linux-kernel - Re: [patch 00/18] CFS Bandwidth Control v7.2

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Date:	Mon, 25 Jul 2011 19:08:14 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Paul Turner <pjt@...gle.com>, linux-kernel@...r.kernel.org,
	Bharata B Rao <bharata@...ux.vnet.ibm.com>,
	Dhaval Giani <dhaval.giani@...il.com>,
	Balbir Singh <bsingharora@...il.com>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	Ingo Molnar <mingo@...e.hu>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Jason Baron <jbaron@...hat.com>
Subject: Re: [patch 00/18] CFS Bandwidth Control v7.2

On Mon, 2011-07-25 at 09:46 -0700, Paul E. McKenney wrote:
> On Mon, Jul 25, 2011 at 06:28:31PM +0200, Peter Zijlstra wrote:
> > On Mon, 2011-07-25 at 09:21 -0700, Paul E. McKenney wrote:
> > > On Mon, Jul 25, 2011 at 05:00:41PM +0200, Peter Zijlstra wrote:
> > > > On Mon, 2011-07-25 at 16:58 +0200, Peter Zijlstra wrote:
> > > > > +               rcu_read_lock();
> > > > > +               ret = walk_tg_tree_from(tg, tg_set_cfs_period_down, NULL, &period);
> > > > > +               rcu_read_unlock(); 
> > > > 
> > > > rcu over a mutex doesn't really work in mainline, bah.. 
> > > 
> > > SRCU can handle that situation, FWIW.  But yes, blocking in an RCU
> > > read-side critical section is a no-no.
> > 
> > Yeah, I know, but didn't notice until after I sent.. SRCU isn't useful
> > though, way too slow due to lacking srcu_call().
> 
> Good point.  How frequently would a call_srcu() be invoked?
> 
> In other words, would a really crude hack involving a globally locked
> per-srcu_struct callback list and a per-srcu_struct kernel thread be
> helpful, or would a slightly less-crude hack involving a per-CPU callback
> list be required?

it would be invoked every time someone kills a cgroup, which I would
consider a slow path, but some folks out there seem to think otherwise
and create/destoy these things like they're free (there was a discussion
about this some time ago about optimizing the cgroup destroy path
etc..).

Anyway, I think I can sort this particular problem by simply wrapping
the whole crap in cgroup_lock(),cgroup_unlock(). If we want to go this
way anyway.

I consider setting the cgroup paramaters an utter slow path, and if
people complain I'll simply tell them to sod off ;-)
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