linux-kernel - Re: [PATCH 5/6] cgroup: implement cgroup v2 thread support

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Date:   Tue, 18 Jul 2017 19:28:01 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Tejun Heo <tj@...nel.org>
Cc:     lizefan@...wei.com, hannes@...xchg.org, mingo@...hat.com,
        longman@...hat.com, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-team@...com, pjt@...gle.com,
        luto@...capital.net, efault@....de, torvalds@...ux-foundation.org,
        guro@...com
Subject: Re: [PATCH 5/6] cgroup: implement cgroup v2 thread support

On Mon, Jul 17, 2017 at 10:26:09AM -0400, Tejun Heo wrote:
> Hello, Peter.
> 
> On Mon, Jul 17, 2017 at 04:14:09PM +0200, Peter Zijlstra wrote:
> > AFAICT this is not in fact what I suggested... :/
> 
> Heh, sorry about misattributing that.  I was mostly referring to the
> overall idea of marking each cgroup domain or threaded rather than
> subtree.
> 
> > My proposal did not have that invalid state. It would simply refuse to
> > change the type from thread to domain in the case where the parent is
> > not a domain.
> > 
> > Also, my proposal maintained the normal property inheritance rules. A
> > child cgroup's creation 'type' would be that of its parent and not
> > always be 'domain'.
> 
> But aren't both of the above get weird when the parent can host both
> domain and threaded children?
> 
> 	  R
>        /
>       A(D)
> 
> If you create another child B under R, it's naturally gonna be a
> domain.  Let's say you turn that to threaded.
> 
> 	   R
>        /   \
>      A(D) B(T)
> 
> And now try to create another child C, should that be a domain or
> threaded?

Domain of course, as R must be a domain, and hence all its children
start out as such.

> If we only inherit from the second level on, which is in itself
> already confusing, that still leads to invalid configs for non-root
> thread roots.

I don't see how. I don't get the example Waiman gave, what is wrong
with:

	R (D)
	|
	A (D)
       / \
     C(D) B(T) 

? Afaict that's a perfectly valid configuration.