linux-kernel - Re: [PATCH v2] sched/core: Fix cpu controller for !RT_GROUP

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Date:   Fri, 19 Jul 2019 10:18:28 +0200
From:   Daniel Bristot de Oliveira <bristot@...hat.com>
To:     Juri Lelli <juri.lelli@...hat.com>, peterz@...radead.org,
        mingo@...hat.com, tj@...nel.org
Cc:     rostedt@...dmis.org, linux-kernel@...r.kernel.org,
        luca.abeni@...tannapisa.it, lizefan@...wei.com, longman@...hat.com,
        cgroups@...r.kernel.org
Subject: Re: [PATCH v2] sched/core: Fix cpu controller for !RT_GROUP_SCHED

On 19/07/2019 08:34, 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.
> 
> Signed-off-by: Juri Lelli <juri.lelli@...hat.com>
> Reviewed-by: Michal Koutný <mkoutny@...e.com>

Reviewed-by: Daniel Bristot de Oliveira <bristot@...hat.com>

Thanks!
-- Daniel