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

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Date:	Thu, 22 May 2008 15:16:49 +0200
From:	Mike Galbraith <efault@....de>
To:	Peter Zijlstra <peterz@...radead.org>
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 14:24 +0200, Peter Zijlstra wrote:
> 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.

Works fine (modulo booboo).

	-Mike


> 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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