From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758585AbcLOPae (ORCPT ); Thu, 15 Dec 2016 10:30:34 -0500 Received: from foss.arm.com ([217.140.101.70]:38644 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755791AbcLOPad (ORCPT ); Thu, 15 Dec 2016 10:30:33 -0500 Date: Thu, 15 Dec 2016 15:29:43 +0000 From: Mark Rutland To: Boqun Feng Cc: linux-kernel@vger.kernel.org, "Paul E . McKenney " , Josh Triplett , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Colin King Subject: Re: [RFC v2.1 1/5] rcu: Introduce for_each_leaf_node_cpu() Message-ID: <20161215152942.GH21758@leverpostej> References: <20161215024204.28620-2-boqun.feng@gmail.com> <20161215152105.30570-1-boqun.feng@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20161215152105.30570-1-boqun.feng@gmail.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Dec 15, 2016 at 11:21:05PM +0800, Boqun Feng wrote: > There are some places inside RCU core, where we need to iterate all mask > (->qsmask, ->expmask, etc) bits in a leaf node, in order to iterate all > corresponding CPUs. The current code iterates all possible CPUs in this > leaf node and then checks with the mask to see whether the bit is set. > > However, given the fact that most bits in cpu_possible_mask are set but > rare bits in an RCU leaf node mask are set(in other words, ->qsmask and > its friends are usually more sparse than cpu_possible_mask), it's better > to iterate in the other way, that is iterating mask bits in a leaf node. > By doing so, we can save several checks in the loop, moreover, that fast > path checking(e.g. ->qsmask == 0) could then be consolidated into the > loop logic. > > This patch introduce for_each_leaf_node_cpu() to iterate mask bits in a > more efficient way. > > By design, The CPUs whose bits are set in the leaf node masks should be > a subset of possible CPUs, so we don't need extra check with > cpu_possible(), however, a WARN_ON_ONCE() is put to check whether there > are some nasty cases we miss, and we skip that "impossible" CPU in that > case. > > Signed-off-by: Boqun Feng Acked-by: Mark Rutland Thanks, Mark. > --- > kernel/rcu/tree.h | 19 +++++++++++++++++++ > 1 file changed, 19 insertions(+) > > diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h > index c0a4bf8f1ed0..b35da5b5dab1 100644 > --- a/kernel/rcu/tree.h > +++ b/kernel/rcu/tree.h > @@ -295,6 +295,25 @@ struct rcu_node { > cpu <= rnp->grphi; \ > cpu = cpumask_next((cpu), cpu_possible_mask)) > > + > +#define MASK_BITS(mask) (BITS_PER_BYTE * sizeof(mask)) > +/* > + * Iterate over all CPUs a leaf RCU node which are still masked in > + * @mask. > + * > + * Note @rnp has to be a leaf node and @mask has to belong to @rnp. And we > + * assume that no CPU is masked in @mask but not set in cpu_possible_mask. IOW, > + * masks of a leaf node never set a bit for an "impossible" CPU. > + */ > +#define for_each_leaf_node_cpu(rnp, mask, cpu) \ > + for ((cpu) = (rnp)->grplo + find_first_bit(&(mask), MASK_BITS(mask)); \ > + (cpu) <= (rnp)->grphi; \ > + (cpu) = (rnp)->grplo + find_next_bit(&(mask), MASK_BITS(mask), \ > + (cpu) - (rnp)->grplo + 1)) \ > + if (WARN_ON_ONCE(!cpu_possible(cpu))) \ > + continue; \ > + else > + > /* > * Union to allow "aggregate OR" operation on the need for a quiescent > * state by the normal and expedited grace periods. > -- > 2.10.2 >