This is a note to let you know that I've just added the patch titled
sched/fair: Fix min_vruntime tracking
to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
sched-fair-fix-min_vruntime-tracking.patch
and it can be found in the queue-4.4 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From b60205c7c558330e4e2b5df498355ec959457358 Mon Sep 17 00:00:00 2001
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date: Tue, 20 Sep 2016 21:58:12 +0200
Subject: sched/fair: Fix min_vruntime tracking
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
commit b60205c7c558330e4e2b5df498355ec959457358 upstream.
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@xxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Mike Galbraith <efault@xxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
Fixes: 1e876231785d ("sched: Fix ->min_vruntime calculation in dequeue_entity()")
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
kernel/sched/fair.c | 29 ++++++++++++++++++++++-------
1 file changed, 22 insertions(+), 7 deletions(-)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -456,17 +456,23 @@ static inline int entity_before(struct s
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);
@@ -3139,9 +3145,10 @@ dequeue_entity(struct cfs_rq *cfs_rq, st
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;
@@ -3149,8 +3156,16 @@ dequeue_entity(struct cfs_rq *cfs_rq, st
/* 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);
}
/*
Patches currently in stable-queue which might be from peterz@xxxxxxxxxxxxx are
queue-4.4/ipc-sem.c-fix-complex_count-vs.-simple-op-race.patch
queue-4.4/x86-e820-don-t-merge-consecutive-e820_pram-ranges.patch
queue-4.4/sched-fair-fix-min_vruntime-tracking.patch
--
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