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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 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 E37E1C433FF for ; Thu, 1 Aug 2019 21:44:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B39A3206B8 for ; Thu, 1 Aug 2019 21:44:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732337AbfHAVom (ORCPT ); Thu, 1 Aug 2019 17:44:42 -0400 Received: from Galois.linutronix.de ([193.142.43.55]:37958 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731418AbfHAVom (ORCPT ); Thu, 1 Aug 2019 17:44:42 -0400 Received: from pd9ef1cb8.dip0.t-ipconnect.de ([217.239.28.184] helo=nanos) by Galois.linutronix.de with esmtpsa (TLS1.2:DHE_RSA_AES_256_CBC_SHA256:256) (Exim 4.80) (envelope-from ) id 1htIs9-0002sZ-6o; Thu, 01 Aug 2019 23:44:29 +0200 Date: Thu, 1 Aug 2019 23:44:28 +0200 (CEST) From: Thomas Gleixner To: Sean Christopherson cc: Oleg Nesterov , LKML , x86@kernel.org, Peter Zijlstra , Ingo Molnar , Sebastian Siewior , Anna-Maria Gleixner , Steven Rostedt , Julia Cartwright , Paul McKenney , Frederic Weisbecker , kvm@vger.kernel.org, Radim Krcmar , Paolo Bonzini , John Stultz , Andy Lutomirski , "Paul E. McKenney" Subject: Re: [patch 2/5] x86/kvm: Handle task_work on VMENTER/EXIT In-Reply-To: <20190801213550.GE6783@linux.intel.com> Message-ID: References: <20190801143250.370326052@linutronix.de> <20190801143657.887648487@linutronix.de> <20190801162451.GE31538@redhat.com> <20190801213550.GE6783@linux.intel.com> User-Agent: Alpine 2.21 (DEB 202 2017-01-01) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 1 Aug 2019, Sean Christopherson wrote: > On Thu, Aug 01, 2019 at 08:34:53PM +0200, Thomas Gleixner wrote: > > On Thu, 1 Aug 2019, Oleg Nesterov wrote: > > > On 08/01, Thomas Gleixner wrote: > > > > > > > > @@ -8172,6 +8174,10 @@ static int vcpu_run(struct kvm_vcpu *vcp > > > > ++vcpu->stat.signal_exits; > > > > break; > > > > } > > > > + > > > > + if (notify_resume_pending()) > > > > + tracehook_handle_notify_resume(); > > > > > > shouldn't you drop kvm->srcu before tracehook_handle_notify_resume() ? > > > > > > I don't understand this code at all, but vcpu_run() does this even before > > > cond_resched(). > > > > Yeah, I noticed that it's dropped around cond_resched(). > > > > My understanding is that for voluntary giving up the CPU via cond_resched() > > it needs to be dropped. > > > > For involuntary preemption (CONFIG_PREEMPT=y) it's not required as the > > whole code section after preempt_enable() is fully preemptible. > > > > Now the 1Mio$ question is whether any of the notify functions invokes > > cond_resched() and whether that really matters. Paolo? > > cond_resched() is called via tracehook_notify_resume()->task_work_run(), > and "kernel code can only call cond_resched() in places where it ... > cannot hold references to any RCU-protected data structures" according to > https://lwn.net/Articles/603252/. Right you are.