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Message-ID: <20190723124446.GE3402@hirez.programming.kicks-ass.net>
Date: Tue, 23 Jul 2019 14:44:46 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Juri Lelli <juri.lelli@...hat.com>
Cc: mingo@...hat.com, tj@...nel.org, rostedt@...dmis.org,
linux-kernel@...r.kernel.org, luca.abeni@...tannapisa.it,
bristot@...hat.com, lizefan@...wei.com, longman@...hat.com,
cgroups@...r.kernel.org
Subject: Re: [PATCH v2] sched/core: Fix cpu controller for !RT_GROUP_SCHED
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@...hat.com>
> Reviewed-by: Michal Koutný <mkoutny@...e.com>
> Acked-by: Tejun Heo <tj@...nel.org>
> ---
> 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
>
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