linux-kernel - [patch] sched: fix select_idle_sibling() regression in selecting an idle SMT sibling

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Date:	Thu, 15 Dec 2011 11:47:00 -0800
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <peterz@...radead.org>,
	Mike Galbraith <efault@....de>, Alex Shi <alex.shi@...el.com>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>
Subject: [patch] sched: fix select_idle_sibling() regression in selecting an
 idle SMT sibling

Ingo, Please queue this scheduler patch for the next 3.2-rc release. We
already have a cleaned up version of this patch queued in -tip. If you
have any merge problems in tip/master, me or peterz can help resolve it.

Thanks.
---
From: Peter Zijlstra <peterz@...radead.org>
Subject: 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>
Acked-by: Suresh Siddha <suresh.b.siddha@...el.com>
Signed-off-by: Peter Zijlstra <peterz@...radead.org>
---
 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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