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@infradead.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@gmx.de>
Reported-by: Alex Shi <alex.shi@intel.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Link: http://lkml.kernel.org/r/1323978421.1984.244.camel@sbsiddha-desk.sc.intel.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 8 insertions, 6 deletions

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index a78ed2736ba7..8a39fa3e3c6c 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();