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=-9.6 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 B660DC2D0E2 for ; Tue, 22 Sep 2020 11:43:41 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5B95123600 for ; Tue, 22 Sep 2020 11:43:41 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=citrix.com header.i=@citrix.com header.b="E4+QSJkD" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726578AbgIVLnV (ORCPT ); Tue, 22 Sep 2020 07:43:21 -0400 Received: from esa1.hc3370-68.iphmx.com ([216.71.145.142]:54054 "EHLO esa1.hc3370-68.iphmx.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726613AbgIVLmx (ORCPT ); Tue, 22 Sep 2020 07:42:53 -0400 X-Greylist: delayed 426 seconds by postgrey-1.27 at vger.kernel.org; Tue, 22 Sep 2020 07:42:51 EDT DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=citrix.com; s=securemail; t=1600774974; h=date:from:to:cc:subject:message-id:references: mime-version:content-transfer-encoding:in-reply-to; bh=LEpVh9z5B/gjx7BbT67s1F5sJUZuubzPdeIHh8AvYIU=; b=E4+QSJkDGVIyKge3e/crXRWyeaGzVYzkgy/34BK23ydP/4nKTDWqOzNr X/jmGo1DW5rkOaUC/OYpTjtHXVWK7wBdyalkbw3pxBWffxkwRgCt0F2wq /r1dQoj/LjI9BRobNG+uqasdLU38pXN7zDJYayQkm//w3PX4811isyU7Z E=; Authentication-Results: esa1.hc3370-68.iphmx.com; dkim=none (message not signed) header.i=none IronPort-SDR: GCLSnh8hOs3Ru+ShdXfoPjUH80O9R5XsE/lEZntN3FnkZr1g9JCKxXw2X94MqBmGI2JLO0taMh n1Y9VkXYPeXaxZyiZf5xKe/CD00oCtgBkkCLw3vMI9mQftR/SHpgiVGhgga/BlrF83lH/CJfUQ rH+ZG5UBlRB969GjJ3qt+gG8R1YpiVSEdg+79iuJ/+UOShlUtWU7eyPFrfzW8wtVVjhbENCt7F enBthT9jDzyXT64pUHO+Q0v9P+/9hPrG0BdDZDtsLyscMGnpVv6CQ3AoUcAgh1P7Uxe1mac4Ro ztc= X-SBRS: 2.7 X-MesageID: 27578698 X-Ironport-Server: esa1.hc3370-68.iphmx.com X-Remote-IP: 162.221.158.21 X-Policy: $RELAYED X-IronPort-AV: E=Sophos;i="5.77,290,1596513600"; d="scan'208";a="27578698" Date: Tue, 22 Sep 2020 13:35:11 +0200 From: Roger Pau =?utf-8?B?TW9ubsOp?= To: SeongJae Park CC: , , SeongJae Park , , , , , , , , Subject: Re: [PATCH v2 1/3] xen-blkback: add a parameter for disabling of persistent grants Message-ID: <20200922113511.GK19254@Air-de-Roger> References: <20200922111259.GJ19254@Air-de-Roger> <20200922112638.14238-1-sjpark@amazon.com> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20200922112638.14238-1-sjpark@amazon.com> X-ClientProxiedBy: AMSPEX02CAS02.citrite.net (10.69.22.113) To FTLPEX02CL06.citrite.net (10.13.108.179) Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org 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. 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? Thanks, Roger.