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

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Date:   Tue, 26 Jul 2022 07:33:22 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Michal Koutný <mkoutny@...e.com>
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

Hello,

On Tue, Jul 26, 2022 at 04:32:57PM +0200, Michal Koutný wrote:
> 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.

Yeah, I was disappoinetd that it wasn't being used by systemd already. It'd
be great if the glibc situation can be rectified soon because this is a much
better interface.

> > +	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?

To distinguish core and per-controller flags.

Thanks.

-- 
tejun