linux-kernel - Re: [RFC PATCH 1/1] kernel/rcu/tree.c: simplify force_quiescent

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Date:	Mon, 23 Jun 2014 10:33:08 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Waiman Long <waiman.long@...com>,
	Romanov Arya <romanov.arya@...il.com>,
	Pranith Kumar <bobby.prani@...il.com>,
	Josh Triplett <josh@...htriplett.org>,
	LKML <linux-kernel@...r.kernel.org>,
	torvalds@...ux-foundation.org
Subject: Re: [RFC PATCH 1/1] kernel/rcu/tree.c: simplify
 force_quiescent_state()

On Mon, Jun 23, 2014 at 08:57:50AM -0700, Paul E. McKenney wrote:
> On Mon, Jun 23, 2014 at 12:28:50PM +0200, Peter Zijlstra wrote:
> > On Tue, Jun 17, 2014 at 10:37:17AM -0700, Paul E. McKenney wrote:
> > > Oh, and to answer the implicit question...  A properly configured 4096-CPU
> > > system will have two funnel levels, with 64 nodes at the leaf level
> > > and a single node at the root level.  If the system is not properly
> > > configured, it will have three funnel levels.  The maximum number of
> > > funnel levels is four, which would handle more than four million CPUs
> > > (sixteen million if properly configured), so we should be good.  ;-)
> > > 
> > > The larger numbers of levels are intended strictly for testing.  I set
> > > CONFIG_RCU_FANOUT_LEAF=2 and CONFIG_RCU_FANOUT=2 on a 16-CPU system just
> > > to make sure that I am testing something uglier than what will be running
> > > in production.  A large system should have both of these set to 64,
> > > though this requires also booting with skew_tick=1 as well.
> > 
> > Right, and I think we talked about this before; the first thing one
> > should do is align the RCU fanout masks with the actual machine
> > topology. Because currently they can be all over the place.
> 
> And we also talked before about how it would make a lot more sense to
> align the CPU numbering with the actual machine topology, as that would
> fix the problem in one place.  But either way, in the particular case
> of the RCU fanout, does anyone have any real data showing that this is
> a real problem?  Given that the rcu_node accesses are quite a ways off
> of any fastpath, I remain skeptical.

And one way to test for this is to set CONFIG_RCU_FANOUT to the number of
cores in a socket (or to the number of hardware threads per socket for
systems that number their hardware threads consecutively), then specify
CONFIG_RCU_FANOUT_EXACT=y.  This will align the rcu_node structures with
the sockets.  If the number of cores/threads per socket is too large,
you can of course use a smaller number that exactly divides the number
of cores/threads per socket.

If this does turn out to improve performance, I would be happy to create
a boot parameter for CONFIG_RCU_FANOUT, perhaps also some mechanism to
allow the architecture to tell RCU what the fanout should be.

							Thanx, Paul

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