sched/fair: Adhere to place_entity() constraints

Mike reports that commit 6d71a9c61604 ("sched/fair: Fix EEVDF entity
placement bug causing scheduling lag") relies on commit 4423af84b297
("sched/fair: optimize the PLACE_LAG when se->vlag is zero") to not
trip a WARN in place_entity().

What happens is that the lag of the very last entity is 0 per
definition -- the average of one element matches the value of that
element. Therefore place_entity() will match the condition skipping
the lag adjustment:

  if (sched_feat(PLACE_LAG) && cfs_rq->nr_queued && se->vlag) {

Without the 'se->vlag' condition -- it will attempt to adjust the zero
lag even though we're inserting into an empty tree.

Notably, we should have failed the 'cfs_rq->nr_queued' condition, but
don't because they didn't get updated.

Additionally, move update_load_add() after placement() as is
consistent with other place_entity() users -- this change is
non-functional, place_entity() does not use cfs_rq->load.

Fixes: 6d71a9c61604 ("sched/fair: Fix EEVDF entity placement bug causing scheduling lag")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reported-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Signed-off-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/c216eb4ef0e0e0029c600aefc69d56681cee5581.camel@gmx.de

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 5e1bd9e8464cddee5d9307437344a3390de0a2e3..eb5a2572b4f8b6b5517befc299312b6ae7476e88 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3795,6 +3795,7 @@ static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
                update_entity_lag(cfs_rq, se);
                se->deadline -= se->vruntime;
                se->rel_deadline = 1;
+               cfs_rq->nr_queued--;
                if (!curr)
                        __dequeue_entity(cfs_rq, se);
                update_load_sub(&cfs_rq->load, se->load.weight);
@@ -3821,10 +3822,11 @@ static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
 
        enqueue_load_avg(cfs_rq, se);
        if (se->on_rq) {
-               update_load_add(&cfs_rq->load, se->load.weight);
                place_entity(cfs_rq, se, 0);
+               update_load_add(&cfs_rq->load, se->load.weight);
                if (!curr)
                        __enqueue_entity(cfs_rq, se);
+               cfs_rq->nr_queued++;
 
                /*
                 * The entity's vruntime has been adjusted, so let's check