The following commit has been merged in the linux branch: commit 92f6a5e37a2e2d3342dafb2b39c2f8bc340bbf84 Author: Peter Zijlstra a.p.zijlstra@chello.nl Date: Fri Oct 9 12:43:07 2009 +0200
sched: Do less agressive buddy clearing
Yanmin reported a hackbench regression due to:
> commit de69a80be32445b0a71e8e3b757e584d7beb90f7 > 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
diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c index 4e777b4..c32c3e6 100644 --- a/kernel/sched_fair.c +++ b/kernel/sched_fair.c @@ -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);