[PATCH 1/6] sched: Fix ttwu_queue_cond()

[Peter Zijlstra]

Where the condition:

  !cpus_share_cache(smp_processor_id(), cpu)

already implies 'cpu != smp_processor_id()', because a CPU always
shares cache with itself, the secondary condition added in commit:

  2ebb17717550 ("sched/core: Offload wakee task activation if it the wakee is descheduling")

voids that implication, resulting in attempting to do local wake-ups
through the queue mechanism.

Fixes: 2ebb17717550 ("sched/core: Offload wakee task activation if it the wakee is descheduling")
Reported-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Tested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 kernel/sched/core.c |   13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2356,11 +2356,22 @@ bool cpus_share_cache(int this_cpu, int
 
 static inline bool ttwu_queue_cond(int cpu, int wake_flags)
 {
+	int this_cpu = smp_processor_id();
+
+	/*
+	 * Only ever queue for remote wakeups. The on_cpu case can only ever
+	 * happen remotely, and for the normal case it makes no sense to
+	 * involve IPIs here, and would be broken, as many architectures cannot
+	 * trivially IPI self in any case.
+	 */
+	if (cpu == this_cpu)
+		return false;
+
 	/*
 	 * If the CPU does not share cache, then queue the task on the
 	 * remote rqs wakelist to avoid accessing remote data.
 	 */
-	if (!cpus_share_cache(smp_processor_id(), cpu))
+	if (!cpus_share_cache(this_cpu, cpu))
 		return true;
 
 	/*