From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932713AbcERQOI (ORCPT ); Wed, 18 May 2016 12:14:08 -0400 Received: from userp1040.oracle.com ([156.151.31.81]:30571 "EHLO userp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932568AbcERQOG convert rfc822-to-8bit (ORCPT ); Wed, 18 May 2016 12:14:06 -0400 Subject: Re: [PATCH] xen: add steal_clock support on x86 To: Juergen Gross , David Vrabel , xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org References: <1463573758-11441-1-git-send-email-jgross@suse.com> <573C804F.6020708@oracle.com> <573C81D1.9040309@suse.com> <573C896F.4090303@oracle.com> <573C8D4A.1000005@suse.com> <573C8E16.5040409@citrix.com> <573C8FEA.2070407@oracle.com> <573C9188.7010303@suse.com> Cc: sstabellini@kernel.org From: Boris Ostrovsky Message-ID: <573C94BF.70508@oracle.com> Date: Wed, 18 May 2016 12:13:51 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.1.0 MIME-Version: 1.0 In-Reply-To: <573C9188.7010303@suse.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 8BIT X-Source-IP: aserv0022.oracle.com [141.146.126.234] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/18/2016 12:00 PM, Juergen Gross wrote: > On 18/05/16 17:53, Boris Ostrovsky wrote: >> On 05/18/2016 11:45 AM, David Vrabel wrote: >>> On 18/05/16 16:42, Juergen Gross wrote: >>>> On 18/05/16 17:25, Boris Ostrovsky wrote: >>>>> On 05/18/2016 10:53 AM, Juergen Gross wrote: >>>>>> On 18/05/16 16:46, Boris Ostrovsky wrote: >>>>>>> On 05/18/2016 08:15 AM, Juergen Gross wrote: >>>>>>>> } >>>>>>>> >>>>>>>> +void __init xen_time_setup_guest(void) >>>>>>>> +{ >>>>>>>> + pv_time_ops.steal_clock = xen_steal_clock; >>>>>>>> + >>>>>>>> + static_key_slow_inc(¶virt_steal_enabled); >>>>>>>> + /* >>>>>>>> + * We can't set paravirt_steal_rq_enabled as this would require the >>>>>>>> + * capability to read another cpu's runstate info. >>>>>>>> + */ >>>>>>>> +} >>>>>>> Won't we be accounting for stolen cycles twice now --- once from >>>>>>> steal_account_process_tick()->steal_clock() and second time from >>>>>>> do_stolen_accounting()? >>>>>> Uuh, yes. >>>>>> >>>>>> I guess I should rip do_stolen_accounting() out, too? >>>>> I don't think PARAVIRT_TIME_ACCOUNTING is always selected for Xen. If >>>> This is easy to accomplish. :-) >> >> I looked at KVM code (PARAVIRT_TIME_ACCOUNTING is not selected there >> neither) and in their case that's presumably because stealing accounting >> is a CPUID bit, i.e. it might not be supported. In Xen case we always >> have this interface. > So they added it later and the default is to keep the old behavior. > >>>>> that's indeed the case then we should ifndef do_stolen_accounting(). Or >>>>> maybe check for paravirt_steal_enabled. >>>> Is this really a sensible thing to do? There is a mechanism used by KVM >>>> to do the stolen accounting. I think we should use it instead of having >>>> a second implementation doing the same thing in case the generic one >>>> isn't enabled. >>> I agree. >>> >>> Although I don't think selecting PARAVIRT_TIME_ACC' is necessary -- I >>> don't think it's essential (or is it?). >> Looks like it's useful only if paravirt_steal_rq_enabled, which we don't >> support yet. > I think the patch is still useful. It is reducing code size and > it is removing arch-specific Xen-hack(s). With the patch Xen's > solution for arm and x86 is common and the same as for KVM. Adding > paravirt_steal_rq_enabled later will be much easier as only one > function needs substantial modification. I am not arguing against having a patch that will remove do_stolen_accounting(). I was responding to David's statement about whether we need to select CONFIG_PARAVIRT_TIME_ACCOUNTING, and I am not sure this is necessary since steal_account_process_tick() (that will take case of things that do_stolen_accounting() currently does) doesn't need it. (And if it is indeed needed --- can we have Xen's Kconfig select it instead of "default y if XEN" ?) -boris