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From: Michal =?iso-8859-1?Q?Koutn=FD?= <mkoutny@suse.com>
Subject: Re: [PATCH RESEND 3/3 cgroup/for-5.20] cgroup: Make !percpu
 threadgroup_rwsem operations optional
Date: Tue, 26 Jul 2022 16:32:57 +0200
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To: Tejun Heo <tj@kernel.org>
Cc: Christian Brauner <brauner@kernel.org>, Peter Zijlstra <peterz@infradead.org>, John Stultz <john.stultz@linaro.org>, Dmitry Shmidt <dimitrysh@google.com>, Oleg Nesterov <oleg@redhat.com>, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org


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On Sat, Jul 23, 2022 at 04:28:28AM -1000, Tejun Heo <tj@kernel.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 =3D (1 << 4),
> +
> +	/*
>  	 * Enable cpuset controller in v1 cgroup to use v2 behavior.
>  	 */
> -	CGRP_ROOT_CPUSET_V2_MODE =3D (1 << 4),
> +	CGRP_ROOT_CPUSET_V2_MODE =3D (1 << 16),
> =20
>  	/*
>  	 * Enable legacy local memory.events.
>  	 */
> -	CGRP_ROOT_MEMORY_LOCAL_EVENTS =3D (1 << 5),
> +	CGRP_ROOT_MEMORY_LOCAL_EVENTS =3D (1 << 17),
> =20
>  	/*
>  	 * Enable recursive subtree protection
>  	 */
> -	CGRP_ROOT_MEMORY_RECURSIVE_PROT =3D (1 << 6),
> +	CGRP_ROOT_MEMORY_RECURSIVE_PROT =3D (1 << 18),

Why this new gap in flag bits?

[1] https://github.com/systemd/systemd/pull/16706
[2] https://github.com/search?q=3Dorg%3Aopencontainers+CLONE_INTO_CGROUP&ty=
pe=3Dall (empty)

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