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=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,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 00286C7618F for ; Mon, 22 Jul 2019 16:14:07 +0000 (UTC) Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id C927921955 for ; Mon, 22 Jul 2019 16:14:07 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="ptHfwiBK" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org C927921955 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20170209; h=Sender: Content-Transfer-Encoding:Content-Type:Cc:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References: Message-ID:Subject:To:From:Date:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=EU8R2YX+JgCzRjWJFmkJIO4j4I4llODiXPyD6f2FQtU=; b=ptHfwiBKtEQQ4M pjIwSVrflpLF1KbZepvaapX5w7kFEwYpDduzIMw+iBR4DoVvralSSdtuNZ7r0qtIh6WauPvFyCVAt eIpAo6HIQbF4ZQ6Oil5dchPYHNvZ52ivWsGQ/XIImwYERWkxZ6WjvHY3vT1mAl0ILQiTQ+smhHL90 TB4bpILMdX6xltUPOxG8DLVKrCbJUN7OL9SpW/FzEZJKMYHHrgsDSDy9wC3lua7vm2Ed7XLxb+CNj 0r/9677Z1p/oPkjbifavHQr/uOdUDYTXjxOvwvSuUTt/nvcpiB+b/Wu/vUmgsn5jAV/+AFHZ4YTcb qkgm30E6vX/Be5kxBCog==; Received: from localhost ([127.0.0.1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.92 #3 (Red Hat Linux)) id 1hpawn-0003sd-Hy; Mon, 22 Jul 2019 16:13:57 +0000 Received: from mx1.redhat.com ([209.132.183.28]) by bombadil.infradead.org with esmtps (Exim 4.92 #3 (Red Hat Linux)) id 1hpawj-0003rU-J6 for linux-arm-kernel@lists.infradead.org; Mon, 22 Jul 2019 16:13:54 +0000 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id BAFB030B8DE2; Mon, 22 Jul 2019 16:13:50 +0000 (UTC) Received: from redhat.com (ovpn-124-54.rdu2.redhat.com [10.10.124.54]) by smtp.corp.redhat.com (Postfix) with SMTP id AE2A260BEC; Mon, 22 Jul 2019 16:13:42 +0000 (UTC) Date: Mon, 22 Jul 2019 12:13:40 -0400 From: "Michael S. Tsirkin" To: "Paul E. McKenney" Subject: Re: RFC: call_rcu_outstanding (was Re: WARNING in __mmdrop) Message-ID: <20190722120011-mutt-send-email-mst@kernel.org> References: <0000000000008dd6bb058e006938@google.com> <000000000000964b0d058e1a0483@google.com> <20190721044615-mutt-send-email-mst@kernel.org> <20190721081933-mutt-send-email-mst@kernel.org> <20190721131725.GR14271@linux.ibm.com> <20190721210837.GC363@bombadil.infradead.org> <20190721233113.GV14271@linux.ibm.com> <20190722151439.GA247639@google.com> <20190722114612-mutt-send-email-mst@kernel.org> <20190722155534.GG14271@linux.ibm.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20190722155534.GG14271@linux.ibm.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.47]); Mon, 22 Jul 2019 16:13:51 +0000 (UTC) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20190722_091353_680032_7261C5E2 X-CRM114-Status: GOOD ( 20.14 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: mhocko@suse.com, peterz@infradead.org, jasowang@redhat.com, ldv@altlinux.org, james.bottomley@hansenpartnership.com, linux-mm@kvack.org, namit@vmware.com, Joel Fernandes , mingo@kernel.org, elena.reshetova@intel.com, aarcange@redhat.com, davem@davemloft.net, Matthew Wilcox , hch@infradead.org, linux-arm-kernel@lists.infradead.org, keescook@chromium.org, syzkaller-bugs@googlegroups.com, jglisse@redhat.com, viro@zeniv.linux.org.uk, christian@brauner.io, wad@chromium.org, linux-parisc@vger.kernel.org, linux-kernel@vger.kernel.org, luto@amacapital.net, ebiederm@xmission.com, akpm@linux-foundation.org, guro@fb.com Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Mon, Jul 22, 2019 at 08:55:34AM -0700, Paul E. McKenney wrote: > On Mon, Jul 22, 2019 at 11:47:24AM -0400, Michael S. Tsirkin wrote: > > On Mon, Jul 22, 2019 at 11:14:39AM -0400, Joel Fernandes wrote: > > > [snip] > > > > > Would it make sense to have call_rcu() check to see if there are many > > > > > outstanding requests on this CPU and if so process them before returning? > > > > > That would ensure that frequent callers usually ended up doing their > > > > > own processing. > > > > > > Other than what Paul already mentioned about deadlocks, I am not sure if this > > > would even work for all cases since call_rcu() has to wait for a grace > > > period. > > > > > > So, if the number of outstanding requests are higher than a certain amount, > > > then you *still* have to wait for some RCU configurations for the grace > > > period duration and cannot just execute the callback in-line. Did I miss > > > something? > > > > > > Can waiting in-line for a grace period duration be tolerated in the vhost case? > > > > > > thanks, > > > > > > - Joel > > > > No, but it has many other ways to recover (try again later, drop a > > packet, use a slower copy to/from user). > > True enough! And your idea of taking recovery action based on the number > of callbacks seems like a good one while we are getting RCU's callback > scheduling improved. > > By the way, was this a real problem that you could make happen on real > hardware? > If not, I would suggest just letting RCU get improved over > the next couple of releases. So basically use kfree_rcu but add a comment saying e.g. "WARNING: in the future callers of kfree_rcu might need to check that not too many callbacks get queued. In that case, we can disable the optimization, or recover in some other way. Watch this space." > If it is something that you actually made happen, please let me know > what (if anything) you need from me for your callback-counting EBUSY > scheme. > > Thanx, Paul If you mean kfree_rcu causing OOM then no, it's all theoretical. If you mean synchronize_rcu stalling to the point where guest will OOPs, then yes, that's not too hard to trigger. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel