From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752927AbbCGVwa (ORCPT ); Sat, 7 Mar 2015 16:52:30 -0500 Received: from e31.co.us.ibm.com ([32.97.110.149]:41502 "EHLO e31.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752647AbbCGVw2 (ORCPT ); Sat, 7 Mar 2015 16:52:28 -0500 Date: Sat, 7 Mar 2015 13:52:23 -0800 From: "Paul E. McKenney" To: Alexander Gordeev Cc: linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/9] rcu: Panic if RCU tree can not accommodate all CPUs Message-ID: <20150307215223.GQ5236@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20150307174234.GL5236@linux.vnet.ibm.com> <20150307184821.GA15033@agordeev.usersys.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150307184821.GA15033@agordeev.usersys.redhat.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15030721-8236-0000-0000-00000A0346D8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Mar 07, 2015 at 06:48:21PM +0000, Alexander Gordeev wrote: > On Sat, Mar 07, 2015 at 09:42:34AM -0800, Paul E. McKenney wrote: > > On Sat, Mar 07, 2015 at 06:03:36PM +0100, Alexander Gordeev wrote: > > > Currently a condition when RCU tree is unable to accommodate > > > the configured number of CPUs is not permitted and causes > > > a fall back to compile-time values. However, the code has no > > > means to exceed the RCU tree capacity neither at compile-time > > > nor in run-time. Therefore, if the condition is met in run- > > > time then it indicates a serios problem elsewhere and should > > > be handled with a panic. > > > > > > Cc: "Paul E. McKenney" > > > Signed-off-by: Alexander Gordeev > > > > The place to put a check like this is in the code that calculates > > nr_cpu_ids. And at least some (perhaps all) are set up so that nr_cpu_ids > > cannot exceed NR_CPUS, which would render this check redundant. > > The emphasis here the existing check (... n > rcu_capacity[MAX_RCU_LVLS]) > (below as [1]) should not cause the fall back to compiled-time values. > It either must panic or, as you say - redundant. You are right, I responded too early on a Saturday. The point of the check below is indeed to verify that RCU's calculations are correct. So do the testing with CONFIG_RCU_FANOUT and CONFIG_RCU_FANOUT_LEAF both equal to five, and rebase to the rcu/next branch of: git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git And I will give them a spin. Thanx, Paul > > So I have to say "no" to this one. > > > > Thanx, Paul > > > > > --- > > > kernel/rcu/tree.c | 15 +++++++++------ > > > 1 file changed, 9 insertions(+), 6 deletions(-) > > > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > > index 48d640c..7588c7f 100644 > > > --- a/kernel/rcu/tree.c > > > +++ b/kernel/rcu/tree.c > > > @@ -3889,16 +3889,19 @@ static void __init rcu_init_geometry(void) > > > rcu_capacity[i] = rcu_capacity[i - 1] * CONFIG_RCU_FANOUT; > > > > > > /* > > > + * The tree must be able to accommodate the configured number of CPUs. > > > + * If this limit is exceeded than we have a serious problem elsewhere. > > > + * > > > * The boot-time rcu_fanout_leaf parameter is only permitted > > > * to increase the leaf-level fanout, not decrease it. Of course, > > > * the leaf-level fanout cannot exceed the number of bits in > > > - * the rcu_node masks. Finally, the tree must be able to accommodate > > > - * the configured number of CPUs. Complain and fall back to the > > > - * compile-time values if these limits are exceeded. > > > + * the rcu_node masks. Complain and fall back to the compile- > > > + * time values if these limits are exceeded. > > > */ > > > - if (rcu_fanout_leaf < CONFIG_RCU_FANOUT_LEAF || > > > - rcu_fanout_leaf > sizeof(unsigned long) * 8 || > > > - n > rcu_capacity[MAX_RCU_LVLS]) { > > [1] > > > > + if (n > rcu_capacity[MAX_RCU_LVLS]) > > > + panic("rcu_init_geometry: rcu_capacity[] is too small"); > > > + else if (rcu_fanout_leaf < CONFIG_RCU_FANOUT_LEAF || > > > + rcu_fanout_leaf > sizeof(unsigned long) * 8) { > > > WARN_ON(1); > > > return; > > > } > > > -- > > > 1.8.3.1 > > > > > > > -- > Regards, > Alexander Gordeev > agordeev@redhat.com >