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authorPeter Zijlstra <peterz@infradead.org>2016-06-16 18:51:48 +0200
committerIngo Molnar <mingo@kernel.org>2016-06-27 12:17:50 +0200
commite210bffd39d01b649c94b820c28ff112673266dd (patch)
treedb99be48b206fb9fd2f43c65499a7433ddd54d54 /kernel/sched
parent630741fb60ac4e286f5396403c0d864d924c02bc (diff)
sched/fair: Fix and optimize the fork() path
The task_fork_fair() callback already calls __set_task_cpu() and takes rq->lock. If we move the sched_class::task_fork callback in sched_fork() under the existing p->pi_lock, right after its set_task_cpu() call, we can avoid doing two such calls and omit the IRQ disabling on the rq->lock. Change to __set_task_cpu() to skip the migration bits, this is a new task, not a migration. Similarly, make wake_up_new_task() use __set_task_cpu() for the same reason, the task hasn't actually migrated as it hasn't ever ran. This cures the problem of calling migrate_task_rq_fair(), which does remove_entity_from_load_avg() on tasks that have never been added to the load avg to begin with. This bug would result in transiently messed up load_avg values, averaged out after a few dozen milliseconds. This is probably the reason why this bug was not found for such a long time. Reported-by: Vincent Guittot <vincent.guittot@linaro.org> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Mike Galbraith <efault@gmx.de> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: linux-kernel@vger.kernel.org Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'kernel/sched')
-rw-r--r--kernel/sched/core.c16
-rw-r--r--kernel/sched/fair.c27
2 files changed, 17 insertions, 26 deletions
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index e406ba0c6891..fa3434dffbbb 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2383,9 +2383,6 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
p->sched_class = &fair_sched_class;
}
- if (p->sched_class->task_fork)
- p->sched_class->task_fork(p);
-
/*
* The child is not yet in the pid-hash so no cgroup attach races,
* and the cgroup is pinned to this child due to cgroup_fork()
@@ -2394,7 +2391,13 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
* Silence PROVE_RCU.
*/
raw_spin_lock_irqsave(&p->pi_lock, flags);
- set_task_cpu(p, cpu);
+ /*
+ * We're setting the cpu for the first time, we don't migrate,
+ * so use __set_task_cpu().
+ */
+ __set_task_cpu(p, cpu);
+ if (p->sched_class->task_fork)
+ p->sched_class->task_fork(p);
raw_spin_unlock_irqrestore(&p->pi_lock, flags);
#ifdef CONFIG_SCHED_INFO
@@ -2534,8 +2537,11 @@ void wake_up_new_task(struct task_struct *p)
* Fork balancing, do it here and not earlier because:
* - cpus_allowed can change in the fork path
* - any previously selected cpu might disappear through hotplug
+ *
+ * Use __set_task_cpu() to avoid calling sched_class::migrate_task_rq,
+ * as we're not fully set-up yet.
*/
- set_task_cpu(p, select_task_rq(p, task_cpu(p), SD_BALANCE_FORK, 0));
+ __set_task_cpu(p, select_task_rq(p, task_cpu(p), SD_BALANCE_FORK, 0));
#endif
rq = __task_rq_lock(p, &rf);
post_init_entity_util_avg(&p->se);
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 73063560b9ec..994f5493ee5b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4448,7 +4448,7 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
*
* note: in the case of encountering a throttled cfs_rq we will
* post the final h_nr_running increment below.
- */
+ */
if (cfs_rq_throttled(cfs_rq))
break;
cfs_rq->h_nr_running++;
@@ -8289,31 +8289,17 @@ static void task_fork_fair(struct task_struct *p)
{
struct cfs_rq *cfs_rq;
struct sched_entity *se = &p->se, *curr;
- int this_cpu = smp_processor_id();
struct rq *rq = this_rq();
- unsigned long flags;
-
- raw_spin_lock_irqsave(&rq->lock, flags);
+ raw_spin_lock(&rq->lock);
update_rq_clock(rq);
cfs_rq = task_cfs_rq(current);
curr = cfs_rq->curr;
-
- /*
- * Not only the cpu but also the task_group of the parent might have
- * been changed after parent->se.parent,cfs_rq were copied to
- * child->se.parent,cfs_rq. So call __set_task_cpu() to make those
- * of child point to valid ones.
- */
- rcu_read_lock();
- __set_task_cpu(p, this_cpu);
- rcu_read_unlock();
-
- update_curr(cfs_rq);
-
- if (curr)
+ if (curr) {
+ update_curr(cfs_rq);
se->vruntime = curr->vruntime;
+ }
place_entity(cfs_rq, se, 1);
if (sysctl_sched_child_runs_first && curr && entity_before(curr, se)) {
@@ -8326,8 +8312,7 @@ static void task_fork_fair(struct task_struct *p)
}
se->vruntime -= cfs_rq->min_vruntime;
-
- raw_spin_unlock_irqrestore(&rq->lock, flags);
+ raw_spin_unlock(&rq->lock);
}
/*