linux-kernel - Re: PostgreSQL pgbench performance regression in 2.6.23+

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Date:	Thu, 22 May 2008 14:24:41 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mike Galbraith <efault@....de>
Cc:	Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
	Greg Smith <gsmith@...gsmith.com>,
	lkml <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
Subject: Re: PostgreSQL pgbench performance regression in 2.6.23+

On Thu, 2008-05-22 at 14:09 +0200, Mike Galbraith wrote:
> On Thu, 2008-05-22 at 13:44 +0200, Peter Zijlstra wrote:
> 
> > Humm,.. how to fix this.. we'd need to somehow detect the 1:n nature of
> > its operation - I'm sure there are other scenarios that could benefit
> > from this.
> 
> Maybe simple (minded): cache waker's last non-interrupt context wakee,
> if the wakee != cached, ignore SYNC_WAKEUP unless sync was requested at
> call time?

Yeah, something like so - or perhaps like you say cache the wakee.

I picked the wake_affine() condition, because I think that is the
biggest factor in this behaviour. You could of course also disable all
of sync.



diff --git a/include/linux/sched.h b/include/linux/sched.h
index c86c5c5..856c2a8 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -950,6 +950,8 @@ struct sched_entity {
 	u64			last_wakeup;
 	u64			avg_overlap;
 
+	struct sched_entity 	*waker;
+
 #ifdef CONFIG_SCHEDSTATS
 	u64			wait_start;
 	u64			wait_max;
diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index 894a702..8971044 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -1036,7 +1036,8 @@ wake_affine(struct rq *rq, struct sched_domain *this_sd, struct rq *this_rq,
 	 * a reasonable amount of time then attract this newly
 	 * woken task:
 	 */
-	if (sync && curr->sched_class == &fair_sched_class) {
+	if (sync && curr->sched_class == &fair_sched_class &&
+	    p->se.waker == curr->se->waker) {
 		if (curr->se.avg_overlap < sysctl_sched_migration_cost &&
 				p->se.avg_overlap < sysctl_sched_migration_cost)
 			return 1;
@@ -1210,6 +1211,7 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p)
 	if (unlikely(se == pse))
 		return;
 
+	se->waker = pse;
 	cfs_rq_of(pse)->next = pse;
 
 	/*



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