From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-1.2 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_PASS,UNPARSEABLE_RELAY autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8BE74C43381 for ; Wed, 20 Feb 2019 20:05:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 54DD72147A for ; Wed, 20 Feb 2019 20:05:36 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=oracle.com header.i=@oracle.com header.b="LZyZw0Wg" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726358AbfBTUFe (ORCPT ); Wed, 20 Feb 2019 15:05:34 -0500 Received: from userp2120.oracle.com ([156.151.31.85]:51408 "EHLO userp2120.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725847AbfBTUFe (ORCPT ); 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Wed, 20 Feb 2019 20:05:01 +0000 Received: from aserv0121.oracle.com (aserv0121.oracle.com [141.146.126.235]) by userv0021.oracle.com (8.14.4/8.14.4) with ESMTP id x1KK50Mj015829 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Wed, 20 Feb 2019 20:05:00 GMT Received: from abhmp0003.oracle.com (abhmp0003.oracle.com [141.146.116.9]) by aserv0121.oracle.com (8.14.4/8.13.8) with ESMTP id x1KK4xGo011297; Wed, 20 Feb 2019 20:04:59 GMT Received: from dhcp-burlington7-2nd-B-east-10-152-55-162.usdhcp.oraclecorp.com (/10.152.32.65) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Wed, 20 Feb 2019 12:04:59 -0800 Subject: Re: xen/evtchn and forced threaded irq To: Julien Grall Cc: Juergen Gross , Stefano Stabellini , xen-devel , "linux-kernel@vger.kernel.org" , Dave P Martin References: <5e256d9a-572c-e01e-7706-407f99245b00@arm.com> <20190220000209.GA4091@localhost.localdomain> <21214d47-9c68-e6bf-007a-4047cc2b02f9@oracle.com> <8f7445d7-fa50-f3e9-44f5-cc2aebd020f4@arm.com> From: Boris Ostrovsky Openpgp: preference=signencrypt Autocrypt: addr=boris.ostrovsky@oracle.com; 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Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0 MIME-Version: 1.0 In-Reply-To: <8f7445d7-fa50-f3e9-44f5-cc2aebd020f4@arm.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Content-Language: en-US X-Proofpoint-Virus-Version: vendor=nai engine=5900 definitions=9173 signatures=668683 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1902200137 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2/20/19 1:05 PM, Julien Grall wrote: > Hi, > > On 20/02/2019 17:07, Boris Ostrovsky wrote: >> On 2/20/19 9:15 AM, Julien Grall wrote: >>> Hi Boris, >>> >>> Thank you for your answer. >>> >>> On 20/02/2019 00:02, Boris Ostrovsky wrote: >>>> On Tue, Feb 19, 2019 at 05:31:10PM +0000, Julien Grall wrote: >>>>> Hi all, >>>>> >>>>> I have been looking at using Linux RT in Dom0. Once the guest is >>>>> started, >>>>> the console is ending to have a lot of warning (see trace below). >>>>> >>>>> After some investigation, this is because the irq handler will now >>>>> be threaded. >>>>> I can reproduce the same error with the vanilla Linux when passing >>>>> the option >>>>> 'threadirqs' on the command line (the trace below is from 5.0.0-rc7 >>>>> that has >>>>> not RT support). >>>>> >>>>> FWIW, the interrupt for port 6 is used to for the guest to >>>>> communicate with >>>>> xenstore. >>>>> >>>>>   From my understanding, this is happening because the interrupt >>>>> handler is now >>>>> run in a thread. So we can have the following happening. >>>>> >>>>>      Interrupt context            |     Interrupt thread >>>>>                                   | >>>>>      receive interrupt port 6     | >>>>>      clear the evtchn port        | >>>>>      set IRQF_RUNTHREAD            | >>>>>      kick interrupt thread        | >>>>>                                   |    clear IRQF_RUNTHREAD >>>>>                                   |    call evtchn_interrupt >>>>>      receive interrupt port 6     | >>>>>      clear the evtchn port        | >>>>>      set IRQF_RUNTHREAD           | >>>>>      kick interrupt thread        | >>>>>                                   |    disable interrupt port 6 >>>>>                                   |    evtchn->enabled = false >>>>>                                   |    [....] >>>>>                                   | >>>>>                                   |    *** Handling the second >>>>> interrupt *** >>>>>                                   |    clear IRQF_RUNTHREAD >>>>>                                   |    call evtchn_interrupt >>>>>                                   |    WARN(...) >>>>> >>>>> I am not entirely sure how to fix this. I have two solutions in mind: >>>>> >>>>> 1) Prevent the interrupt handler to be threaded. We would also >>>>> need to >>>>> switch from spin_lock to raw_spin_lock as the former may sleep on >>>>> RT-Linux. >>>>> >>>>> 2) Remove the warning >>>> >>>> I think access to evtchn->enabled is racy so (with or without the >>>> warning) we can't use it reliably. >>> >>> Thinking about it, it would not be the only issue. The ring is sized >>> to contain only one instance of the same event. So if you receive >>> twice the event, you may overflow the ring. >> >> Hm... That's another argument in favor of "unthreading" the handler. > > I first thought it would be possible to unthread it. However, > wake_up_interruptible is using a spin_lock. On RT spin_lock can sleep, > so this cannot be used in an interrupt context. > > So I think "unthreading" the handler is not an option here. That sounds like a different problem. I.e. there are two issues: * threaded interrupts don't work properly (races, ring overflow) * evtchn_interrupt() (threaded or not) has spin_lock(), which is not going to work for RT The first can be fixed by using non-threaded handlers. > >> >>> >>>> >>>> Another alternative could be to queue the irq if !evtchn->enabled and >>>> handle it in evtchn_write() (which is where irq is supposed to be >>>> re-enabled). >>> What do you mean by queue? Is it queueing in the ring? >> >> >> No, I was thinking about having a new structure for deferred interrupts. > > Hmmm, I am not entirely sure what would be the structure here. Could > you expand your thinking? Some sort of a FIFO that stores {irq, data} tuple. It could obviously be implemented as a ring but not necessarily as Xen shared ring (if that's what you were referring to). -boris