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

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Message-ID: <20190605133442.GJ374014@devbig004.ftw2.facebook.com>
Date:   Wed, 5 Jun 2019 06:34:42 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Juri Lelli <juri.lelli@...hat.com>
Cc:     peterz@...radead.org, mingo@...hat.com, rostedt@...dmis.org,
        linux-kernel@...r.kernel.org, luca.abeni@...tannapisa.it,
        bristot@...hat.com, lizefan@...wei.com, cgroups@...r.kernel.org
Subject: Re: [PATCH] sched/core: Fix cpu controller for !RT_GROUP_SCHED

Hello,

On Wed, Jun 05, 2019 at 01:49:35PM +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
> 
> 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>
> ---
> Hi,
> 
> Although I'm pretty assertive in the changelog, I actually wonder what
> am I missing here and why (if) current behavior is needed and makes
> sense.
> 
> Any input?

Yeah, RT tasks being transprent to the cpu controller when
!RT_GROUP_SCHED makes sense to me, especially given that the rules
around it are already inconsistent.  Please feel free to add

  Acked-by: Tejun Heo <tj@...nel.org>

Thanks.

-- 
tejun