tjh.dev / kernel
Linux kernel mirror (for testing) git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
kernel os linux
1
fork

Configure Feed

Select the types of activity you want to include in your feed.

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

Peter Zijlstra c70fc32f 44671e21

+3 -1
+3 -1
kernel/sched/fair.c
··· 3795 3795 update_entity_lag(cfs_rq, se); 3796 3796 se->deadline -= se->vruntime; 3797 3797 se->rel_deadline = 1; 3798 + cfs_rq->nr_queued--; 3798 3799 if (!curr) 3799 3800 __dequeue_entity(cfs_rq, se); 3800 3801 update_load_sub(&cfs_rq->load, se->load.weight); ··· 3822 3821 3823 3822 enqueue_load_avg(cfs_rq, se); 3824 3823 if (se->on_rq) { 3825 - update_load_add(&cfs_rq->load, se->load.weight); 3826 3824 place_entity(cfs_rq, se, 0); 3825 + update_load_add(&cfs_rq->load, se->load.weight); 3827 3826 if (!curr) 3828 3827 __enqueue_entity(cfs_rq, se); 3828 + cfs_rq->nr_queued++; 3829 3829 3830 3830 /* 3831 3831 * The entity's vruntime has been adjusted, so let's check