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 mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id A29B5C433EF for ; Wed, 29 Sep 2021 09:20:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7FEA8613A9 for ; Wed, 29 Sep 2021 09:20:31 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S245041AbhI2JWL (ORCPT ); Wed, 29 Sep 2021 05:22:11 -0400 Received: from [119.249.100.165] ([119.249.100.165]:15915 "EHLO mx423.baidu.com" rhost-flags-FAIL-FAIL-OK-FAIL) by vger.kernel.org with ESMTP id S229487AbhI2JWJ (ORCPT ); Wed, 29 Sep 2021 05:22:09 -0400 Received: from bjhw-sys-rpm015653cc5.bjhw.baidu.com (bjhw-sys-rpm015653cc5.bjhw.baidu.com [10.227.53.39]) by mx423.baidu.com (Postfix) with ESMTP id D45D316E01154; Wed, 29 Sep 2021 17:20:14 +0800 (CST) Received: from localhost (localhost [127.0.0.1]) by bjhw-sys-rpm015653cc5.bjhw.baidu.com (Postfix) with ESMTP id CCAC4D9932; Wed, 29 Sep 2021 17:20:14 +0800 (CST) From: Li RongQing To: lirongqing@baidu.com, linux-kernel@vger.kernel.org, tglx@linutronix.de Subject: [PATCH] KVM: Micro-optimize pvclock_clocksource_read Date: Wed, 29 Sep 2021 17:20:14 +0800 Message-Id: <1632907214-29633-1-git-send-email-lirongqing@baidu.com> X-Mailer: git-send-email 1.7.1 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Compare the return of atomic64_cmpxchg with previous save for last, to reduce the number of while-loop iterations with atomic operations from two to one in the most common situation Original patch at: https://kvm.vger.kernel.narkive.com/WraXedaQ/patch-kvm-pvclock-clocksource-read-while-loop-optimization Signed-off-by: Li RongQing --- arch/x86/kernel/pvclock.c | 5 +++-- 1 files changed, 3 insertions(+), 2 deletions(-) diff --git a/arch/x86/kernel/pvclock.c b/arch/x86/kernel/pvclock.c index eda37df..98d106f 100644 --- a/arch/x86/kernel/pvclock.c +++ b/arch/x86/kernel/pvclock.c @@ -67,8 +67,8 @@ u8 pvclock_read_flags(struct pvclock_vcpu_time_info *src) u64 pvclock_clocksource_read(struct pvclock_vcpu_time_info *src) { unsigned version; + u64 last, save; u64 ret; - u64 last; u8 flags; do { @@ -104,8 +104,9 @@ u64 pvclock_clocksource_read(struct pvclock_vcpu_time_info *src) do { if (ret < last) return last; + save = last; last = atomic64_cmpxchg(&last_value, last, ret); - } while (unlikely(last != ret)); + } while (unlikely(last != save)); return ret; } -- 1.7.1