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

[Conor Dooley]

On Mon, Sep 09, 2024 at 08:14:16PM +0800, 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.

Yes, this is the way I understood things to be. IOW, when Heinrich wrote
ba6cfef057e1 ("riscv: enable Docker requirements in defconfig"), it was
a requirement for docker. The requirement later being removed doesn't
make his patch incorrect, which is why I don't think this is a fix.

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