Date: Mon, 23 Jun 2014 12:28:50 +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 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.
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