->se.group_node);
+ p->on_rq = ONRQ_QUEUED;
+ activate_task(env->dst_rq, p, 0);
+ check_preempt_curr(env->dst_rq, p, 0);
+ }
}
#ifdef CONFIG_FAIR_GROUP_SCHED
@@ -6561,6 +6582,7 @@ static int load_balance(int this_cpu, struct rq *this_rq,
.loop_break = sched_nr_migrate_break,
.cpus = cpus,
.fbq_type = all,
+ .tasks = LIST_HEAD_INIT(env.tasks),
};
/*
@@ -6610,16 +6632,35 @@ static int load_balance(int this_cpu, struct rq *this_rq,
env.loop_max = min(sysctl_sched_nr_migrate, busiest->nr_running);
more_balance:
- local_irq_save(flags);
- double_rq_lock(env.dst_rq, busiest);
+ raw_spin_lock_irqsave(&busiest->lock, flags);
/*
* cur_ld_moved - load moved in current iteration
* ld_moved - cumulative load moved across iterations
*/
- cur_ld_moved = move_tasks(&env);
- ld_moved += cur_ld_moved;
- double_rq_unlock(env.dst_rq, busiest);
+ cur_ld_moved = detach_tasks(&env);
+
+ /*
+ * We've detached some tasks from busiest_rq. Every
+ * task is masked "ONRQ_MIGRATED", so we can safely
+ * unlock busiest->lock, and we are able to be sure
+ * that nobody can manipulate the tasks in parallel.
+ * See task_rq_lock() family for the details.
+ */
+
+ raw_spin_unlock(&busiest->lock);
+
+ if (cur_ld_moved) {
+ raw_spin_lock(&env.dst_rq->lock);
+ /*
+ * Attach the tasks to env->dst_rq
+ * and mask them "ONRQ_QUEUED".
+ */
+ attach_tasks(&env);
+ raw_spin_unlock(&env.dst_rq->lock);
+ ld_moved += cur_ld_moved;
+ }
+
local_irq_restore(flags);
/*
@@ -6755,7 +6796,7 @@ static int load_balance(int this_cpu, struct rq *this_rq,
* If we've begun active balancing, start to back off. This
* case may not be covered by the all_pinned logic if there
* is only 1 task on the busy runqueue (because we don't call
- * move_tasks).
+ * detach_tasks).
*/
if (sd->balance_interval < sd->max_interval)
sd->balance_interval *= 2;
--
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