Commit de69a80b authored Sep 17, 2009 by Peter Zijlstra Committed by Ingo Molnar Sep 17, 2009

sched: Stop buddies from hogging the system



Clear buddies more agressively.

The (theoretical, haven't actually observed any of this) problem is
that when we do not select either buddy in pick_next_entity()
because they are too far ahead of the left-most task, we do not
clear the buddies.

This means that as soon as we service the left-most task, these
same buddies will be tried again on the next schedule. Now if the
left-most task was a pure hog, it wouldn't have done any wakeups
and it wouldn't have set buddies of its own. That leads to the old
buddies dominating, which would lead to bad latencies.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

parent ad4b78bb

kernel/sched_fair.c

+8 −3

Original line number	Diff line number	Diff line
		@@ -764,10 +764,10 @@ enqueue_entity(struct cfs_rq cfs_rq, struct sched_entity se, int wakeup)

		static void __clear_buddies(struct cfs_rq cfs_rq, struct sched_entity se)
		{
		if (cfs_rq->last == se)
		if (!se \|\| cfs_rq->last == se)
		cfs_rq->last = NULL;

		if (cfs_rq->next == se)
		if (!se \|\| cfs_rq->next == se)
		cfs_rq->next = NULL;
		}

		@@ -1646,8 +1646,13 @@ static struct task_struct pick_next_task_fair(struct rq rq)
		/*
		* If se was a buddy, clear it so that it will have to earn
		* the favour again.
		*
		* If se was not a buddy, clear the buddies because neither
		* was elegible to run, let them earn it again.
		*
		* IOW. unconditionally clear buddies.
		*/
		__clear_buddies(cfs_rq, se);
		__clear_buddies(cfs_rq, NULL);
		set_next_entity(cfs_rq, se);
		cfs_rq = group_cfs_rq(se);
		} while (cfs_rq);

Admin message