On Wed, Oct 16, 2013 at 11:16:27AM -0700, Ben Segall wrote: > From: Paul Turner <[email protected]> > > Currently, group entity load-weights are initialized to zero. This > admits some races with respect to the first time they are re-weighted in > earlty use. ( Let g[x] denote the se for "g" on cpu "x". ) > > Suppose that we have root->a and that a enters a throttled state, > immediately followed by a[0]->t1 (the only task running on cpu[0]) > blocking: > > put_prev_task(group_cfs_rq(a[0]), t1) > put_prev_entity(..., t1) > check_cfs_rq_runtime(group_cfs_rq(a[0])) > throttle_cfs_rq(group_cfs_rq(a[0])) > > Then, before unthrottling occurs, let a[0]->b[0]->t2 wake for the first > time: > > enqueue_task_fair(rq[0], t2) > enqueue_entity(group_cfs_rq(b[0]), t2) > enqueue_entity_load_avg(group_cfs_rq(b[0]), t2) > account_entity_enqueue(group_cfs_ra(b[0]), t2) > update_cfs_shares(group_cfs_rq(b[0])) > < skipped because b is part of a throttled hierarchy > > enqueue_entity(group_cfs_rq(a[0]), b[0]) > ... > > We now have b[0] enqueued, yet group_cfs_rq(a[0])->load.weight == 0 > which violates invariants in several code-paths. Eliminate the > possibility of this by initializing group entity weight. > > Signed-off-by: Paul Turner <[email protected]> > --- > kernel/sched/fair.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index fc44cc3..424c294 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -7207,7 +7207,8 @@ void init_tg_cfs_entry(struct task_group *tg, struct > cfs_rq *cfs_rq, > se->cfs_rq = parent->my_q; > > se->my_q = cfs_rq; > - update_load_set(&se->load, 0); > + /* guarantee group entities always have weight */ > + update_load_set(&se->load, NICE_0_LOAD); > se->parent = parent; > }
Hurm.. this gives new groups a massive weight; nr_cpus * NICE_0. ISTR there being some issues with this; or was that on the wakeup path where a task woke on a cpu who's group entity had '0' load because it used to run on another cpu -- I can't remember. But please do expand how this isn't a problem. I suppose for the regular cgroup case, group creation is a rare event so nobody cares, but autogroups can come and go far too quickly I think. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
