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[209.132.183.28]) by mx.google.com with ESMTPS id 51si20447817qvo.107.2019.07.26.05.53.34 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 26 Jul 2019 05:53:34 -0700 (PDT) Received-SPF: pass (google.com: domain of jasowang@redhat.com designates 209.132.183.28 as permitted sender) client-ip=209.132.183.28; Authentication-Results: mx.google.com; spf=pass (google.com: domain of jasowang@redhat.com designates 209.132.183.28 as permitted sender) smtp.mailfrom=jasowang@redhat.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com 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 AE5F6300D1CA; Fri, 26 Jul 2019 12:53:33 +0000 (UTC) Received: from [10.72.12.238] (ovpn-12-238.pek2.redhat.com [10.72.12.238]) by smtp.corp.redhat.com (Postfix) with ESMTP id 3F4645DE6F; Fri, 26 Jul 2019 12:53:19 +0000 (UTC) Subject: Re: WARNING in __mmdrop To: "Michael S. Tsirkin" 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 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> <20190726082837-mutt-send-email-mst@kernel.org> From: Jason Wang Message-ID: Date: Fri, 26 Jul 2019 20:53:18 +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: <20190726082837-mutt-send-email-mst@kernel.org> 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 [10.5.110.40]); Fri, 26 Jul 2019 12:53:34 +0000 (UTC) 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 2019/7/26 下午8:38, Michael S. Tsirkin wrote: > 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 ... Ok, let me retry if necessary (but I do remember I end up with deadlocks last try). > > >> If we just worry about the IPI, > With synchronize_rcu what I would worry about is that guest is stalled Can this synchronize_rcu() be triggered by guest? If yes, there are several other MMU notifiers that can block. Is vhost something special here? > because system is busy because of other guests. > With expedited it's the IPIs... > The current synchronize_rcu()  can force a expedited grace period: void synchronize_rcu(void) {         ...         if (rcu_blocking_is_gp()) return;         if (rcu_gp_is_expedited()) synchronize_rcu_expedited(); else wait_rcu_gp(call_rcu); } EXPORT_SYMBOL_GPL(synchronize_rcu); >> 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? No faster but no IPI. > > >>> 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. Yes, that's possible but should be done on top. Thanks