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: 1e876231785d ("sched: Fix ->min_vruntime calculation in dequeue_entity()")
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 22 insertions, 7 deletions

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a6789485fcae..543b2f291152 100644
--- a/kernel/sched/fair.c
+++ b/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);
 }
 
 /*