On 20 September 2016 at 13:54, Peter Zijlstra <pet...@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 <pet...@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; > + 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); > @@ -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); > } > > /*