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=unavailable 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 2B285C4727D for ; Tue, 22 Sep 2020 12:07:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C930220936 for ; Tue, 22 Sep 2020 12:07:39 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=amazon.com header.i=@amazon.com header.b="CxHmzwgb" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726621AbgIVMHj (ORCPT ); Tue, 22 Sep 2020 08:07:39 -0400 Received: from smtp-fw-9101.amazon.com ([207.171.184.25]:64402 "EHLO smtp-fw-9101.amazon.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726533AbgIVMHi (ORCPT ); Tue, 22 Sep 2020 08:07:38 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=amazon.com; i=@amazon.com; q=dns/txt; s=amazon201209; t=1600776457; x=1632312457; h=from:to:cc:subject:date:message-id:mime-version: in-reply-to:content-transfer-encoding; bh=EbMxVxxFklRKS3zGijxq8O1nhhE8A4FeT/IHUzpIBpg=; b=CxHmzwgbsYbcuRpq5gRr0I/ee9JDRtaJVuGTgq/tAO7/0jE+LTKRQe9S em81zz7F/7DkGCpYPJZKmh1bodtdzQYAN5SdoEm6dfqS6wxrgDpR1bv9o ReUP/4iQ9HCJhoRWfkNTktocXWKCkeVYL+0+0kYAZUDDahW7vTxWtYES+ c=; X-IronPort-AV: E=Sophos;i="5.77,290,1596499200"; d="scan'208";a="70077024" Received: from sea32-co-svc-lb4-vlan3.sea.corp.amazon.com (HELO email-inbound-relay-2a-90c42d1d.us-west-2.amazon.com) ([10.47.23.38]) by smtp-border-fw-out-9101.sea19.amazon.com with ESMTP; 22 Sep 2020 12:07:33 +0000 Received: from EX13D31EUA004.ant.amazon.com (pdx4-ws-svc-p6-lb7-vlan3.pdx.amazon.com [10.170.41.166]) by email-inbound-relay-2a-90c42d1d.us-west-2.amazon.com (Postfix) with ESMTPS id 790E9A18C9; Tue, 22 Sep 2020 12:07:32 +0000 (UTC) Received: from u3f2cd687b01c55.ant.amazon.com (10.43.160.185) by EX13D31EUA004.ant.amazon.com (10.43.165.161) with Microsoft SMTP Server (TLS) id 15.0.1497.2; Tue, 22 Sep 2020 12:07:25 +0000 From: SeongJae Park To: =?UTF-8?q?Roger=20Pau=20Monn=C3=A9?= CC: SeongJae Park , , , SeongJae Park , , , , , , , , Subject: Re: [PATCH v2 1/3] xen-blkback: add a parameter for disabling of persistent grants Date: Tue, 22 Sep 2020 14:07:10 +0200 Message-ID: <20200922120710.24685-1-sjpark@amazon.com> X-Mailer: git-send-email 2.17.1 MIME-Version: 1.0 In-Reply-To: <20200922113511.GK19254@Air-de-Roger> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit X-Originating-IP: [10.43.160.185] 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 13:35:11 +0200 "Roger Pau Monné" wrote: > On Tue, Sep 22, 2020 at 01:26:38PM +0200, 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. > > If possible I prefer to avoid reading from xenstore because it's slow, > very slow compared to storing this somewhere in the blkback > structure. Agreed. > > Could you use the feature_gnt_persistent field in xen_vbd maybe as a > place to cache the value before it's set with the final negotiated > value between the frontend and the backend? Ok, I will do so in the next version. Thanks, SeongJae Park