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

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Message-ID: <20110725164619.GK2327@linux.vnet.ibm.com>
Date:	Mon, 25 Jul 2011 09:46:19 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
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, 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?

							Thanx, Paul
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