[tip:sched/urgent] sched/numa: Avoid selecting oneself as swap target

[tip-bot for Peter Zijlstra]

Commit-ID:  7af683350cb0ddd0e9d3819b4eb7abe9e2d3e709
Gitweb:     http://git.kernel.org/tip/7af683350cb0ddd0e9d3819b4eb7abe9e2d3e709
Author:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate: Mon, 10 Nov 2014 10:54:35 +0100
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Sun, 16 Nov 2014 10:04:17 +0100

sched/numa: Avoid selecting oneself as swap target

Because the whole numa task selection stuff runs with preemption
enabled (its long and expensive) we can end up migrating and selecting
oneself as a swap target. This doesn't really work out well -- we end
up trying to acquire the same lock twice for the swap migrate -- so
avoid this.

Reported-and-Tested-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/20141110100328.GF29390@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 kernel/sched/fair.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 34baa60..3af3d1e 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1180,6 +1180,13 @@ static void task_numa_compare(struct task_numa_env *env,
 	raw_spin_unlock_irq(&dst_rq->lock);
 
 	/*
+	 * Because we have preemption enabled we can get migrated around and
+	 * end try selecting ourselves (current == env->p) as a swap candidate.
+	 */
+	if (cur == env->p)
+		goto unlock;
+
+	/*
 	 * "imp" is the fault differential for the source task between the
 	 * source and destination node. Calculate the total differential for
 	 * the source task and potential destination task. The more negative
--
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