Re: [PATCH v2] sched/core: Fix cpu controller for !RT_GROUP_SCHED

[Peter Zijlstra]

On Fri, Jul 19, 2019 at 08:34:55AM +0200, Juri Lelli wrote:
> On !CONFIG_RT_GROUP_SCHED configurations it is currently not possible to
> move RT tasks between cgroups to which cpu controller has been attached;
> but it is oddly possible to first move tasks around and then make them
> RT (setschedule to FIFO/RR).
> 
> E.g.:
> 
>   # mkdir /sys/fs/cgroup/cpu,cpuacct/group1
>   # chrt -fp 10 $$
>   # echo $$ > /sys/fs/cgroup/cpu,cpuacct/group1/tasks
>   bash: echo: write error: Invalid argument
>   # chrt -op 0 $$
>   # echo $$ > /sys/fs/cgroup/cpu,cpuacct/group1/tasks
>   # chrt -fp 10 $$
>   # cat /sys/fs/cgroup/cpu,cpuacct/group1/tasks
>   2345
>   2598
>   # chrt -p 2345
>   pid 2345's current scheduling policy: SCHED_FIFO
>   pid 2345's current scheduling priority: 10
> 
> Also, as Michal noted, it is currently not possible to enable cpu
> controller on unified hierarchy with !CONFIG_RT_GROUP_SCHED (if there
> are any kernel RT threads in root cgroup, they can't be migrated to the
> newly created cpu controller's root in cgroup_update_dfl_csses()).
> 
> Existing code comes with a comment saying the "we don't support RT-tasks
> being in separate groups". Such comment is however stale and belongs to
> pre-RT_GROUP_SCHED times. Also, it doesn't make much sense for
> !RT_GROUP_ SCHED configurations, since checks related to RT bandwidth
> are not performed at all in these cases.
> 
> Make moving RT tasks between cpu controller groups viable by removing
> special case check for RT (and DEADLINE) tasks.

I have very vague memories of there being a matching test in
sched_setscheduler(), but clearly that's no longer there.

Thanks!

> Signed-off-by: Juri Lelli <juri.lelli@xxxxxxxxxx>
> Reviewed-by: Michal Koutný <mkoutny@xxxxxxxx>
> Acked-by: Tejun Heo <tj@xxxxxxxxxx>
> ---
> v1 -> v2: added comment about unified hierachy in changelog (Michal)
>           collected acks/reviews
> ---
>  kernel/sched/core.c | 4 ----
>  1 file changed, 4 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index fa43ce3962e7..be041dc7d313 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -6934,10 +6934,6 @@ static int cpu_cgroup_can_attach(struct cgroup_taskset *tset)
>  #ifdef CONFIG_RT_GROUP_SCHED
>  		if (!sched_rt_can_attach(css_tg(css), task))
>  			return -EINVAL;
> -#else
> -		/* We don't support RT-tasks being in separate groups */
> -		if (task->sched_class != &fair_sched_class)
> -			return -EINVAL;
>  #endif
>  		/*
>  		 * Serialize against wake_up_new_task() such that if its
> -- 
> 2.17.2
>