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=-14.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_GIT autolearn=ham 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 12753C2D0E2 for ; Tue, 22 Sep 2020 12:08:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B556F2395C for ; Tue, 22 Sep 2020 12:08:46 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=amazon.com header.i=@amazon.com header.b="CXKxGxyZ" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726593AbgIVMIp (ORCPT ); Tue, 22 Sep 2020 08:08:45 -0400 Received: from smtp-fw-9101.amazon.com ([207.171.184.25]:64716 "EHLO smtp-fw-9101.amazon.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726531AbgIVMIp (ORCPT ); Tue, 22 Sep 2020 08:08:45 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=amazon.com; i=@amazon.com; q=dns/txt; s=amazon201209; t=1600776524; x=1632312524; h=from:to:cc:subject:date:message-id:mime-version: in-reply-to:content-transfer-encoding; bh=Iu17TnezrR9cCfm/mX2pd36Twy2NLDZBTF2SbIDH3UQ=; b=CXKxGxyZTYCTm4WIambZmZP50xwAdBYJjY00gOwelHXkA/MSGkzG4ELL Oyk4p4kIe+82c8JMID3z6/25CBr8tznq3qLdG1BzM0tqbB0eVG97umGnA hVsy9WQvaLc2kdobhEWYgATiD0GT/a2636KDrDZj4GzgEbSoT6Qzf6CD7 g=; X-IronPort-AV: E=Sophos;i="5.77,290,1596499200"; d="scan'208";a="70077461" Received: from sea32-co-svc-lb4-vlan3.sea.corp.amazon.com (HELO email-inbound-relay-2a-6e2fc477.us-west-2.amazon.com) ([10.47.23.38]) by smtp-border-fw-out-9101.sea19.amazon.com with ESMTP; 22 Sep 2020 12:08:43 +0000 Received: from EX13D31EUA004.ant.amazon.com (pdx4-ws-svc-p6-lb7-vlan2.pdx.amazon.com [10.170.41.162]) by email-inbound-relay-2a-6e2fc477.us-west-2.amazon.com (Postfix) with ESMTPS id 216E3A227B; Tue, 22 Sep 2020 12:08:43 +0000 (UTC) Received: from u3f2cd687b01c55.ant.amazon.com (10.43.162.43) by EX13D31EUA004.ant.amazon.com (10.43.165.161) with Microsoft SMTP Server (TLS) id 15.0.1497.2; Tue, 22 Sep 2020 12:08:36 +0000 From: SeongJae Park To: =?UTF-8?q?J=C3=BCrgen=20Gro=C3=9F?= CC: SeongJae Park , =?UTF-8?q?Roger=20Pau=20Monn=C3=A9?= , , SeongJae Park , , , , , , , , Subject: Re: [PATCH v2 1/3] xen-blkback: add a parameter for disabling of persistent grants Date: Tue, 22 Sep 2020 14:08:20 +0200 Message-ID: <20200922120820.24978-1-sjpark@amazon.com> X-Mailer: git-send-email 2.17.1 MIME-Version: 1.0 In-Reply-To: <33a8d880-d8ce-0cf7-d115-ca6938889f27@suse.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit X-Originating-IP: [10.43.162.43] X-ClientProxiedBy: EX13D50UWC002.ant.amazon.com (10.43.162.189) To EX13D31EUA004.ant.amazon.com (10.43.165.161) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 22 Sep 2020 13:35:30 +0200 "Jürgen Groß" wrote: > On 22.09.20 13:26, SeongJae Park wrote: > > On Tue, 22 Sep 2020 13:12:59 +0200 "Roger Pau Monné" wrote: > > > >> On Tue, Sep 22, 2020 at 12:52:07PM +0200, SeongJae Park wrote: > >>> From: SeongJae Park > >>> > >>> Persistent grants feature provides high scalability. On some small > >>> systems, however, it could incur data copy overheads[1] and thus it is > >>> required to be disabled. But, there is no option to disable it. For > >>> the reason, this commit adds a module parameter for disabling of the > >>> feature. > >>> > >>> [1] https://wiki.xen.org/wiki/Xen_4.3_Block_Protocol_Scalability > >>> > >>> Signed-off-by: Anthony Liguori > >>> Signed-off-by: SeongJae Park > >>> --- > >>> .../ABI/testing/sysfs-driver-xen-blkback | 9 ++++++ > >>> drivers/block/xen-blkback/xenbus.c | 28 ++++++++++++++----- > >>> 2 files changed, 30 insertions(+), 7 deletions(-) > >>> > >>> diff --git a/Documentation/ABI/testing/sysfs-driver-xen-blkback b/Documentation/ABI/testing/sysfs-driver-xen-blkback > >>> index ecb7942ff146..ac2947b98950 100644 > >>> --- a/Documentation/ABI/testing/sysfs-driver-xen-blkback > >>> +++ b/Documentation/ABI/testing/sysfs-driver-xen-blkback > >>> @@ -35,3 +35,12 @@ Description: > >>> controls the duration in milliseconds that blkback will not > >>> cache any page not backed by a grant mapping. > >>> The default is 10ms. > >>> + > >>> +What: /sys/module/xen_blkback/parameters/feature_persistent > >>> +Date: September 2020 > >>> +KernelVersion: 5.10 > >>> +Contact: SeongJae Park > >>> +Description: > >>> + Whether to enable the persistent grants feature or not. Note > >>> + that this option only takes effect on newly created backends. > >>> + The default is Y (enable). > >>> diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c > >>> index b9aa5d1ac10b..8a95ddd08b13 100644 > >>> --- a/drivers/block/xen-blkback/xenbus.c > >>> +++ b/drivers/block/xen-blkback/xenbus.c > >>> @@ -879,6 +879,12 @@ static void reclaim_memory(struct xenbus_device *dev) > >>> > >>> /* ** Connection ** */ > >>> > >>> +/* Enable the persistent grants feature. */ > >>> +static bool feature_persistent = true; > >>> +module_param(feature_persistent, bool, 0644); > >>> +MODULE_PARM_DESC(feature_persistent, > >>> + "Enables the persistent grants feature"); > >>> + > >>> /* > >>> * Write the physical details regarding the block device to the store, and > >>> * switch to Connected state. > >>> @@ -906,11 +912,15 @@ static void connect(struct backend_info *be) > >>> > >>> xen_blkbk_barrier(xbt, be, be->blkif->vbd.flush_support); > >>> > >>> - err = xenbus_printf(xbt, dev->nodename, "feature-persistent", "%u", 1); > >>> - if (err) { > >>> - xenbus_dev_fatal(dev, err, "writing %s/feature-persistent", > >>> - dev->nodename); > >>> - goto abort; > >>> + if (feature_persistent) { > >>> + err = xenbus_printf(xbt, dev->nodename, "feature-persistent", > >>> + "%u", feature_persistent); > >>> + if (err) { > >>> + xenbus_dev_fatal(dev, err, > >>> + "writing %s/feature-persistent", > >>> + dev->nodename); > >>> + goto abort; > >>> + } > >>> } > >>> > >>> err = xenbus_printf(xbt, dev->nodename, "sectors", "%llu", > >>> @@ -1093,8 +1103,12 @@ static int connect_ring(struct backend_info *be) > >>> xenbus_dev_fatal(dev, err, "unknown fe protocol %s", protocol); > >>> return -ENOSYS; > >>> } > >>> - pers_grants = xenbus_read_unsigned(dev->otherend, "feature-persistent", > >>> - 0); > >>> + if (feature_persistent) > >>> + pers_grants = xenbus_read_unsigned(dev->otherend, > >>> + "feature-persistent", 0); > >>> + else > >>> + pers_grants = 0; > >>> + > >> > >> Sorry for not realizing earlier, but looking at it again I think you > >> need to cache the value of feature_persistent when it's first used in > >> the blkback state data, so that it's consistent. > >> > >> What would happen for example with the following flow (assume a > >> persistent grants enabled frontend): > >> > >> feature_persistent = false > >> > >> connect(...) > >> feature-persistent is not written to xenstore > >> > >> User changes feature_persistent = true > >> > >> connect_ring(...) > >> pers_grants = true, because feature-persistent is set unconditionally > >> by the frontend and feature_persistent variable is now true. > >> > >> Then blkback will try to use persistent grants and the whole > >> connection will malfunction because the frontend won't. > > > > Ah, you're right. I should also catch this before but didn't, sorry. > > > >> > >> The other option is to prevent changing the variable when there are > >> blkback instances already running. > > > > I think storing the option value in xenstore would be simpler. That said, if > > you prefer this way, please let me know. > > No, Xenstore isn't the right place for that. This is a local > implementation detail of blkback and shouldn't be exported to Xenstore. Agreed. I will try using the 'feature_gnt_persistent' as Roger recommended. Thanks, SeongJae Park