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Sender: Tejun Heo <htejun@gmail.com>
Date:   Sun, 4 Apr 2021 19:39:03 -0400
From:   Tejun Heo <tj@kernel.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     joel@joelfernandes.org, chris.hyser@oracle.com, joshdon@google.com,
        mingo@kernel.org, vincent.guittot@linaro.org,
        valentin.schneider@arm.com, mgorman@suse.de,
        linux-kernel@vger.kernel.org, tglx@linutronix.de,
        Michal =?iso-8859-1?Q?Koutn=FD?= <mkoutny@suse.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Zefan Li <lizefan.x@bytedance.com>
Subject: Re: [PATCH 0/9] sched: Core scheduling interfaces
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cc'ing Michal and Christian who've been spending some time on cgroup
interface issues recently and Li Zefan for cpuset.

On Thu, Apr 01, 2021 at 03:10:12PM +0200, Peter Zijlstra wrote:
> The cgroup interface now uses a 'core_sched' file, which still takes 0,1. It is
> however changed such that you can have nested tags. The for any given task, the
> first parent with a cookie is the effective one. The rationale is that this way
> you can delegate subtrees and still allow them some control over grouping.

I find it difficult to like the proposed interface from the name (the term
"core" is really confusing given how the word tends to be used internally)
to the semantics (it isn't like anything else) and even the functionality
(we're gonna have fixed processors at some point, right?).

Here are some preliminary thoughts:

* Are both prctl and cgroup based interfaces really necessary? I could be
  being naive but given that we're (hopefully) working around hardware
  deficiencies which will go away in time, I think there's a strong case for
  minimizing at least the interface to the bare minimum.

  Given how cgroups are set up (membership operations happening only for
  seeding, especially with the new clone interface), it isn't too difficult
  to synchronize process tree and cgroup hierarchy where it matters - ie.
  given the right per-process level interface, restricting configuration for
  a cgroup sub-hierarchy may not need any cgroup involvement at all. This
  also nicely gets rid of the interaction between prctl and cgroup bits.

* If we *have* to have cgroup interface, I wonder whether this would fit a
  lot better as a part of cpuset. If you squint just right, this can be
  viewed as some dynamic form of cpuset. Implementation-wise, it probably
  won't integrate with the rest but I think the feature will be less jarring
  as a part of cpuset, which already is a bit of kitchensink anyway.

> The cgroup thing also '(ab)uses' cgroup_mutex for serialization because it
> needs to ensure continuity between ss->can_attach() and ss->attach() for the
> memory allocation. If the prctl() were allowed to interleave it might steal the
> memory.
> 
> Using cgroup_mutex feels icky, but is not without precedent,
> kernel/bpf/cgroup.c does the same thing afaict.
> 
> TJ, can you please have a look at this?

Yeah, using cgroup_mutex for stabilizing cgroup hierarchy for consecutive
operations is fine. It might be worthwhile to break that out into a proper
interface but that's the least of concerns here.

Can someone point me to a realistic and concrete usage scenario for this
feature?

Thanks.

-- 
tejun