linux-kernel - Re: [PATCH RESEND 3/3 cgroup/for-5.20] cgroup: Make !percpu threadgroup

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Message-ID: <20220726143257.GA23882@blackbody.suse.cz>
Date:   Tue, 26 Jul 2022 16:32:57 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Christian Brauner <brauner@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        John Stultz <john.stultz@...aro.org>,
        Dmitry Shmidt <dimitrysh@...gle.com>,
        Oleg Nesterov <oleg@...hat.com>, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org
Subject: Re: [PATCH RESEND 3/3 cgroup/for-5.20] cgroup: Make !percpu
 threadgroup_rwsem operations optional

On Sat, Jul 23, 2022 at 04:28:28AM -1000, Tejun Heo <tj@...nel.org> wrote:
> This makes the hotter paths - fork and exit - slower as they're always
> forced into the slow path. There is no reason to force this on everyone
> especially given that more common static usage pattern can now completely
> avoid write-locking the rwsem. Write-locking is elided when turning on and
> off controllers on empty sub-trees and CLONE_INTO_CGROUP enables seeding a
> cgroup without grabbing the rwsem.

Just a practical note that CLONE_INTO_CGROUP may not be so widespread
yet [1][2].
But generally, the change makes sense to me.


> +	CGRP_ROOT_FAVOR_DYNMODS = (1 << 4),
> +
> +	/*
>  	 * Enable cpuset controller in v1 cgroup to use v2 behavior.
>  	 */
> -	CGRP_ROOT_CPUSET_V2_MODE = (1 << 4),
> +	CGRP_ROOT_CPUSET_V2_MODE = (1 << 16),
>  
>  	/*
>  	 * Enable legacy local memory.events.
>  	 */
> -	CGRP_ROOT_MEMORY_LOCAL_EVENTS = (1 << 5),
> +	CGRP_ROOT_MEMORY_LOCAL_EVENTS = (1 << 17),
>  
>  	/*
>  	 * Enable recursive subtree protection
>  	 */
> -	CGRP_ROOT_MEMORY_RECURSIVE_PROT = (1 << 6),
> +	CGRP_ROOT_MEMORY_RECURSIVE_PROT = (1 << 18),

Why this new gap in flag bits?

[1] https://github.com/systemd/systemd/pull/16706
[2] https://github.com/search?q=org%3Aopencontainers+CLONE_INTO_CGROUP&type=all (empty)

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