From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758381AbcLOPLy (ORCPT ); Thu, 15 Dec 2016 10:11:54 -0500 Received: from foss.arm.com ([217.140.101.70]:38194 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755576AbcLOPLw (ORCPT ); Thu, 15 Dec 2016 10:11:52 -0500 Date: Thu, 15 Dec 2016 15:10:15 +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 1/5] rcu: Introduce for_each_leaf_node_cpu() Message-ID: <20161215151015.GG21758@leverpostej> References: <20161215024204.28620-1-boqun.feng@gmail.com> <20161215024204.28620-2-boqun.feng@gmail.com> <20161215114351.GA21758@leverpostej> <20161215143858.GM9728@tardis.cn.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20161215143858.GM9728@tardis.cn.ibm.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 10:38:58PM +0800, Boqun Feng wrote: > On Thu, Dec 15, 2016 at 11:43:52AM +0000, Mark Rutland wrote: > > On Thu, Dec 15, 2016 at 10:42:00AM +0800, Boqun Feng wrote: > > > +#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. > > > > Not a big deal, but perhaps it's worth enforcing this? If we took just > > the name of the mask here, (e.g. qsmask rather than rnp->qsmask), we > > could have the macro always use (rnp)->(mask). That would also make the > > invocations shorter. > > I thought about this approach, but there may be some cases it seems > inappropriate, see patch #5, passing "qsmaskinitnext" directly to the > for_each_leaf_node_cpu() might be OK, but it just break another > abstraction layer which rcu_rnp_online_cpus() provides. I had missed that. Given that, not enforcingi t makes sense to me. > > > 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 && !WARN_ON_ONCE(!cpu_possible(cpu)); \ > > > > If this happens, we'll exit the loop. If there are any reamining > > possible CPUs, we'll skip them, which would be less than ideal. > > > > I guess this shouldn't happen anyway, but it might be worth continuing. > > > > I chose to break if we met impossible only because I wanted to avoid > using that "if(...) else" trick in an iteration macro ;-) Understandable. ;) > I don't know whether this is the first time something like this is > brought into kernel, so I'm kinda hesitating to bring this in. But seems > I got you as one supporter ;-) > > Certainly, skip is better than stop. >>From a quick look around, I found at least a few instances of the pattern. e.g. include/linux/cpufreq.h: #define cpufreq_for_each_valid_entry(pos, table) \ for (pos = table; pos->frequency != CPUFREQ_TABLE_END; pos++) \ if (pos->frequency == CPUFREQ_ENTRY_INVALID) \ continue; \ else tools/perf/util/build-id.c: #define dsos__for_each_with_build_id(pos, head) \ list_for_each_entry(pos, head, node) \ if (!pos->has_build_id) \ continue; \ else Some drivers, like drivers/net/ethernet/broadcom/bnx2x/bnx2x.h really love it! Thanks, Mark.