Date: Mon, 23 Jun 2014 20:57:35 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Waiman Long <waiman.long@hp.com>, Romanov Arya <romanov.arya@gmail.com>,
        Pranith Kumar <bobby.prani@gmail.com>,
        Josh Triplett <josh@joshtriplett.org>,
        LKML <linux-kernel@vger.kernel.org>, torvalds@linux-foundation.org
Subject: Re: [RFC PATCH 1/1] kernel/rcu/tree.c: simplify
 force_quiescent_state()
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On Mon, Jun 23, 2014 at 10:33:08AM -0700, Paul E. McKenney wrote:
> 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.

Typical Intel cpu numbering is [0..n) for SMT0 and [n..2*n) for SMT1, so
that'll fall flat on its face at try 1.
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