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>
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7306356..994f549 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4448,7 +4448,7 @@
*
* 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 @@
{
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 @@
}
se->vruntime -= cfs_rq->min_vruntime;
-
- raw_spin_unlock_irqrestore(&rq->lock, flags);
+ raw_spin_unlock(&rq->lock);
}
/*