sched/fair: Restore previous rq_flags when migrating tasks in hotplug

__migrate_task() can return with a different runqueue locked than the
one we passed as an argument. So that we can repin the lock in
migrate_tasks() (and keep the update_rq_clock() bit) we need to
restore the old rq_flags before repinning.

Note that it wouldn't be correct to change move_queued_task() to repin
because of the change of runqueue and the fact that having an
up-to-date clock on the initial rq doesn't mean the new rq has one
too.

Signed-off-by: Matt Fleming <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Mike Galbraith <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>

kernel/sched/core.c

@@ -5608,7 +5608,7 @@ static void migrate_tasks(struct rq *dead_rq)
 {
 	struct rq *rq = dead_rq;
 	struct task_struct *next, *stop = rq->stop;
-	struct rq_flags rf;
+	struct rq_flags rf, old_rf;
 	int dest_cpu;
 
 	/*
@@ -5669,6 +5669,13 @@ static void migrate_tasks(struct rq *dead_rq)
 			continue;
 		}
 
+		/*
+		 * __migrate_task() may return with a different
+		 * rq->lock held and a new cookie in 'rf', but we need
+		 * to preserve rf::clock_update_flags for 'dead_rq'.
+		 */
+		old_rf = rf;
+
 		/* Find suitable destination for @next, with force if needed. */
 		dest_cpu = select_fallback_rq(dead_rq->cpu, next);
 
@@ -5677,6 +5684,7 @@ static void migrate_tasks(struct rq *dead_rq)
 			raw_spin_unlock(&rq->lock);
 			rq = dead_rq;
 			raw_spin_lock(&rq->lock);
+			rf = old_rf;
 		}
 		raw_spin_unlock(&next->pi_lock);
 	}