sched/fair: Fix min_vruntime tracking

While going through enqueue/dequeue to review the movement of
set_curr_task() I noticed that the (2nd) update_min_vruntime() call in
dequeue_entity() is suspect.

It turns out, its actually wrong because it will consider
cfs_rq->curr, which could be the entry we just normalized. This mixes
different vruntime forms and leads to fail.

The purpose of the second update_min_vruntime() is to move
min_vruntime forward if the entity we just removed is the one that was
holding it back; _except_ for the DEQUEUE_SAVE case, because then we
know its a temporary removal and it will come back.

However, since we do put_prev_task() _after_ dequeue(), cfs_rq->curr
will still be set (and per the above, can be tranformed into a
different unit), so update_min_vruntime() should also consider
curr->on_rq. This also fixes another corner case where the enqueue
(which also does update_curr()->update_min_vruntime()) happens on the
rq->lock break in schedule(), between dequeue and put_prev_task.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Fixes: 1e876231 ("sched: Fix ->min_vruntime calculation in dequeue_entity()")
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/fair.c

@@ -460,17 +460,23 @@ static inline int entity_before(struct sched_entity *a,
 
 static void update_min_vruntime(struct cfs_rq *cfs_rq)
 {
+	struct sched_entity *curr = cfs_rq->curr;
+
 	u64 vruntime = cfs_rq->min_vruntime;
 
-	if (cfs_rq->curr)
-		vruntime = cfs_rq->curr->vruntime;
+	if (curr) {
+		if (curr->on_rq)
+			vruntime = curr->vruntime;
+		else
+			curr = NULL;
+	}
 
 	if (cfs_rq->rb_leftmost) {
 		struct sched_entity *se = rb_entry(cfs_rq->rb_leftmost,
 						   struct sched_entity,
 						   run_node);
 
-		if (!cfs_rq->curr)
+		if (!curr)
 			vruntime = se->vruntime;
 		else
 			vruntime = min_vruntime(vruntime, se->vruntime);
@@ -3478,9 +3484,10 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 	account_entity_dequeue(cfs_rq, se);
 
 	/*
-	 * Normalize the entity after updating the min_vruntime because the
-	 * update can refer to the ->curr item and we need to reflect this
-	 * movement in our normalized position.
+	 * Normalize after update_curr(); which will also have moved
+	 * min_vruntime if @se is the one holding it back. But before doing
+	 * update_min_vruntime() again, which will discount @se's position and
+	 * can move min_vruntime forward still more.
 	 */
 	if (!(flags & DEQUEUE_SLEEP))
 		se->vruntime -= cfs_rq->min_vruntime;
@@ -3488,8 +3495,16 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 	/* return excess runtime on last dequeue */
 	return_cfs_rq_runtime(cfs_rq);
 
-	update_min_vruntime(cfs_rq);
 	update_cfs_shares(cfs_rq);
+
+	/*
+	 * Now advance min_vruntime if @se was the entity holding it back,
+	 * except when: DEQUEUE_SAVE && !DEQUEUE_MOVE, in this case we'll be
+	 * put back on, and if we advance min_vruntime, we'll be placed back
+	 * further than we started -- ie. we'll be penalized.
+	 */
+	if ((flags & (DEQUEUE_SAVE | DEQUEUE_MOVE)) == DEQUEUE_SAVE)
+		update_min_vruntime(cfs_rq);
 }
 
 /*