Subject: Re: [tip:sched/core] sched/core: Add debugging code to catch missing update_rq_clock() calls

From: Peter Zijlstra <[email-masked]>
Date: Thu, 2 Feb 2017 16:55:06 +0100

On Tue, Jan 31, 2017 at 10:22:47AM -0700, Ross Zwisler wrote:
> On Tue, Jan 31, 2017 at 4:48 AM, Mike Galbraith <[email-masked]> wrote:
> > On Tue, 2017-01-31 at 16:30 +0530, Sachin Sant wrote:

Could some of you test this? It seems to cure things in my (very)
limited testing.

---
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 96e4ccc..b773821 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5609,7 +5609,7 @@ static void migrate_tasks(struct rq *dead_rq)
 {
 	struct rq *rq = dead_rq;
 	struct task_struct *next, *stop = rq->stop;
-	struct rq_flags rf, old_rf;
+	struct rq_flags rf;
 	int dest_cpu;
 
 	/*
@@ -5628,7 +5628,9 @@ static void migrate_tasks(struct rq *dead_rq)
 	 * class method both need to have an up-to-date
 	 * value of rq->clock[_task]
 	 */
+	rq_pin_lock(rq, &rf);
 	update_rq_clock(rq);
+	rq_unpin_lock(rq, &rf);
 
 	for (;;) {
 		/*
@@ -5641,7 +5643,7 @@ static void migrate_tasks(struct rq *dead_rq)
 		/*
 		 * pick_next_task assumes pinned rq->lock.
 		 */
-		rq_pin_lock(rq, &rf);
+		rq_repin_lock(rq, &rf);
 		next = pick_next_task(rq, &fake_task, &rf);
 		BUG_ON(!next);
 		next->sched_class->put_prev_task(rq, next);
@@ -5670,13 +5672,6 @@ 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);
 
@@ -5685,7 +5680,6 @@ 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);
 	}