linux-kernel - Re: sched: Avoid SMT siblings in select_idle

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Date:	Thu, 17 Nov 2011 17:38:37 +0100
From:	Mike Galbraith <efault@....de>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Suresh Siddha <suresh.b.siddha@...el.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>, Paul Turner <pjt@...gle.com>
Subject: Re: sched: Avoid SMT siblings in select_idle_sibling() if possible

On Thu, 2011-11-17 at 16:56 +0100, Peter Zijlstra wrote:
> On Thu, 2011-11-17 at 16:38 +0100, Mike Galbraith wrote:
> > On Thu, 2011-11-17 at 02:59 +0100, Mike Galbraith wrote:
> > > On Wed, 2011-11-16 at 10:37 -0800, Suresh Siddha wrote:
> > 
> > > > It should be ok. Isn't it?
> > > 
> > > Nope, wasn't ok.
> > 
> > Because you can't get to again: with a single E5620 package, it having
> > only SIBLING and MC domains.
> >  
> > again:
> >         for_each_domain(target, sd) {
> >                 if (!smt && (sd->flags & SD_SHARE_CPUPOWER))
> >                         continue;
> > 
> >                 if (!(sd->flags & SD_SHARE_PKG_RESOURCES)) {
> >                         if (!smt) {
> >                                 smt = 1;
> >                                 goto again;
> > 
> 
> D'0h, indeed.. 
> 
> Something like the below maybe, although I'm certain it all can be
> written much nicer indeed.

I'll give it a go.

Squabbling with bouncing buddies in an isolated and otherwise idle
cpuset ate my day.
 
> ---
>  kernel/sched/fair.c |   14 ++++++++------
>  1 files changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index cd3b642..340e62e 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -2676,13 +2676,11 @@ static int select_idle_sibling(struct task_struct *p, int target)
>  		if (!smt && (sd->flags & SD_SHARE_CPUPOWER))
>  			continue;
>  
> -		if (!(sd->flags & SD_SHARE_PKG_RESOURCES)) {
> -			if (!smt) {
> -				smt = 1;
> -				goto again;
> -			}
> +		if (smt && !(sd->flags & SD_SHARE_CPUPOWER))
> +			break;
> +
> +		if (!(sd->flags & SD_SHARE_PKG_RESOURCES))
>  			break;
> -		}
>  
>  		sg = sd->groups;
>  		do {
> @@ -2702,6 +2700,10 @@ static int select_idle_sibling(struct task_struct *p, int target)
>  			sg = sg->next;
>  		} while (sg != sd->groups);
>  	}
> +	if (!smt) {
> +		smt = 1;
> +		goto again;
> +	}
>  done:
>  	rcu_read_unlock();
>  
> 
> 
> --
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