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 D673AC433FF for ; Thu, 1 Aug 2019 21:35:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A720F206A2 for ; Thu, 1 Aug 2019 21:35:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2389187AbfHAVfx (ORCPT ); Thu, 1 Aug 2019 17:35:53 -0400 Received: from mga06.intel.com ([134.134.136.31]:35131 "EHLO mga06.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728248AbfHAVfw (ORCPT ); Thu, 1 Aug 2019 17:35:52 -0400 X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from fmsmga008.fm.intel.com ([10.253.24.58]) by orsmga104.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 01 Aug 2019 14:35:52 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.64,335,1559545200"; d="scan'208";a="173038368" Received: from sjchrist-coffee.jf.intel.com (HELO linux.intel.com) ([10.54.74.41]) by fmsmga008.fm.intel.com with ESMTP; 01 Aug 2019 14:35:51 -0700 Date: Thu, 1 Aug 2019 14:35:50 -0700 From: Sean Christopherson To: Thomas Gleixner 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 Message-ID: <20190801213550.GE6783@linux.intel.com> References: <20190801143250.370326052@linutronix.de> <20190801143657.887648487@linutronix.de> <20190801162451.GE31538@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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/.