From mboxrd@z Thu Jan 1 00:00:00 1970 From: Marcelo Tosatti Subject: Re: [PATCH 0/5 V5] Avoid soft lockup message when KVM is stopped by host Date: Wed, 14 Dec 2011 10:18:10 -0200 Message-ID: <20111214121810.GC18317@amt.cnet> References: <1323116344-17911-1-git-send-email-emunson@mgebm.net> <4EDF7B0D.4060001@redhat.com> <4EE4A4DA.90100@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <4EE4A4DA.90100@redhat.com> Sender: linux-kernel-owner@vger.kernel.org To: Dor Laor Cc: Avi Kivity , Eric B Munson , mingo@redhat.com, hpa@zytor.com, arnd@arndb.de, ryanh@linux.vnet.ibm.com, aliguori@us.ibm.com, jeremy.fitzhardinge@citrix.com, levinsasha928@gmail.com, Jan Kiszka , kvm@vger.kernel.org, linux-arch@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org List-Id: linux-arch.vger.kernel.org On Sun, Dec 11, 2011 at 02:40:58PM +0200, Dor Laor wrote: > >>When a guest kernel is stopped by the host hypervisor it can look like a soft > >>lockup to the guest kernel. This false warning can mask later soft lockup > >>warnings which may be real. This patch series adds a method for a host > >>hypervisor to communicate to a guest kernel that it is being stopped. The > >>final patch in the series has the watchdog check this flag when it goes to > >>issue a soft lockup warning and skip the warning if the guest knows it was > >>stopped. > >> > >>It was attempted to solve this in Qemu, but the side effects of saving and > >>restoring the clock and tsc for each vcpu put the wall clock of the guest behind > >>by the amount of time of the pause. This forces a guest to have ntp running > >>in order to keep the wall clock accurate. > > Guests need to run NTP regardless, not only the virtualization layer > add some skew, the physical world is not that perfect. > btw: traditional NTP client won't sync the time automatically if the > diff is > 0.5%. > > > > >Having this controlled from userspace means it doesn't work for SIGSTOP > >or for long scheduling delays. What about doing this automatically > >based on preempt notifiers? > > > > > > Isn't it solved by steal time? No. From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx1.redhat.com ([209.132.183.28]:35235 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755446Ab1LNMTn (ORCPT ); Wed, 14 Dec 2011 07:19:43 -0500 Date: Wed, 14 Dec 2011 10:18:10 -0200 From: Marcelo Tosatti Subject: Re: [PATCH 0/5 V5] Avoid soft lockup message when KVM is stopped by host Message-ID: <20111214121810.GC18317@amt.cnet> References: <1323116344-17911-1-git-send-email-emunson@mgebm.net> <4EDF7B0D.4060001@redhat.com> <4EE4A4DA.90100@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4EE4A4DA.90100@redhat.com> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Dor Laor Cc: Avi Kivity , Eric B Munson , mingo@redhat.com, hpa@zytor.com, arnd@arndb.de, ryanh@linux.vnet.ibm.com, aliguori@us.ibm.com, jeremy.fitzhardinge@citrix.com, levinsasha928@gmail.com, Jan Kiszka , kvm@vger.kernel.org, linux-arch@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org Message-ID: <20111214121810.OSzFfy4cUwhb8aYRO0iOMVbD5aUX8VnS5-vkHaQKwTU@z> On Sun, Dec 11, 2011 at 02:40:58PM +0200, Dor Laor wrote: > >>When a guest kernel is stopped by the host hypervisor it can look like a soft > >>lockup to the guest kernel. This false warning can mask later soft lockup > >>warnings which may be real. This patch series adds a method for a host > >>hypervisor to communicate to a guest kernel that it is being stopped. The > >>final patch in the series has the watchdog check this flag when it goes to > >>issue a soft lockup warning and skip the warning if the guest knows it was > >>stopped. > >> > >>It was attempted to solve this in Qemu, but the side effects of saving and > >>restoring the clock and tsc for each vcpu put the wall clock of the guest behind > >>by the amount of time of the pause. This forces a guest to have ntp running > >>in order to keep the wall clock accurate. > > Guests need to run NTP regardless, not only the virtualization layer > add some skew, the physical world is not that perfect. > btw: traditional NTP client won't sync the time automatically if the > diff is > 0.5%. > > > > >Having this controlled from userspace means it doesn't work for SIGSTOP > >or for long scheduling delays. What about doing this automatically > >based on preempt notifiers? > > > > > > Isn't it solved by steal time? No.