linux-kernel - [tip:sched/urgent] sched: Fix select_idle_sibling() regression in selecting an idle SMT sibling

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Date:	Fri, 16 Dec 2011 03:36:10 -0800
From:	tip-bot for Peter Zijlstra <peterz@...radead.org>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, hpa@...or.com, mingo@...hat.com,
	efault@....de, peterz@...radead.org, alex.shi@...el.com,
	suresh.b.siddha@...el.com, tglx@...utronix.de, mingo@...e.hu
Subject: [tip:sched/urgent] sched: Fix select_idle_sibling()
  regression in selecting an idle SMT sibling

Commit-ID:  ab2789213d224202237292d78aaa0c386c7b28b2
Gitweb:     http://git.kernel.org/tip/ab2789213d224202237292d78aaa0c386c7b28b2
Author:     Peter Zijlstra <peterz@...radead.org>
AuthorDate: Thu, 15 Dec 2011 11:47:00 -0800
Committer:  Ingo Molnar <mingo@...e.hu>
CommitDate: Fri, 16 Dec 2011 09:44:58 +0100

sched: Fix select_idle_sibling() regression in selecting an idle SMT sibling

Mike Galbraith reported that this recent commit:

   commit 4dcfe1025b513c2c1da5bf5586adb0e80148f612
   Author: Peter Zijlstra <peterz@...radead.org>
   Date:   Thu Nov 10 13:01:10 2011 +0100

       sched: Avoid SMT siblings in select_idle_sibling() if possible

stopped selecting an idle SMT sibling when there are no idle
cores in a single socket system.

Intent of the select_idle_sibling() was to fallback to an idle
SMT sibling, if it fails to identify an idle core. But this
fallback was not happening on systems where all the scheduler
domains had `SD_SHARE_PKG_RESOURCES' flag set.

Fix it. Slightly bigger patch of cleaning all these goto's etc
is queued up for the next release.

Reported-by: Mike Galbraith <efault@....de>
Reported-by: Alex Shi <alex.shi@...el.com>
Signed-off-by: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Suresh Siddha <suresh.b.siddha@...el.com>
Link: http://lkml.kernel.org/r/1323978421.1984.244.camel@sbsiddha-desk.sc.intel.com
Signed-off-by: Ingo Molnar <mingo@...e.hu>
---
 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 a78ed27..8a39fa3 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -2352,13 +2352,11 @@ again:
 		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 {
@@ -2378,6 +2376,10 @@ next:
 			sg = sg->next;
 		} while (sg != sd->groups);
 	}
+	if (!smt) {
+		smt = 1;
+		goto again;
+	}
 done:
 	rcu_read_unlock();
 
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