superrob1500
/
mirror-linux
mirror of https://github.com/torvalds/linux
1
0
Fork 0
Code Issues Packages Projects Releases Wiki Activity

sched/fair: Adhere to place_entity() constraints 

Mike reports that commit 6d71a9c616 ("sched/fair: Fix EEVDF entity
placement bug causing scheduling lag") relies on commit 4423af84b2
("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: 6d71a9c616 ("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
pull/1250/head
Peter Zijlstra 2025-01-28 15:39:49 +01:00
parent 44671e21e3
commit c70fc32f44
1 changed files with 3 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

4
kernel/sched/fair.c
View File

@ -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