From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752651AbcL2Itf (ORCPT ); Thu, 29 Dec 2016 03:49:35 -0500 Received: from mga01.intel.com ([192.55.52.88]:4917 "EHLO mga01.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752424AbcL2Itd (ORCPT ); Thu, 29 Dec 2016 03:49:33 -0500 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.33,425,1477983600"; d="scan'208";a="47716130" Date: Thu, 29 Dec 2016 16:59:02 +0800 From: Chen Yu To: Marcelo Tosatti Cc: kvm@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org, "Rafael J. Wysocki" , Len Brown , Paolo Bonzini , Radim Krcmar , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Wanpeng Li Subject: Re: [PATCH][RFC] VM: x86: Return ealier if clocksource has not changed Message-ID: <20161229085902.GA2474@yu-desktop-1.sh.intel.com> References: <1482482513-24151-1-git-send-email-yu.c.chen@intel.com> <20161226194422.GA30796@amt.cnet> <20161227080644.GA5370@yu-desktop-1.sh.intel.com> <20161227153244.GA14267@amt.cnet> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20161227153244.GA14267@amt.cnet> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Dec 27, 2016 at 01:32:47PM -0200, Marcelo Tosatti wrote: > On Tue, Dec 27, 2016 at 04:06:44PM +0800, Chen Yu wrote: > > Hi Marcelo, > > On Mon, Dec 26, 2016 at 05:44:25PM -0200, Marcelo Tosatti wrote: > > > On Fri, Dec 23, 2016 at 04:41:53PM +0800, Chen Yu wrote: > > > > Currently the notifier of pvclock_gtod_notify() get invoked > > > > frequently due to the periodic update_wall_time(). This might > > > > slow down the system a little bit as there might be redundant > > > > execution code path and unnecessary lock contention > > > > in update_pvclock_gtod(), which was found when I was doing > > > > suspend/resume speed testings. As pvclock_gtod_notify() > > > > should be invoked only when clocksource has changed, according to > > > > Commit 16e8d74d2da9 ("KVM: x86: notifier for clocksource changes") > > > > , either we can add a new notifier for clocksource switch, > > > > or we can simply bypass the following code in pvclock_gtod_notify() > > > > earlier if there is no clocksource switch. > > > > > > > > Cc: Paolo Bonzini > > > > Cc: "Radim Krcmar" > > > > Cc: Marcelo Tosatti > > > > Cc: Thomas Gleixner > > > > Cc: Ingo Molnar > > > > Cc: "H. Peter Anvin" > > > > Cc: Wanpeng Li > > > > Signed-off-by: Chen Yu > > > > --- > > > > arch/x86/kvm/x86.c | 5 +++-- > > > > 1 file changed, 3 insertions(+), 2 deletions(-) > > > > > > > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > > > > index 445c51b..54aa32d 100644 > > > > --- a/arch/x86/kvm/x86.c > > > > +++ b/arch/x86/kvm/x86.c > > > > @@ -5961,13 +5961,14 @@ static int pvclock_gtod_notify(struct notifier_block *nb, unsigned long unused, > > > > struct pvclock_gtod_data *gtod = &pvclock_gtod_data; > > > > struct timekeeper *tk = priv; > > > > > > > > + if (likely(gtod->clock.vclock_mode == VCLOCK_TSC)) > > > > + return 0; > > > > > > I think this is only safe if any of the values in "struct > > > pvclock_gtod_data" are unchanged. Otherwise the local (KVM) copy is > > > kept incorrect. > > I missread the code previously and I thought only under the condition > > the clocksource has been switched to another one will the KVM copy > > be touched. Apparently it is not the case because the copy should > > be updated on-time during normal tick, right? > > thanks for your reply, > > Yes, it is updated during the normal tick, and mult/freq values change. > > However, if none of them change, its not necessary to call the callback. > Perhaps you can check if any of the values changed and only > invoke the callback in that case? > Yes, this should be an optimization, but most of the callers(workload) come from update_wall_time(), and in this code path the clock source's cycle should already be updated in most cases, so this optimization should not take much effect to reduce the burden I guess? Thanks, Yu