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Commit e210bffd authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
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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: default avatarVincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: default avatarPeter 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: default avatarIngo Molnar <mingo@kernel.org>
parent 630741fb

kernel/sched/core.c

+11 −5
Original line number Diff line number Diff line
@@ -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);

kernel/sched/fair.c

+6 −21
Original line number Diff line number Diff line
@@ -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();



	if (curr) {

		update_curr(cfs_rq);



	if (curr)

		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);

}



/*