On 20 September 2016 at 13:54, Peter Zijlstra peterz@infradead.org wrote:
On Fri, Sep 16, 2016 at 04:23:16PM +0200, Vincent Guittot wrote:
On 16 September 2016 at 14:16, Peter Zijlstra peterz@infradead.org wrote:
Also, the normalize comment in dequeue_entity() worries me, 'someone' didn't update that when he moved update_min_vruntime() around.
I now worry more, so we do:
dequeue_task := dequeue_task_fair (p == current) dequeue_entity update_curr() update_min_vruntime() vruntime -= min_vruntime update_min_vruntime() // use cfs_rq->curr, which we just normalized !yes but does it really change the cfs_rq->min_vruntime in this case ?
So let me see; it does:
vruntime = cfs_rq->min_vruntime; if (curr) // true vruntime = curr->vruntime; // == vruntime - min_vruntime if (leftmost) // possible if (curr) // true vruntime = min_vruntime(vruntime, se->vruntime); if (se->vruntime - (curr->vruntime - min_vruntime)) < 0 // false min_vruntime = max_vruntime(min_vruntime, vruntime); if ((curr->vruntime - min_vruntime) - min_vruntime) > 0)The problem is that double subtraction of min_vruntime can wrap. The thing is, min_vruntime is the 0-point in our modular space, it normalizes vruntime (ideally min_vruntime would be our 0-lag point, resulting in vruntime - min_vruntime being the lag).
The moment min_vruntime grows past S64_MAX/2 -2*min_vruntime wraps into
fair enough
positive space again and the test above becomes true and we'll select the normalized @curr vruntime as new min_vruntime and weird stuff will happen.
Also, even it things magically worked out, its still very icky to mix the normalized vruntime into things.
I agree
put_prev_task := put_prev_task_fair put_prev_entity cfs_rq->curr = NULL;Now the point of the latter update_min_vruntime() is to advance min_vruntime when the task we removed was the one holding it back.
However, it means that if we do dequeue+enqueue, we're further in the future (ie. we get penalized).
So I'm inclined to simply remove the (2nd) update_min_vruntime() call. But as said above, my brain isn't co-operating much today.
OK, so not sure we can actually remove it, we do want it to move min_vruntime forward (sometimes). We just don't want it to do so when DEQUEUE_SAVE -- we want to get back where we left off, nor do we want to muck about with touching normalized values.
Another fun corner case is DEQUEUE_SLEEP; in that case we do not normalize, but we still want advance min_vruntime if this was the one holding it back.
I ended up with the below, but I'm not sure I like it much. Let me prod a wee bit more to see if there's not something else we can do.
Google has this patch-set replacing min_vruntime with an actual global 0-lag, which greatly simplifies things. If only they'd post it sometime :/ /me prods pjt and ben with a sharp stick :-)
kernel/sched/fair.c | 22 ++++++++++++++++++---- 1 file changed, 18 insertions(+), 4 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 986c10c25176..77566a340cbf 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -462,17 +462,23 @@ 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;elsecurr = 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);@@ -3483,8 +3489,16 @@ 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);}
/*