Re: [PATCH v2 1/4] riscv: defconfig: drop RT_GROUP_SCHED=y

[Heinrich Schuchardt]

On 09.09.24 14:14, Celeste Liu wrote:
> On 2024-09-09 19:53, Conor Dooley wrote:
>
> > On Fri, Aug 23, 2024 at 01:43:26AM +0800, Celeste Liu wrote:
> > > Commit ba6cfef057e1 ("riscv: enable Docker requirements in defconfig")
> > > introduced it because of Docker, but Docker has removed this requirement
> > > since [1] (2023-04-19).
> > >
> > > For cgroup v1, if turned on, and there's any cgroup in the "cpu" hierarchy it
> > > needs an RT budget assigned, otherwise the processes in it will not be able to
> > > get RT at all. The problem with RT group scheduling is that it requires the
> > > budget assigned but there's no way we could assign a default budget, since the
> > > values to assign are both upper and lower time limits, are absolute, and need to
> > > be sum up to < 1 for each individal cgroup. That means we cannot really come up
> > > with values that would work by default in the general case.[2]
> > >
> > > For cgroup v2, it's almost unusable as well. If it turned on, the cpu controller
> > > can only be enabled when all RT processes are in the root cgroup. But it will
> > > lose the benefits of cgroup v2 if all RT process were placed in the same cgroup.
> > >
> > > Red Hat, Gentoo, Arch Linux and Debian all disable it. systemd also doesn't
> > > support it.[3]
> > >
> > > [1]: https://github.com/moby/moby/commit/005150ed69c540fb0b5323e0f2208608c1204536
> > > [2]: https://bugzilla.redhat.com/show_bug.cgi?id=1229700
> > > [3]: https://github.com/systemd/systemd/issues/13781#issuecomment-549164383
> > >
> > > Fixes: ba6cfef057e1 ("riscv: enable Docker requirements in defconfig")
> >
> > I don't think this fixes tag is suitable, the commit you cite in
> > moby/docker is a year younger than the one in the fixes tag, so it was
> > correct at the time it was written. I think the fixes tag should just be
> > removed, since that commit was not wrong. Or am I missing something?
>
> The Docker commit I cited introduce the fix, not the bug. So it should be later
> than commit in fixes tag, otherwise Heinrich Schuchardt won't pick the wrong
> config from upstream.
>
> Timeline is:
> 1) 2015-06-19 Qiang Huang introduce RT_GROUP_SCHED check in Docker.[1]
> 2) 2022-06-08 Heinrich Schuchardt pick this to defconfig in commit
>                in fixes tag.
> 3) 2023-04-19 Florian Schmaus remove this requirement from Docker in
>                commit I cited.
>
> [1]: https://github.com/moby/moby/commit/fb85a99f5899df7475ba6b1ac4b2e1d32974eca2

I experience no problems running Docker without CONFIG_RT_GROUP_SCHED on 
Ubuntu 24.04.

The Docker patch is only for contrib/check-config.sh. Probably Docker at 
the time of my patch would have worked without CONFIG_RT_GROUP_SCHED, 
too. I have no issue with the Fixes: tag in this patch.

Acked-by: Heinrich Schuchardt <heinrich.schuchardt@xxxxxxxxxxxxx>

> > > Signed-off-by: Celeste Liu <CoelacanthusHex@xxxxxxxxx>
> > > ---
> > >   arch/riscv/configs/defconfig | 1 -
> > >   1 file changed, 1 deletion(-)
> > >
> > > diff --git a/arch/riscv/configs/defconfig b/arch/riscv/configs/defconfig
> > > index 12dc8c73a8ac..de85c3ab261e 100644
> > > --- a/arch/riscv/configs/defconfig
> > > +++ b/arch/riscv/configs/defconfig
> > > @@ -9,7 +9,6 @@ CONFIG_CGROUPS=y
> > >   CONFIG_MEMCG=y
> > >   CONFIG_CGROUP_SCHED=y
> > >   CONFIG_CFS_BANDWIDTH=y
> > > -CONFIG_RT_GROUP_SCHED=y
> > >   CONFIG_CGROUP_PIDS=y
> > >   CONFIG_CGROUP_FREEZER=y
> > >   CONFIG_CGROUP_HUGETLB=y
> > >
> > > --
> > > 2.46.0