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 92D57C54EED for ; Mon, 30 Jan 2023 14:15:07 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237296AbjA3OPG (ORCPT ); Mon, 30 Jan 2023 09:15:06 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59040 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237310AbjA3OPF (ORCPT ); Mon, 30 Jan 2023 09:15:05 -0500 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 6E2567698 for ; Mon, 30 Jan 2023 06:15:04 -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 1DBEFB80DEB for ; Mon, 30 Jan 2023 14:15:03 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1ED4AC433EF; Mon, 30 Jan 2023 14:15:00 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1675088101; bh=PiQIi1sGWYNyhdFwLS8tLrQSCzg4TBXd2U/CVxrfxjI=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=eSa3rLhVsOhY2s52waTbcvXuxi2vXCspn4fEyAO1xGf6VyfKPUedRhSu5Ya6Hr4zs KkT3iMsexFuce3QXmegpWulk3Q/JlnwOloWG5KLEtRqV82grOoGNUhcwVoI2uIZgw6 SvIux2kcAePnjrQ4RfAbUvhfk5zYNIJ4Jx1cfrX4= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Mateusz Guzik , Peter Zijlstra , Linus Torvalds , Sasha Levin , Tony Luck , Nicholas Piggin , Will Deacon Subject: [PATCH 5.15 116/204] lockref: stop doing cpu_relax in the cmpxchg loop Date: Mon, 30 Jan 2023 14:51:21 +0100 Message-Id: <20230130134321.567176074@linuxfoundation.org> X-Mailer: git-send-email 2.39.1 In-Reply-To: <20230130134316.327556078@linuxfoundation.org> References: <20230130134316.327556078@linuxfoundation.org> User-Agent: quilt/0.67 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org From: Mateusz Guzik [ Upstream commit f5fe24ef17b5fbe6db49534163e77499fb10ae8c ] On the x86-64 architecture even a failing cmpxchg grants exclusive access to the cacheline, making it preferable to retry the failed op immediately instead of stalling with the pause instruction. To illustrate the impact, below are benchmark results obtained by running various will-it-scale tests on top of the 6.2-rc3 kernel and Cascade Lake (2 sockets * 24 cores * 2 threads) CPU. All results in ops/s. Note there is some variance in re-runs, but the code is consistently faster when contention is present. open3 ("Same file open/close"): proc stock no-pause 1 805603 814942 (+%1) 2 1054980 1054781 (-0%) 8 1544802 1822858 (+18%) 24 1191064 2199665 (+84%) 48 851582 1469860 (+72%) 96 609481 1427170 (+134%) fstat2 ("Same file fstat"): proc stock no-pause 1 3013872 3047636 (+1%) 2 4284687 4400421 (+2%) 8 3257721 5530156 (+69%) 24 2239819 5466127 (+144%) 48 1701072 5256609 (+209%) 96 1269157 6649326 (+423%) Additionally, a kernel with a private patch to help access() scalability: access2 ("Same file access"): proc stock patched patched +nopause 24 2378041 2005501 5370335 (-15% / +125%) That is, fixing the problems in access itself *reduces* scalability after the cacheline ping-pong only happens in lockref with the pause instruction. Note that fstat and access benchmarks are not currently integrated into will-it-scale, but interested parties can find them in pull requests to said project. Code at hand has a rather tortured history. First modification showed up in commit d472d9d98b46 ("lockref: Relax in cmpxchg loop"), written with Itanium in mind. Later it got patched up to use an arch-dependent macro to stop doing it on s390 where it caused a significant regression. Said macro had undergone revisions and was ultimately eliminated later, going back to cpu_relax. While I intended to only remove cpu_relax for x86-64, I got the following comment from Linus: I would actually prefer just removing it entirely and see if somebody else hollers. You have the numbers to prove it hurts on real hardware, and I don't think we have any numbers to the contrary. So I think it's better to trust the numbers and remove it as a failure, than say "let's just remove it on x86-64 and leave everybody else with the potentially broken code" Additionally, Will Deacon (maintainer of the arm64 port, one of the architectures previously benchmarked): So, from the arm64 side of the fence, I'm perfectly happy just removing the cpu_relax() calls from lockref. As such, come back full circle in history and whack it altogether. Signed-off-by: Mateusz Guzik Link: https://lore.kernel.org/all/CAGudoHHx0Nqg6DE70zAVA75eV-HXfWyhVMWZ-aSeOofkA_=WdA@mail.gmail.com/ Acked-by: Tony Luck # ia64 Acked-by: Nicholas Piggin # powerpc Acked-by: Will Deacon # arm64 Acked-by: Peter Zijlstra Signed-off-by: Linus Torvalds Signed-off-by: Sasha Levin --- lib/lockref.c | 1 - 1 file changed, 1 deletion(-) diff --git a/lib/lockref.c b/lib/lockref.c index 5b34bbd3eba8..81ac5f355242 100644 --- a/lib/lockref.c +++ b/lib/lockref.c @@ -24,7 +24,6 @@ } \ if (!--retry) \ break; \ - cpu_relax(); \ } \ } while (0) -- 2.39.0