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=-3.7 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 01B67C55186 for ; Wed, 22 Apr 2020 10:51:13 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E0C812073A for ; Wed, 22 Apr 2020 10:51:12 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732183AbgDVKvI (ORCPT ); Wed, 22 Apr 2020 06:51:08 -0400 Received: from disco-boy.misterjones.org ([51.254.78.96]:48482 "EHLO disco-boy.misterjones.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728702AbgDVKvF (ORCPT ); Wed, 22 Apr 2020 06:51:05 -0400 X-Greylist: delayed 2170 seconds by postgrey-1.27 at vger.kernel.org; Wed, 22 Apr 2020 06:51:03 EDT Received: from disco-boy.misterjones.org ([51.254.78.96] helo=www.loen.fr) by disco-boy.misterjones.org with esmtpsa (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.92) (envelope-from ) id 1jRCOi-005RP3-VM; Wed, 22 Apr 2020 11:14:29 +0100 MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 7bit Date: Wed, 22 Apr 2020 11:14:28 +0100 From: Marc Zyngier To: Davidlohr Bueso Cc: tglx@linutronix.de, pbonzini@redhat.com, kvm@vger.kernel.org, Davidlohr Bueso , peterz@infradead.org, torvalds@linux-foundation.org, bigeasy@linutronix.de, linux-kernel@vger.kernel.org, rostedt@goodmis.org, linux-mips@vger.kernel.org, Paul Mackerras , joel@joelfernandes.org, will@kernel.org, kvmarm@lists.cs.columbia.edu Subject: Re: [PATCH 4/5] kvm: Replace vcpu->swait with rcuwait In-Reply-To: <20200422040739.18601-5-dave@stgolabs.net> References: <20200422040739.18601-1-dave@stgolabs.net> <20200422040739.18601-5-dave@stgolabs.net> Message-ID: X-Sender: maz@misterjones.org User-Agent: Roundcube Webmail/1.3.10 X-SA-Exim-Connect-IP: 51.254.78.96 X-SA-Exim-Rcpt-To: dave@stgolabs.net, tglx@linutronix.de, pbonzini@redhat.com, kvm@vger.kernel.org, dbueso@suse.de, peterz@infradead.org, torvalds@linux-foundation.org, bigeasy@linutronix.de, linux-kernel@vger.kernel.org, rostedt@goodmis.org, linux-mips@vger.kernel.org, paulus@ozlabs.org, joel@joelfernandes.org, will@kernel.org, kvmarm@lists.cs.columbia.edu X-SA-Exim-Mail-From: maz@misterjones.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2020-04-22 05:07, Davidlohr Bueso wrote: > The use of any sort of waitqueue (simple or regular) for > wait/waking vcpus has always been an overkill and semantically > wrong. Because this is per-vcpu (which is blocked) there is > only ever a single waiting vcpu, thus no need for any sort of > queue. > > As such, make use of the rcuwait primitive, with the following > considerations: > > - rcuwait already provides the proper barriers that serialize > concurrent waiter and waker. > > - Task wakeup is done in rcu read critical region, with a > stable task pointer. > > - Because there is no concurrency among waiters, we need > not worry about rcuwait_wait_event() calls corrupting > the wait->task. As a consequence, this saves the locking > done in swait when modifying the queue. This also applies > to per-vcore wait for powerpc kvm-hv. > > The x86 tscdeadline_latency test mentioned in 8577370fb0cb > ("KVM: Use simple waitqueue for vcpu->wq") shows that, on avg, > latency is reduced by around 15-20% with this change. > > Cc: Paul Mackerras > Cc: kvmarm@lists.cs.columbia.edu > Cc: linux-mips@vger.kernel.org > Signed-off-by: Davidlohr Bueso Reviewed-by: Marc Zyngier Thanks, M. -- Who you jivin' with that Cosmik Debris?