From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 40B5AC32750 for ; Wed, 14 Aug 2019 00:12:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 110E0206C2 for ; Wed, 14 Aug 2019 00:12:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726753AbfHNAMi (ORCPT ); Tue, 13 Aug 2019 20:12:38 -0400 Received: from lgeamrelo12.lge.com ([156.147.23.52]:45020 "EHLO lgeamrelo11.lge.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726512AbfHNAMi (ORCPT ); Tue, 13 Aug 2019 20:12:38 -0400 Received: from unknown (HELO lgeamrelo02.lge.com) (156.147.1.126) by 156.147.23.52 with ESMTP; 14 Aug 2019 09:12:35 +0900 X-Original-SENDERIP: 156.147.1.126 X-Original-MAILFROM: byungchul.park@lge.com Received: from unknown (HELO X58A-UD3R) (10.177.222.33) by 156.147.1.126 with ESMTP; 14 Aug 2019 09:12:35 +0900 X-Original-SENDERIP: 10.177.222.33 X-Original-MAILFROM: byungchul.park@lge.com Date: Wed, 14 Aug 2019 09:11:03 +0900 From: Byungchul Park To: "Paul E. McKenney" Cc: Joel Fernandes , Byungchul Park , LKML , Rao Shoaib , kernel-team@android.com, kernel-team , Davidlohr Bueso , Josh Triplett , Lai Jiangshan , Mathieu Desnoyers , rcu , Steven Rostedt Subject: Re: [PATCH RFC v1 1/2] rcu/tree: Add basic support for kfree_rcu batching Message-ID: <20190814001103.GA31884@X58A-UD3R> References: <20190808125607.GB261256@google.com> <20190808180916.GP28441@linux.ibm.com> <20190811083626.GA9486@X58A-UD3R> <20190811084950.GB9486@X58A-UD3R> <20190811234939.GC28441@linux.ibm.com> <20190812101052.GA10478@X58A-UD3R> <20190812131234.GC27552@google.com> <20190813052954.GA18373@X58A-UD3R> <20190813154145.GE28441@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190813154145.GE28441@linux.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Aug 13, 2019 at 08:41:45AM -0700, Paul E. McKenney wrote: > On Tue, Aug 13, 2019 at 02:29:54PM +0900, Byungchul Park wrote: > > On Mon, Aug 12, 2019 at 09:12:34AM -0400, Joel Fernandes wrote: > > > On Mon, Aug 12, 2019 at 07:10:52PM +0900, Byungchul Park wrote: > > > > On Sun, Aug 11, 2019 at 04:49:39PM -0700, Paul E. McKenney wrote: > > > > > Maybe. Note well that I said "potential issue". When I checked a few > > > > > years ago, none of the uses of rcu_barrier() cared about kfree_rcu(). > > > > > They cared instead about call_rcu() callbacks that accessed code or data > > > > > that was going to disappear soon, for example, due to module unload or > > > > > filesystem unmount. > > > > > > > > > > So it -might- be that rcu_barrier() can stay as it is, but with changes > > > > > as needed to documentation. > > > > > > Right, we should update the docs. Byungchul, do you mind sending a patch that > > > documents the rcu_barrier() behavior? > > > > Are you trying to give me the chance? I feel thankful. It doens't matter > > to try it at the moment though, I can't follow-up until September. I'd > > better do that in Septamber or give it up this time. > > Which reminds me... I recall your asking if the kfree_rcu() patch > might be sensitive to the exact hardware, but I cannot locate that > email right off-hand. This is an excellent question! When faced with > floods of kfree_rcu() calls, I would expect some hardware, compiler, > and kernel-configuration sensitivity. Which is why it will likely be Yes. > necessary to do a few more improvements over time -- for but one example, > accumulating callbacks into vectors in order to reduce the number of > kfree()-time cache misses. Yes. That would be a pretty good way to mitigate the problem. I hope the simple way we've done works well enough so it would never happen though. Or I would check the condition of all system resourses e.g. CPU and memory and control the bandwith of them, of course only if that actually happens. Thanks a lot for sharing your opinion on it! Thanks, Byungchul > Thanx, Paul > > > Thanks, > > Byungchul > > > > > > > It also -might- be, maybe now or maybe some time in the future, that > > > > > there will need to be a kfree_rcu_barrier() or some such. But if so, > > > > > let's not create it until it is needed. For one thing, it is reasonably > > > > > likely that something other than a kfree_rcu_barrier() would really > > > > > be what was needed. After all, the main point would be to make sure > > > > > that the old memory really was freed before allocating new memory. > > > > > > > > Now I fully understand what you meant thanks to you. Thank you for > > > > explaining it in detail. > > > > > > > > > But if the system had ample memory, why wait? In that case you don't > > > > > really need to wait for all the old memory to be freed, but rather for > > > > > sufficient memory to be available for allocation. > > > > > > > > Agree. Totally make sense. > > > > > > Agreed, all makes sense. > > > > > > thanks, > > > > > > - Joel > > > > > > [snip] > >