linux-kernel - Re: [PATCH] cgroup: Simplify code in css_set_move

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Message-ID: <20221031131140.GC27841@blackbody.suse.cz>
Date:   Mon, 31 Oct 2022 14:11:40 +0100
From:   Michal Koutný <mkoutny@...e.com>
To:     Haifeng Xu <haifeng.xu@...pee.com>
Cc:     tj@...nel.org, lizefan.x@...edance.com, hannes@...xchg.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cgroup: Simplify code in css_set_move_task

Hello.

> 1) If calls 'css_set_update_populated' , the cset is either getting the
> first task or losing the last. There is a need to update the populated
> state of the cset only when 'css_set_populated' returns false.
> In other words, the last has been deleted from from_cset and the first
> hasn't been added to to_cset yet.

I've likely misread the condition previously. I see how this works now
(update happens after "from_cset" but before "to_cset" migration).

> 3) In order to update the populated state of to_cset the same way
> from_cset does, 'css_set_update_populated' is also invoked during the
> process of moving a task to to_cset.

As I think more about this in the context of vertical migrations
(ancestor<->descendant, such as during controller dis- or enablement),
I'm afraid the inverted order would lead to "spurious" emptiness
notifications in ancestors (in the case a there is just a single task
that migrates parent->child, parent/cgroup.populated would generate and
event that'd be nullified by the subsequent population of the child).

So I'm not sure the change is worth it.

Michal

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