Commit 92f6a5e3 authored Oct 09, 2009 by Peter Zijlstra Committed by Ingo Molnar Oct 14, 2009

sched: Do less agressive buddy clearing



Yanmin reported a hackbench regression due to:

 > commit de69a80b
 > Author: Peter Zijlstra <a.p.zijlstra@chello.nl>
 > Date:   Thu Sep 17 09:01:20 2009 +0200
 >
 >     sched: Stop buddies from hogging the system

I really liked de69a80b, and it affecting hackbench shows I wasn't
crazy ;-)

So hackbench is a multi-cast, with one sender spraying multiple
receivers, who in their turn don't spray back.

This would be exactly the scenario that patch 'cures'. Previously
we would not clear the last buddy after running the next task,
allowing the sender to get back to work sooner than it otherwise
ought to have been, increasing latencies for other tasks.

Now, since those receivers don't poke back, they don't enforce the
buddy relation, which means there's nothing to re-elect the sender.

Cure this by less agressively clearing the buddy stats. Only clear
buddies when they were not chosen. It should still avoid a buddy
sticking around long after its served its time.

Reported-by: "Zhang, Yanmin" <yanmin_zhang@linux.intel.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
CC: Mike Galbraith <efault@gmx.de>
LKML-Reference: <1255084986.8802.46.camel@laptop>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

parent 799e2205

kernel/sched_fair.c

+13 −14

Original line number	Diff line number	Diff line
		@@ -861,12 +861,21 @@ wakeup_preempt_entity(struct sched_entity curr, struct sched_entity se);
		static struct sched_entity pick_next_entity(struct cfs_rq cfs_rq)
		{
		struct sched_entity *se = __pick_next_entity(cfs_rq);
		struct sched_entity *buddy;

		if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, se) < 1)
		return cfs_rq->next;
		if (cfs_rq->next) {
		buddy = cfs_rq->next;
		cfs_rq->next = NULL;
		if (wakeup_preempt_entity(buddy, se) < 1)
		return buddy;
		}

		if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, se) < 1)
		return cfs_rq->last;
		if (cfs_rq->last) {
		buddy = cfs_rq->last;
		cfs_rq->last = NULL;
		if (wakeup_preempt_entity(buddy, se) < 1)
		return buddy;
		}

		return se;
		}
		@@ -1654,16 +1663,6 @@ static struct task_struct pick_next_task_fair(struct rq rq)

		do {
		se = pick_next_entity(cfs_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, NULL);
		set_next_entity(cfs_rq, se);
		cfs_rq = group_cfs_rq(se);
		} while (cfs_rq);

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