From patchwork Mon Jan 23 20:50:11 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Peter Zijlstra X-Patchwork-Id: 13113017 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 157CEC38142 for ; Mon, 23 Jan 2023 20:59:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232804AbjAWU67 (ORCPT ); Mon, 23 Jan 2023 15:58:59 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59246 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232710AbjAWU6r (ORCPT ); Mon, 23 Jan 2023 15:58:47 -0500 Received: from desiato.infradead.org (desiato.infradead.org [IPv6:2001:8b0:10b:1:d65d:64ff:fe57:4e05]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id AB2D616315; Mon, 23 Jan 2023 12:58:24 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; h=Content-Type:MIME-Version:References: Subject:Cc:To:From:Date:Message-ID:Sender:Reply-To:Content-Transfer-Encoding: Content-ID:Content-Description:In-Reply-To; bh=LKrP1wtwDwtGfmqJdEblQavVdQZHYUX7Rw9EhYSY0po=; b=LthrIcTz/vqS1NeYOD0uryIINK L5KA0bgSmcB9KMp+I7aQ5EwxmG5qOTNqwwDMaLS8PmkuBC4HSTBPEsoS2bdKiHPtGPN4OFBPidnr2 5zbVMyZP0Y8odEb60DwuQfxT2Eaq/NUfnc1krSs7fj+svrKOAiOd/c5YRTY+/4PYC1xoidVDbYSli ORaftVjMZwg4jYvWLzMGWTGKhY1mBn3TA0jGQksHIcZZqt2zQz/rCGybV3JcBvrHQ6IxXmwVkth8l d1NqgwluXNaHMVCrlyKf/lsJuQ5S//P74orZosbW8IQc/Q1n2PXtJaHc/YqKiZHWCmUT8RmH7O4/Q FhSIaXXw==; Received: from j130084.upc-j.chello.nl ([24.132.130.84] helo=noisy.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.96 #2 (Red Hat Linux)) id 1pK3sA-001dtm-02; Mon, 23 Jan 2023 20:57:07 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 6025C3002BF; Mon, 23 Jan 2023 21:57:24 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 0) id C4CC620754993; Mon, 23 Jan 2023 21:57:24 +0100 (CET) Message-ID: <20230123205515.001913664@infradead.org> User-Agent: quilt/0.66 Date: Mon, 23 Jan 2023 21:50:11 +0100 From: Peter Zijlstra To: mingo@kernel.org Cc: will@kernel.org, peterz@infradead.org, boqun.feng@gmail.com, mark.rutland@arm.com, tglx@linutronix.de, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, seanjc@google.com, pbonzini@redhat.com, jgross@suse.com, srivatsa@csail.mit.edu, amakhalov@vmware.com, pv-drivers@vmware.com, rostedt@goodmis.org, mhiramat@kernel.org, wanpengli@tencent.com, vkuznets@redhat.com, boris.ostrovsky@oracle.com, rafael@kernel.org, daniel.lezcano@linaro.org, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, bsegall@google.com, mgorman@suse.de, bristot@redhat.com, vschneid@redhat.com, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, virtualization@lists.linux-foundation.org, linux-trace-kernel@vger.kernel.org, linux-pm@vger.kernel.org, Uros Bizjak Subject: [PATCH 2/6] x86/pvclock: improve atomic update of last_value in pvclock_clocksource_read References: <20230123205009.790550642@infradead.org> MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org From: Uros Bizjak Improve atomic update of last_value in pvclock_clocksource_read: - Atomic update can be skipped if the "last_value" is already equal to "ret". - The detection of atomic update failure is not correct. The value, returned by atomic64_cmpxchg should be compared to the old value from the location to be updated. If these two are the same, then atomic update succeeded and "last_value" location is updated to "ret" in an atomic way. Otherwise, the atomic update failed and it should be retried with the value from "last_value" - exactly what atomic64_try_cmpxchg does in a correct and more optimal way. Signed-off-by: Uros Bizjak Signed-off-by: Peter Zijlstra (Intel) Link: https://lkml.kernel.org/r/20230118202330.3740-1-ubizjak@gmail.com --- arch/x86/kernel/pvclock.c | 5 ++--- 1 file changed, 2 insertions(+), 3 deletions(-) diff --git a/arch/x86/kernel/pvclock.c b/arch/x86/kernel/pvclock.c index eda37df016f0..5a2a517dd61b 100644 --- a/arch/x86/kernel/pvclock.c +++ b/arch/x86/kernel/pvclock.c @@ -102,10 +102,9 @@ u64 pvclock_clocksource_read(struct pvclock_vcpu_time_info *src) */ last = atomic64_read(&last_value); do { - if (ret < last) + if (ret <= last) return last; - last = atomic64_cmpxchg(&last_value, last, ret); - } while (unlikely(last != ret)); + } while (!atomic64_try_cmpxchg(&last_value, &last, ret)); return ret; }