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

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Message-ID: <1321545376.2495.1.camel@laptop>
Date:	Thu, 17 Nov 2011 16:56:16 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mike Galbraith <efault@....de>
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 16:38 +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.

---
 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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