From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753026AbbAMQRq (ORCPT ); Tue, 13 Jan 2015 11:17:46 -0500 Received: from aserp1040.oracle.com ([141.146.126.69]:47582 "EHLO aserp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751452AbbAMQRp (ORCPT ); Tue, 13 Jan 2015 11:17:45 -0500 Message-ID: <54B54513.8060604@oracle.com> Date: Tue, 13 Jan 2015 11:17:23 -0500 From: Boris Ostrovsky User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.3.0 MIME-Version: 1.0 To: David Vrabel , Imre Palik , xen-devel@lists.xenproject.org CC: "Palik, Imre" , x86@kernel.org, linux-kernel@vger.kernel.org, Ingo Molnar , Anthony Liguori , "H. Peter Anvin" , Thomas Gleixner Subject: Re: [Xen-devel] [PATCH] xen-time: decreasing the rating of the xen clocksource below that of the tsc clocksource for dom0's References: <1421136862-15083-1-git-send-email-imrep.amz@gmail.com> <54B4EAC7.2080105@citrix.com> <54B53CDD.7070400@oracle.com> <54B542C0.4070701@citrix.com> In-Reply-To: <54B542C0.4070701@citrix.com> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit X-Source-IP: acsinet21.oracle.com [141.146.126.237] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 01/13/2015 11:07 AM, David Vrabel wrote: > On 13/01/15 15:42, Boris Ostrovsky wrote: >> On 01/13/2015 04:52 AM, David Vrabel wrote: >>> On 13/01/15 08:14, Imre Palik wrote: >>>> From: "Palik, Imre" >>>> >>>> In Dom0's the use of the TSC clocksource (whenever it is stable >>>> enough to >>>> be used) instead of the Xen clocksource should not cause any issues, as >>>> Dom0 VMs never live-migrated. The TSC clocksource is somewhat more >>>> efficient than the Xen paravirtualised clocksource, thus it should have >>>> higher rating. >>>> >>>> This patch decreases the rating of the Xen clocksource in Dom0s to 275. >>>> Which is half-way between the rating of the TSC clocksource (300) and >>>> the >>>> hpet clocksource (250). >>> I'm happy with this but would like to see acks from those who objected >>> to v1. >>> >>> David >>> >>>> --- a/arch/x86/xen/time.c >>>> +++ b/arch/x86/xen/time.c >>>> @@ -487,6 +487,10 @@ static void __init xen_time_init(void) >>>> int cpu = smp_processor_id(); >>>> struct timespec tp; >>>> + /* As Dom0 is never moved, no penalty on using TSC there */ >> Again, why not any PV guest with TSC_MODE_NEVER_EMULATE? > Surely if TSC_MODE_NEVER_EMULATE is set then the TSC is /not/ stable > across a guest save/restore thus the PV clocksource must be used? TSC is declared stable when !d->disable_migrate && !d->arch.vtsc, with vtsc being 0 with TSC_MODE_NEVER_EMULATE (per domain_cpuid()). And if TSC is not stable as seen by CPUID (which would be the case if disable_migrate is not set) then kernel won't use TSC as clocksource anyway, regardless of rating value, won't it? -boris > > I don't think we want to assume that TSC_MODE_NEVER_EMULATE => never > migrate. > > David