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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 CB6DEC76190 for ; Fri, 26 Jul 2019 12:38:25 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 99E6721951 for ; Fri, 26 Jul 2019 12:38:25 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 99E6721951 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 1545B6B0003; Fri, 26 Jul 2019 08:38:25 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 1061A8E0002; Fri, 26 Jul 2019 08:38:25 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id F37126B0008; Fri, 26 Jul 2019 08:38:24 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from mail-qk1-f197.google.com (mail-qk1-f197.google.com [209.85.222.197]) by kanga.kvack.org (Postfix) with ESMTP id DCBF66B0003 for ; Fri, 26 Jul 2019 08:38:24 -0400 (EDT) Received: by mail-qk1-f197.google.com with SMTP id n190so45088810qkd.5 for ; Fri, 26 Jul 2019 05:38:24 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-original-authentication-results:x-gm-message-state:date:from:to :cc:subject:message-id:references:mime-version:content-disposition :content-transfer-encoding:in-reply-to; bh=7AAahzA3S5OmAp3WHwhPen11x5qmdYBmT/Y66i6DKGs=; b=e1VGQRn/M3xGcQEUXS3L5iASLNaopzoU+JLR9HbGbF6Kxl1PQ1jm8JcveNyQqYmFgP HgvS192/0c3Qv5SHr0Ep3NvZ365YcVyBps31Vp/qX/5OvNCQkvrwE6d5sXJG0Ap+ccji R4HXyJvh55HfL81y8bHpsg37VVPMkd+ZQnAxU+D5Vcu2iJ4WYZaccdxlCl7mZygvgDkK 9VntQDaVkA+mZAnb94rHJPnvItVYh3+wdpZPQn+reIsgXjGaaRv4sicWjtsEAnVy72h1 xK61eYAOwhFXlTdUZkz33zjjoBaG7YwtCFXYV7T92n4sm5RMU69OB+ren0LHYveKPNOA g0jA== X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of mst@redhat.com designates 209.85.220.65 as permitted sender) smtp.mailfrom=mst@redhat.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com X-Gm-Message-State: APjAAAWfRg23S+NxEDvacEh6VMIMpx3a/aWpfstlWEA19GFIgesjw8bW 90vwHSyxHUtKwH9oeCwLPv41K3zwTtP0aTuV/UcG7DN7Pv2EiP5isX3km0ViBM9rTrnILyewCrJ gTiVqdKHomTVvKYCrvNoj0xvIM1AkiAEoR7HXNYjj+kVnXKrOGKjm2ewx/S85OpFKqw== X-Received: by 2002:a0c:c688:: with SMTP id d8mr67895738qvj.86.1564144704666; Fri, 26 Jul 2019 05:38:24 -0700 (PDT) X-Received: by 2002:a0c:c688:: with SMTP id d8mr67895706qvj.86.1564144704075; Fri, 26 Jul 2019 05:38:24 -0700 (PDT) ARC-Seal: i=1; a=rsa-sha256; t=1564144704; cv=none; d=google.com; s=arc-20160816; b=sj699GJmDp2oi8dmW0cyvWNdhjJZSsCR0INlO4WpPKt4YLrm3HTFyOpeZrCePhD/q+ sgirk2pLCkKbpZd9lemXoKzwYAkkdeBsHR8ttjFT4MsssvNvDhByAqSBb+J38rPru/nN nu8xx37u2MErZ6CKCPKDipJXKtUPjakJHK3yBumhJ2RJX+yEGK2IrWLvgFaeNDVYKegQ kj/pHia0vHXoGVo4fS/phomBcP+n5N0pVooo8p80NEFRO0gvLdiWp1v+3QsTZdXcUgig m2jAk89MIymsnb4apAW9EpuyWqBoiYNSM4SRGRoOUWqQF7jvGpLCFWLtOBqcX3mrhaG4 X+WQ== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=in-reply-to:content-transfer-encoding:content-disposition :mime-version:references:message-id:subject:cc:to:from:date; bh=7AAahzA3S5OmAp3WHwhPen11x5qmdYBmT/Y66i6DKGs=; b=c0fllbFJrgndipy1kDnxyxfA8K2M3IyYS+a/0tMPNOGnMRfCNX6b3Qs4ifmLwf7CcR cb/ndkZ+smrtPBC5vgoXZu4JMDOHS04SZYe2PBwjcCu1UVza9Q+Mx8H35jckXJ+PJo6i KcAxXQk4Be1Hc4l7eVrKunFpJSXAmdvATcjYBMZICzVUWJPq1kMYhEZ4dgRVAnbEMnBM QfnDOpzRk2YEMUa2NKwat34b0mv5foneoQ4rCjlx7Y/cZB7wpotao9C1tf3h/agBoNtd SxGFVQEiDFgCRWCDo9F8hBSd1B9phX6vIe9C9ynDass1JEn/o4KRye7ldph+jnTJRBGg YnDg== ARC-Authentication-Results: i=1; mx.google.com; spf=pass (google.com: domain of mst@redhat.com designates 209.85.220.65 as permitted sender) smtp.mailfrom=mst@redhat.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com Received: from mail-sor-f65.google.com (mail-sor-f65.google.com. [209.85.220.65]) by mx.google.com with SMTPS id b15sor61208618qtt.9.2019.07.26.05.38.24 for (Google Transport Security); Fri, 26 Jul 2019 05:38:24 -0700 (PDT) Received-SPF: pass (google.com: domain of mst@redhat.com designates 209.85.220.65 as permitted sender) client-ip=209.85.220.65; Authentication-Results: mx.google.com; spf=pass (google.com: domain of mst@redhat.com designates 209.85.220.65 as permitted sender) smtp.mailfrom=mst@redhat.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com X-Google-Smtp-Source: APXvYqzmq5+4vWxaDwMY+05UXlQp7huBRpq3YhuBZEQ2PajFPNo/CMYsaRV1rBY4ljt21ew7Zf/M7w== X-Received: by 2002:ac8:37b8:: with SMTP id d53mr65290476qtc.227.1564144703827; Fri, 26 Jul 2019 05:38:23 -0700 (PDT) Received: from redhat.com ([212.92.104.165]) by smtp.gmail.com with ESMTPSA id v7sm25082729qte.86.2019.07.26.05.38.16 (version=TLS1_3 cipher=AEAD-AES256-GCM-SHA384 bits=256/256); Fri, 26 Jul 2019 05:38:23 -0700 (PDT) Date: Fri, 26 Jul 2019 08:38:13 -0400 From: "Michael S. Tsirkin" To: Jason Wang Cc: syzbot , aarcange@redhat.com, akpm@linux-foundation.org, christian@brauner.io, davem@davemloft.net, ebiederm@xmission.com, elena.reshetova@intel.com, guro@fb.com, hch@infradead.org, james.bottomley@hansenpartnership.com, jglisse@redhat.com, keescook@chromium.org, ldv@altlinux.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-parisc@vger.kernel.org, luto@amacapital.net, mhocko@suse.com, mingo@kernel.org, namit@vmware.com, peterz@infradead.org, syzkaller-bugs@googlegroups.com, viro@zeniv.linux.org.uk, wad@chromium.org Subject: Re: WARNING in __mmdrop Message-ID: <20190726082837-mutt-send-email-mst@kernel.org> References: <20190723051828-mutt-send-email-mst@kernel.org> <20190725012149-mutt-send-email-mst@kernel.org> <55e8930c-2695-365f-a07b-3ad169654d28@redhat.com> <20190725042651-mutt-send-email-mst@kernel.org> <84bb2e31-0606-adff-cf2a-e1878225a847@redhat.com> <20190725092332-mutt-send-email-mst@kernel.org> <11802a8a-ce41-f427-63d5-b6a4cf96bb3f@redhat.com> <20190726074644-mutt-send-email-mst@kernel.org> <5cc94f15-b229-a290-55f3-8295266edb2b@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <5cc94f15-b229-a290-55f3-8295266edb2b@redhat.com> X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, Jul 26, 2019 at 08:00:58PM +0800, Jason Wang wrote: > > On 2019/7/26 下午7:49, Michael S. Tsirkin wrote: > > On Thu, Jul 25, 2019 at 10:25:25PM +0800, Jason Wang wrote: > > > On 2019/7/25 下午9:26, Michael S. Tsirkin wrote: > > > > > Exactly, and that's the reason actually I use synchronize_rcu() there. > > > > > > > > > > So the concern is still the possible synchronize_expedited()? > > > > I think synchronize_srcu_expedited. > > > > > > > > synchronize_expedited sends lots of IPI and is bad for realtime VMs. > > > > > > > > > Can I do this > > > > > on through another series on top of the incoming V2? > > > > > > > > > > Thanks > > > > > > > > > The question is this: is this still a gain if we switch to the > > > > more expensive srcu? If yes then we can keep the feature on, > > > > > > I think we only care about the cost on srcu_read_lock() which looks pretty > > > tiny form my point of view. Which is basically a READ_ONCE() + WRITE_ONCE(). > > > > > > Of course I can benchmark to see the difference. > > > > > > > > > > if not we'll put it off until next release and think > > > > of better solutions. rcu->srcu is just a find and replace, > > > > don't see why we need to defer that. can be a separate patch > > > > for sure, but we need to know how well it works. > > > > > > I think I get here, let me try to do that in V2 and let's see the numbers. > > > > > > Thanks > > > It looks to me for tree rcu, its srcu_read_lock() have a mb() which is too > expensive for us. I will try to ponder using vq lock in some way. Maybe with trylock somehow ... > If we just worry about the IPI, With synchronize_rcu what I would worry about is that guest is stalled because system is busy because of other guests. With expedited it's the IPIs... > can we do something like in > vhost_invalidate_vq_start()? > >         if (map) { >                 /* In order to avoid possible IPIs with >                  * synchronize_rcu_expedited() we use call_rcu() + >                  * completion. > */ > init_completion(&c.completion); >                 call_rcu(&c.rcu_head, vhost_finish_vq_invalidation); > wait_for_completion(&c.completion); >                 vhost_set_map_dirty(vq, map, index); > vhost_map_unprefetch(map); >         } > > ? Why would that be faster than synchronize_rcu? > > > There's one other thing that bothers me, and that is that > > for large rings which are not physically contiguous > > we don't implement the optimization. > > > > For sure, that can wait, but I think eventually we should > > vmap large rings. > > > Yes, worth to try. But using direct map has its own advantage: it can use > hugepage that vmap can't > > Thanks Sure, so we can do that for small rings. -- MST