[PATCH 3.19.y-ckt 44/86] sched: access local runqueue directly in single_task_running

[Kamal Mostafa]

3.19.8-ckt9 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Dominik Dingel <dingel@xxxxxxxxxxxxxxxxxx>

commit 00cc1633816de8c95f337608a1ea64e228faf771 upstream.

Commit 2ee507c47293 ("sched: Add function single_task_running to let a task
check if it is the only task running on a cpu") referenced the current
runqueue with the smp_processor_id.  When CONFIG_DEBUG_PREEMPT is enabled,
that is only allowed if preemption is disabled or the currrent task is
bound to the local cpu (e.g. kernel worker).

With commit f78195129963 ("kvm: add halt_poll_ns module parameter") KVM
calls single_task_running. If CONFIG_DEBUG_PREEMPT is enabled that
generates a lot of kernel messages.

To avoid adding preemption in that cases, as it would limit the usefulness,
we change single_task_running to access directly the cpu local runqueue.

Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Fixes: 2ee507c472939db4b146d545352b8a7c79ef47f8
Signed-off-by: Dominik Dingel <dingel@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Signed-off-by: Kamal Mostafa <kamal@xxxxxxxxxxxxx>
---
 kernel/sched/core.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 48b14d6..2ef0f5e 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2408,13 +2408,20 @@ unsigned long nr_running(void)
 
 /*
  * Check if only the current task is running on the cpu.
+ *
+ * Caution: this function does not check that the caller has disabled
+ * preemption, thus the result might have a time-of-check-to-time-of-use
+ * race.  The caller is responsible to use it correctly, for example:
+ *
+ * - from a non-preemptable section (of course)
+ *
+ * - from a thread that is bound to a single CPU
+ *
+ * - in a loop with very short iterations (e.g. a polling loop)
  */
 bool single_task_running(void)
 {
-	if (cpu_rq(smp_processor_id())->nr_running == 1)
-		return true;
-	else
-		return false;
+	return raw_rq()->nr_running == 1;
 }
 EXPORT_SYMBOL(single_task_running);
 
-- 
1.9.1

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