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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 7EA5AFA3747 for ; Mon, 24 Oct 2022 16:50:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231955AbiJXQuW (ORCPT ); Mon, 24 Oct 2022 12:50:22 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57932 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235539AbiJXQtt (ORCPT ); Mon, 24 Oct 2022 12:49:49 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id BFA2015A25 for ; Mon, 24 Oct 2022 08:32:45 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1666625448; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=t/jSO0lqWHSQcHiF6BRMSRgd7XZaU/RYEhuGiGA3W4o=; b=dYFGKgACZ4RV49AtiMIbOmhNGbflbNwSseQ9VUzPP2k/rGm4qZ3pY3rMDkn/QGgR+ZtvXv T/s7Xi5eCKZRWwUnmsg14oZspdC5ZFG9rLVobQ82eEG5Z62GHKBbxF8hsqbVOMec8/46NA /eFu4I76ZIP2Wn+/rpNyLkRTFLHmMCw= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-656-BD8qyXrmOD2Ly-xXM2Yriw-1; Mon, 24 Oct 2022 11:30:43 -0400 X-MC-Unique: BD8qyXrmOD2Ly-xXM2Yriw-1 Received: from smtp.corp.redhat.com (int-mx09.intmail.prod.int.rdu2.redhat.com [10.11.54.9]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id BEA24811E87; Mon, 24 Oct 2022 15:30:41 +0000 (UTC) Received: from localhost (unknown [10.39.194.177]) by smtp.corp.redhat.com (Postfix) with ESMTP id 28818492CA2; Mon, 24 Oct 2022 15:30:40 +0000 (UTC) Date: Mon, 24 Oct 2022 11:30:39 -0400 From: Stefan Hajnoczi To: Ming Lei Cc: Jens Axboe , Christoph Hellwig , Bart Van Assche , djeffery@redhat.com, linux-block@vger.kernel.org, linux-scsi@vger.kernel.org, virtualization@lists.linux-foundation.org Subject: Re: [Bug] double ->queue_rq() because of timeout in ->queue_rq() Message-ID: References: MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="p/CIZd9qse9A1PsE" Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.1 on 10.11.54.9 Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org --p/CIZd9qse9A1PsE Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Fri, Oct 21, 2022 at 10:23:57AM +0800, Ming Lei wrote: > On Thu, Oct 20, 2022 at 04:01:11PM -0400, Stefan Hajnoczi wrote: > > On Thu, Oct 20, 2022 at 05:10:13PM +0800, Ming Lei wrote: > > > Hi, > > >=20 > > > David Jeffery found one double ->queue_rq() issue, so far it can > > > be triggered in the following two cases: > > >=20 > > > 1) scsi driver in guest kernel > > >=20 > > > - the story could be long vmexit latency or long preempt latency of > > > vCPU pthread, then IO req is timed out before queuing the request > > > to hardware but after calling blk_mq_start_request() during ->queue_r= q(), > > > then timeout handler handles it by requeue, then double ->queue_rq() = is > > > caused, and kernel panic > > >=20 > > > 2) burst of kernel messages from irq handler=20 > > >=20 > > > For 1), I think it is one reasonable case, given latency from host si= de > > > can come anytime in theory because vCPU is emulated by one normal host > > > pthread which can be preempted anywhere. For 2), I guess kernel messa= ge is > > > supposed to be rate limited. > > >=20 > > > Firstly, is this kind of so long(30sec) random latency when running k= ernel > > > code something normal? Or do we need to take care of it? IMO, it looks > > > reasonable in case of VM, but our VM experts may have better idea abo= ut this > > > situation. Also the default 30sec timeout could be reduced via sysfs = or > > > drivers. > >=20 > > 30 seconds is a long latency that does not occur during normal > > operation, but unfortunately does happen on occasion. >=20 > Thanks for the confirmation! >=20 > >=20 > > I think there's an interest in understanding the root cause and solving > > long latencies (if possible) in the QEMU/KVM communities. We can > > investigate specific cases on kvm@vger.kernel.org and/or > > qemu-devel@nongnu.org. >=20 > The issue was original reported on VMware VM, but maybe David can figure > out how to trigger it on QEMU/KVM. A very basic question: The virtio_blk driver has no q->mq_ops->timeout() callback. Why does the block layer still enable the timeout mechanism when the driver doesn't implement ->timeout()? I saw there was some "idle" hctx logic and I guess the requests are resubmitted (although it wasn't obvious to me how that happens in the code)? Maybe that's why the timer is still used if the driver doesn't care about timeouts... 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