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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4626AC4332F for ; Sat, 26 Nov 2022 18:03:59 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229491AbiKZSD5 (ORCPT ); Sat, 26 Nov 2022 13:03:57 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54444 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229459AbiKZSD5 (ORCPT ); Sat, 26 Nov 2022 13:03:57 -0500 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C704118396 for ; Sat, 26 Nov 2022 10:03:55 -0800 (PST) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id 777A4B80951 for ; Sat, 26 Nov 2022 18:03:54 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 16BDFC433C1; Sat, 26 Nov 2022 18:03:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1669485833; bh=zrz4L2EfzCFnPx1a5xq3YMyQSRr8mpXpGBR71X1rp+E=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=EwYhLJ215JRqPyP1s/PFIw9QYhDSVmaF5Uw9xrgW7b//oIWz6SnqPAgbMVrFbFf5e teGdf17MjkS6g7Zjjlj78auztbPiGTddCf/9hMMd/zIS6TaVOweb2LHqibzxO3m5FQ D6pHlnHrQjK3IO88DV79mqwZ9aofocfS8AG83UhKlbI7CU7TgHwWMf/73Y6mhT/PF9 US0iqTC2To2jRmycH9Ui3AIHPcmcB+SBd2eHQ00mQH1OxPimNj062GrdYVCoV0lPkg hj5FrSU9uAiIZENGXU0nXr+pjyMxDUw/zvbRB52Ob7cqx9oOkIXkwMfpT9Sq75ftRm DWg4yzTM4HRXw== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id AA8405C0BE7; Sat, 26 Nov 2022 10:03:52 -0800 (PST) Date: Sat, 26 Nov 2022 10:03:52 -0800 From: "Paul E. McKenney" To: Pingfan Liu Cc: rcu@vger.kernel.org, Lai Jiangshan , Frederic Weisbecker , Josh Triplett , Steven Rostedt , Mathieu Desnoyers Subject: Re: [PATCHv2 2/2] srcu: Eliminate the case that snp_seq bigger than snap in srcu_funnel_gp_start() Message-ID: <20221126180352.GD4001@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20221123135638.79021-1-kernelfans@gmail.com> <20221123135638.79021-3-kernelfans@gmail.com> <20221123192046.GH4001@paulmck-ThinkPad-P17-Gen-1> <20221124052417.GO4001@paulmck-ThinkPad-P17-Gen-1> <20221124161926.GA3495117@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Fri, Nov 25, 2022 at 08:37:03PM +0800, Pingfan Liu wrote: > On Thu, Nov 24, 2022 at 08:19:26AM -0800, Paul E. McKenney wrote: > > On Wed, Nov 23, 2022 at 09:24:17PM -0800, Paul E. McKenney wrote: > > > On Thu, Nov 24, 2022 at 12:06:39PM +0800, Pingfan Liu wrote: > > > > On Thu, Nov 24, 2022 at 11:33 AM Pingfan Liu wrote: > > > > > On Wed, Nov 23, 2022 at 11:20:46AM -0800, Paul E. McKenney wrote: > > > > > > On Wed, Nov 23, 2022 at 09:56:38PM +0800, Pingfan Liu wrote: > > > > > > > Since the srcu read lock is still held during srcu_funnel_gp_start(), > > > > > > > the seq snap should be the largest number for the slot > > > > > > > srcu_have_cbs[idx]. > > > > > > > > > > > > > > Signed-off-by: Pingfan Liu > > > > > > > Cc: Lai Jiangshan > > > > > > > Cc: "Paul E. McKenney" > > > > > > > Cc: Frederic Weisbecker > > > > > > > Cc: Josh Triplett > > > > > > > Cc: Steven Rostedt > > > > > > > Cc: Mathieu Desnoyers > > > > > > > To: rcu@vger.kernel.org > > > > > > > --- > > > > > > > include/linux/rcupdate.h | 1 + > > > > > > > kernel/rcu/srcutree.c | 11 ++++++----- > > > > > > > 2 files changed, 7 insertions(+), 5 deletions(-) > > > > > > > > > > > > > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h > > > > > > > index 08605ce7379d..a09007236660 100644 > > > > > > > --- a/include/linux/rcupdate.h > > > > > > > +++ b/include/linux/rcupdate.h > > > > > > > @@ -32,6 +32,7 @@ > > > > > > > #include > > > > > > > > > > > > > > #define ULONG_CMP_GE(a, b) (ULONG_MAX / 2 >= (a) - (b)) > > > > > > > +#define ULONG_CMP_GT(a, b) (ULONG_MAX / 2 > (a) - (b)) > > > > > > > > > > > > Please see below... > > > > > > > > > > > > > #define ULONG_CMP_LT(a, b) (ULONG_MAX / 2 < (a) - (b)) > > > > > > > #define ulong2long(a) (*(long *)(&(a))) > > > > > > > #define USHORT_CMP_GE(a, b) (USHRT_MAX / 2 >= (unsigned short)((a) - (b))) > > > > > > > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c > > > > > > > index 2fc0e775ade4..41902b823687 100644 > > > > > > > --- a/kernel/rcu/srcutree.c > > > > > > > +++ b/kernel/rcu/srcutree.c > > > > > > > @@ -905,14 +905,15 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp, > > > > > > > for (snp = snp_leaf; snp != NULL; snp = snp->srcu_parent) { > > > > > > > spin_lock_irqsave_rcu_node(snp, flags); > > > > > > > snp_seq = snp->srcu_have_cbs[idx]; > > > > > > > - if (!srcu_invl_snp_seq(snp_seq) && ULONG_CMP_GE(snp_seq, s)) { > > > > > > > + /* > > > > > > > + * s should be the biggest in the current slot. Hence only LE is > > > > > > > + * valid > > > > > > > + */ > > > > > > > + BUG_ON(ULONG_CMP_GT(snp_seq, s)); > > > > > > > > > > > > Why not this? (Plus adjusting the comment above, of course.) > > > > > > > > > > > > WARN_ON_ONCE(ULONG_CMP_LT(s, snp_seq)); > > > > > > > > > > > > > > > > A neat solution! > > > > > > > > > > And what about the comment > > > > > /* s should be the biggest in the current slot. */ > > > > > > > > > > > That way we don't need ULONG_CMP_GT(). Plus if we are confused about > > > > > > s being the biggest in the current slot, we get a splat and can debug > > > > > > further. We both might be quite sure that we are not confused, but > > > > > > that is exactly when we are most prone to making mistakes. ;-) > > > > > > > > > > > > > > > > Same feeling :) > > > > > > > > > > > I also ask that you run with this check for some time. After all, if > > > > > > > > Forget to ask if the test > > > > "tools/testing/selftests/rcutorture/bin/kvm.sh --allcpus --duration > > > > 10h --configs 18*SRCU-P" > > > > satisfies your requirement? > > > > > > I am thinking more in terms of adding the WARN_ON_ONCE(), letting it be > > > in -next and possibly mainline for a couple of years, and then if there > > > are no splats, start feeling more confident in the asserted relationship. > > > > > > If there was a significant performance, scalability, energy-efficiency, or > > > simplification benefit, I would feel justified in being more aggressive. > > > > > > But I am not seeing a significant benefit. > > > > Ah, and before I forget -again-, have you thought through the counter-wrap > > scenarios? Please keep in mind that on a 32-bit system, those counters > > can wrap quite quickly compared to typical uptimes. > > I think 32-bit has not significant different since the statement > WARN_ON_ONCE(ULONG_CMP_LT(s, snp_seq)); > > where 's < snp_seq' is not associated with bits. Am I missing anything? Your point is that long before there was any danger of wrap, this WARN_ON_ONCE() would trigger? If so, the point is to avoid that WARN_ON_ONCE() from triggering. Or am I missing your point? Thanx, Paul > Thanks, > > Pingfan > > > Thanx, Paul > > > > > > Thanks > > > > > > > > Pingfan > > > > > Sure. I will try it immediately. > > > > > > > > > > > the assumption is incorrect, the resulting SRCU hangs off in various > > > > > > systems around the world will not be so much fun to debug. > > > > > > > > > > > > Given that this is slowpath code, it is much better to take an extra > > > > > > compare and branch than to introduce even an extremely small risk of > > > > > > hanging SRCU. > > > > > > > > > > > > > > > > Agree. > > > > > > > > > > > > > > > Thanks, > > > > > > > > > > Pingfan > > > > > > > > > > > Thanx, Paul > > > > > > > > > > > > > + if (!srcu_invl_snp_seq(snp_seq) && (snp_seq == s)) { > > > > > > > if (snp == snp_leaf && snp_seq == s) > > > > > > > snp->srcu_data_have_cbs[idx] |= sdp->grpmask; > > > > > > > spin_unlock_irqrestore_rcu_node(snp, flags); > > > > > > > - if (snp == snp_leaf && snp_seq != s) { > > > > > > > - srcu_schedule_cbs_sdp(sdp, do_norm ? SRCU_INTERVAL : 0); > > > > > > > - return; > > > > > > > - } > > > > > > > if (!do_norm) > > > > > > > srcu_funnel_exp_start(ssp, snp, s); > > > > > > > return; > > > > > > > -- > > > > > > > 2.31.1 > > > > > > >