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

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Message-ID: <20140623102850.GX19860@laptop.programming.kicks-ass.net>
Date:	Mon, 23 Jun 2014 12:28:50 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
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 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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