Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754094AbaLAPyb (ORCPT ); Mon, 1 Dec 2014 10:54:31 -0500 Received: from smtp02.citrix.com ([66.165.176.63]:56288 "EHLO SMTP02.CITRIX.COM" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754017AbaLAPya (ORCPT ); Mon, 1 Dec 2014 10:54:30 -0500 X-IronPort-AV: E=Sophos;i="5.07,494,1413244800"; d="scan'208";a="198444920" Message-ID: <547C8F30.1010306@citrix.com> Date: Mon, 1 Dec 2014 15:54:24 +0000 From: David Vrabel User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Icedove/31.2.0 MIME-Version: 1.0 To: "Luis R. Rodriguez" CC: Juergen Gross , Peter Zijlstra , Ingo Molnar , Oleg Nesterov , Konrad Rzeszutek Wilk , , , "linux-kernel@vger.kernel.org" , , , Davidlohr Bueso , Joerg Roedel , Borislav Petkov , Jan Beulich , Olaf Hering Subject: Re: [PATCH] xen: privcmd: schedule() after private hypercall when non CONFIG_PREEMPT References: <1417040805-15857-1-git-send-email-mcgrof@do-not-panic.com> <5476C66F.5040308@suse.com> <20141127183616.GV25677@wotan.suse.de> <547C4CEF.1010603@citrix.com> <20141201150546.GC25677@wotan.suse.de> <547C86BF.2040705@citrix.com> In-Reply-To: Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit X-DLP: MIA1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 01/12/14 15:44, Luis R. Rodriguez wrote: > On Mon, Dec 1, 2014 at 10:18 AM, David Vrabel wrote: >> On 01/12/14 15:05, Luis R. Rodriguez wrote: >>> On Mon, Dec 01, 2014 at 11:11:43AM +0000, David Vrabel wrote: >>>> On 27/11/14 18:36, Luis R. Rodriguez wrote: >>>>> On Thu, Nov 27, 2014 at 07:36:31AM +0100, Juergen Gross wrote: >>>>>> On 11/26/2014 11:26 PM, Luis R. Rodriguez wrote: >>>>>>> From: "Luis R. Rodriguez" >>>>>>> >>>>>>> Some folks had reported that some xen hypercalls take a long time >>>>>>> to complete when issued from the userspace private ioctl mechanism, >>>>>>> this can happen for instance with some hypercalls that have many >>>>>>> sub-operations, this can happen for instance on hypercalls that use >>>> [...] >>>>>>> --- a/drivers/xen/privcmd.c >>>>>>> +++ b/drivers/xen/privcmd.c >>>>>>> @@ -60,6 +60,9 @@ static long privcmd_ioctl_hypercall(void __user *udata) >>>>>>> hypercall.arg[0], hypercall.arg[1], >>>>>>> hypercall.arg[2], hypercall.arg[3], >>>>>>> hypercall.arg[4]); >>>>>>> +#ifndef CONFIG_PREEMPT >>>>>>> + schedule(); >>>>>>> +#endif >>>> >>>> As Juergen points out, this does nothing. You need to schedule while in >>>> the middle of the hypercall. >>>> >>>> Remember that Xen's hypercall preemption only preempts the hypercall to >>>> run interrupts in the guest. >>> >>> How is it ensured that when the kernel preempts on this code path on >>> CONFIG_PREEMPT=n kernel that only interrupts in the guest are run? >> >> Sorry, I really didn't describe this very well. >> >> If a hypercall needs a continuation, Xen returns to the guest with the >> IP set to the hypercall instruction, and on the way back to the guest >> Xen may schedule a different VCPU or it will do any upcalls (as per normal). >> >> The guest is free to return from the upcall to the original task >> (continuing the hypercall) or to a different one. > > OK so that addresses what Xen will do when using continuation and > hypercall preemption, my concern here was that using > preempt_schedule_irq() on CONFIG_PREEMPT=n kernels in the middle of a > hypercall on the return from an interrupt (e.g., the timer interrupt) > would still let the kernel preempt to tasks other than those related > to Xen. Um. Why would that be a problem? We do want to switch to any task the Linux scheduler thinks is best. David -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/