linux-kernel - Re: [RFC] cgroup TODOs

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Date:	Mon, 17 Sep 2012 10:21:23 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Glauber Costa <glommer@...allels.com>
Cc:	containers@...ts.linux-foundation.org, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org, Li Zefan <lizefan@...wei.com>,
	Michal Hocko <mhocko@...e.cz>,
	Peter Zijlstra <peterz@...radead.org>,
	Paul Turner <pjt@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Thomas Graf <tgraf@...g.ch>,
	"Serge E. Hallyn" <serue@...ibm.com>,
	Paul Mackerras <paulus@...ba.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Neil Horman <nhorman@...driver.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	"Daniel P. Berrange" <berrange@...hat.com>,
	Lennart Poettering <lennart@...ttering.net>,
	Kay Sievers <kay.sievers@...y.org>
Subject: Re: [RFC] cgroup TODOs

Hello, Glauber.

On Mon, Sep 17, 2012 at 12:50:47PM +0400, Glauber Costa wrote:
> > Can you be a bit more specific?
>
> What I mean is that if some operation needs to operate locked, they will
> have to lock. Whether or not the locking is called from cgroup core or
> not. If the lock is not available outside, people will end up calling a
> core function that locks.

I was asking whether you have certain specific operations on mind.

> >> And the problem is that people need to lock. cgroup_lock is needed
> >> because the data you are accessing is protected by it. The way I see it,
> >> it is incredible how we were able to revive the BKL in the form of
> >> cgroup_lock after we finally manage to successfully get rid of it!
> > 
> > I wouldn't go as far as comparing it to BKL.
>
> Of course not, since it is not system-wide. But I think the comparison
> still holds in spirit...

Subsystem-wide locks covering non-hot paths aren't evil things.  We
have a lot of them and they work fine.  BKL was a completely different
beast initially with implicit locking on kernel entry and unlocking on
sleeping and then got morphed into some chimera inbetween afterwards.

Simple locking is a good thing.  If finer-grained locking is
necessary, we sure do that but please stop throwing over-generalized
half-arguments at it.  It doesn't help anything.

> you seem to hear "comount", and think of unified vision, and that is the
> reason for this discussion to still be going on. Mounting is all about
> the root. And if you comount, hierarchies have the same root.
>
> In your example, the different controllers are comounted. They have not
> the same view, but the possible views are restricted to be a subset of
> the underlying tree - because they are mounted in the same place, forced
> or not.

Heh, I can't really tell whether you understand it or not.  Here and
in the previous thread too.  You seem to understand that there are
different views upto this point.

> In a situation like this, it makes all the sense in the world to use the
> css_id as a primary identifier, because it will be guaranteed to be the

And then you say something like this (or that this would remove
walking different hierarchies in the previous thread - yes, to a
certain point but not completely).  css_id is a per-css attribute.
How can that be the "primariy" identifier when there can be multiple
views?  For each userland-visible cgroup, there must be a css_set
which points to the css's belonging to it, which may not be at the
same level - multiple nodes in the userland visible tree may point to
the same css.

If you mean that css_id would be the primary identifier for that
specific controller's css, why even say that?  That's true now and
won't ever change.

> same. What makes the tree overly flexible, is that you can have multiple
> roots, starting in multiple places, with arbitrary topologies downwards.

And now you seem to be on the same page again.  But then again, you're
asserting that incorporating forced co-mounts *now* is a gradual step
towards the goal, which is utterly bonkers.  I don't know.  I just
can't understand what you're thinking at all.

Thanks.

-- 
tejun
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