Commit 2c13c919 authored Feb 01, 2011 by Rik van Riel Committed by Ingo Molnar Feb 03, 2011

sched: Limit the scope of clear_buddies



The clear_buddies function does not seem to play well with the concept
of hierarchical runqueues.  In the following tree, task groups are
represented by 'G', tasks by 'T', next by 'n' and last by 'l'.

     (nl)
    /    \
   G(nl)  G
   / \     \
 T(l) T(n)  T

This situation can arise when a task is woken up T(n), and the previously
running task T(l) is marked last.

When clear_buddies is called from either T(l) or T(n), the next and last
buddies of the group G(nl) will be cleared.  This is not the desired
result, since we would like to be able to find the other type of buddy
in many cases.

This especially a worry when implementing yield_task_fair through the
buddy system.

The fix is simple: only clear the buddy type that the task itself
is indicated to be.  As an added bonus, we stop walking up the tree
when the buddy has already been cleared or pointed elsewhere.

Signed-off-by: Rik van Riel <riel@redhat.coM>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20110201094837.6b0962a9@annuminas.surriel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

parent 725e7580

kernel/sched_fair.c

+23 −7

Original line number	Original line	Diff line number	Diff line
	@@ -995,19 +995,35 @@ enqueue_entity(struct cfs_rq cfs_rq, struct sched_entity se, int flags)
	list_add_leaf_cfs_rq(cfs_rq);		list_add_leaf_cfs_rq(cfs_rq);
	}		}

	static void __clear_buddies(struct cfs_rq cfs_rq, struct sched_entity se)		static void __clear_buddies_last(struct sched_entity *se)
	{		{
	if (!se \|\| cfs_rq->last == se)		for_each_sched_entity(se) {
			struct cfs_rq *cfs_rq = cfs_rq_of(se);
			if (cfs_rq->last == se)
	cfs_rq->last = NULL;		cfs_rq->last = NULL;
			else
			break;
			}
			}

	if (!se \|\| cfs_rq->next == se)		static void __clear_buddies_next(struct sched_entity *se)
			{
			for_each_sched_entity(se) {
			struct cfs_rq *cfs_rq = cfs_rq_of(se);
			if (cfs_rq->next == se)
	cfs_rq->next = NULL;		cfs_rq->next = NULL;
			else
			break;
			}
	}		}

	static void clear_buddies(struct cfs_rq cfs_rq, struct sched_entity se)		static void clear_buddies(struct cfs_rq cfs_rq, struct sched_entity se)
	{		{
	for_each_sched_entity(se)		if (cfs_rq->last == se)
	__clear_buddies(cfs_rq_of(se), se);		__clear_buddies_last(se);

			if (cfs_rq->next == se)
			__clear_buddies_next(se);
	}		}

	static void		static void

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