Re: [PATCH] sched: Filter root_task_group at the beginning

[Ingo Molnar]

* Haifeng Xu <haifeng.xu@xxxxxxxxxx> wrote:

> We can't change the weight of the root cgroup. Let's handle
> root_task_group before doing any real work including acquiring
> the shares_mutex.
> 
> Signed-off-by: Haifeng Xu <haifeng.xu@xxxxxxxxxx>
> ---
>  kernel/sched/fair.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index a80a73909dc2..1ac2df87e070 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -12594,6 +12594,9 @@ int sched_group_set_shares(struct task_group *tg, unsigned long shares)
>  {
>  	int ret;
>  
> +	if (tg == &root_task_group)
> +		return -EINVAL;
> +
>  	mutex_lock(&shares_mutex);
>  	if (tg_is_idle(tg))
>  		ret = -EINVAL;

So what's the motivation, how common is this case? 

Normally this should be a -EINVAL error code path, which sane user-space
presumably never conscisously tries to call in that fashion, right?

It's not worth optimizing pathological cases, especially
since we check for the root CG inside __sched_group_set_shares()
already:

        /*
         * We can't change the weight of the root cgroup.
         */
        if (!tg->se[0])
                return -EINVAL;


Thanks,

	Ingo