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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 A6B66C433FF for ; Mon, 5 Aug 2019 04:20:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 893942086D for ; Mon, 5 Aug 2019 04:20:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726767AbfHEEUy (ORCPT ); Mon, 5 Aug 2019 00:20:54 -0400 Received: from mx1.redhat.com ([209.132.183.28]:49032 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725902AbfHEEUx (ORCPT ); Mon, 5 Aug 2019 00:20:53 -0400 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 4E8B5A3EB3; Mon, 5 Aug 2019 04:20:53 +0000 (UTC) Received: from [10.72.12.115] (ovpn-12-115.pek2.redhat.com [10.72.12.115]) by smtp.corp.redhat.com (Postfix) with ESMTP id 870955D9E2; Mon, 5 Aug 2019 04:20:47 +0000 (UTC) Subject: Re: [PATCH V2 7/9] vhost: do not use RCU to synchronize MMU notifier with worker To: Jason Gunthorpe Cc: mst@redhat.com, kvm@vger.kernel.org, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <20190731084655.7024-1-jasowang@redhat.com> <20190731084655.7024-8-jasowang@redhat.com> <20190731123935.GC3946@ziepe.ca> <7555c949-ae6f-f105-6e1d-df21ddae9e4e@redhat.com> <20190731193057.GG3946@ziepe.ca> <20190801141512.GB23899@ziepe.ca> <42ead87b-1749-4c73-cbe4-29dbeb945041@redhat.com> <20190802124613.GA11245@ziepe.ca> From: Jason Wang Message-ID: <11b2a930-eae4-522c-4132-3f8a2da05666@redhat.com> Date: Mon, 5 Aug 2019 12:20:45 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <20190802124613.GA11245@ziepe.ca> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.30]); Mon, 05 Aug 2019 04:20:53 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2019/8/2 下午8:46, Jason Gunthorpe wrote: > On Fri, Aug 02, 2019 at 05:40:07PM +0800, Jason Wang wrote: >>> This must be a proper barrier, like a spinlock, mutex, or >>> synchronize_rcu. >> >> I start with synchronize_rcu() but both you and Michael raise some >> concern. > I've also idly wondered if calling synchronize_rcu() under the various > mm locks is a deadlock situation. Maybe, that's why I suggest to use vhost_work_flush() which is much lightweight can can achieve the same function. It can guarantee all previous work has been processed after vhost_work_flush() return. > >> Then I try spinlock and mutex: >> >> 1) spinlock: add lots of overhead on datapath, this leads 0 performance >> improvement. > I think the topic here is correctness not performance improvement But the whole series is to speed up vhost. > >> 2) SRCU: full memory barrier requires on srcu_read_lock(), which still leads >> little performance improvement > >> 3) mutex: a possible issue is need to wait for the page to be swapped in (is >> this unacceptable ?), another issue is that we need hold vq lock during >> range overlap check. > I have a feeling that mmu notififers cannot safely become dependent on > progress of swap without causing deadlock. You probably should avoid > this. Yes, so that's why I try to synchronize the critical region by myself. >>> And, again, you can't re-invent a spinlock with open coding and get >>> something better. >> So the question is if waiting for swap is considered to be unsuitable for >> MMU notifiers. If not, it would simplify codes. If not, we still need to >> figure out a possible solution. >> >> Btw, I come up another idea, that is to disable preemption when vhost thread >> need to access the memory. Then register preempt notifier and if vhost >> thread is preempted, we're sure no one will access the memory and can do the >> cleanup. > I think you should use the spinlock so at least the code is obviously > functionally correct and worry about designing some properly justified > performance change after. > > Jason Spinlock is correct but make the whole series meaningless consider it won't bring any performance improvement. Thanks