Re: [PATCH 4/5] sched: Guarantee new group-entities always have weight

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Oct 16, 2013 at 11:16:27AM -0700, Ben Segall wrote:
> From: Paul Turner <pjt@google.com>
> 
> 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 <pjt@google.com>
> ---
>  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.