From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qt0-f198.google.com (mail-qt0-f198.google.com [209.85.216.198]) by kanga.kvack.org (Postfix) with ESMTP id 696E16B0006 for ; Thu, 2 Aug 2018 11:18:54 -0400 (EDT) Received: by mail-qt0-f198.google.com with SMTP id d25-v6so1838755qtp.10 for ; Thu, 02 Aug 2018 08:18:54 -0700 (PDT) Received: from mx1.redhat.com (mx3-rdu2.redhat.com. [66.187.233.73]) by mx.google.com with ESMTPS id u55-v6si2143322qtj.69.2018.08.02.08.18.52 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 02 Aug 2018 08:18:52 -0700 (PDT) Date: Thu, 2 Aug 2018 18:18:49 +0300 From: "Michael S. Tsirkin" Subject: Re: [PATCH v2 2/2] virtio_balloon: replace oom notifier with shrinker Message-ID: <20180802181309-mutt-send-email-mst@kernel.org> References: <1532683495-31974-1-git-send-email-wei.w.wang@intel.com> <1532683495-31974-3-git-send-email-wei.w.wang@intel.com> <20180730090041.GC24267@dhcp22.suse.cz> <5B619599.1000307@intel.com> <20180801113444.GK16767@dhcp22.suse.cz> <5B62DDCC.3030100@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5B62DDCC.3030100@intel.com> Sender: owner-linux-mm@kvack.org List-ID: To: Wei Wang Cc: Michal Hocko , virtio-dev@lists.oasis-open.org, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, linux-mm@kvack.org, akpm@linux-foundation.org On Thu, Aug 02, 2018 at 06:32:44PM +0800, Wei Wang wrote: > On 08/01/2018 07:34 PM, Michal Hocko wrote: > > On Wed 01-08-18 19:12:25, Wei Wang wrote: > > > On 07/30/2018 05:00 PM, Michal Hocko wrote: > > > > On Fri 27-07-18 17:24:55, Wei Wang wrote: > > > > > The OOM notifier is getting deprecated to use for the reasons mentioned > > > > > here by Michal Hocko: https://lkml.org/lkml/2018/7/12/314 > > > > > > > > > > This patch replaces the virtio-balloon oom notifier with a shrinker > > > > > to release balloon pages on memory pressure. > > > > It would be great to document the replacement. This is not a small > > > > change... > > > OK. I plan to document the following to the commit log: > > > > > > The OOM notifier is getting deprecated to use for the reasons: > > > - As a callout from the oom context, it is too subtle and easy to > > > generate bugs and corner cases which are hard to track; > > > - It is called too late (after the reclaiming has been performed). > > > Drivers with large amuont of reclaimable memory is expected to be > > > released them at an early age of memory pressure; > > > - The notifier callback isn't aware of the oom contrains; > > > Link: https://lkml.org/lkml/2018/7/12/314 > > > > > > This patch replaces the virtio-balloon oom notifier with a shrinker > > > to release balloon pages on memory pressure. Users can set the amount of > > > memory pages to release each time a shrinker_scan is called via the > > > module parameter balloon_pages_to_shrink, and the default amount is 256 > > > pages. Historically, the feature VIRTIO_BALLOON_F_DEFLATE_ON_OOM has > > > been used to release balloon pages on OOM. We continue to use this > > > feature bit for the shrinker, so the shrinker is only registered when > > > this feature bit has been negotiated with host. > > Do you have any numbers for how does this work in practice? > > It works in this way: for example, we can set the parameter, > balloon_pages_to_shrink, to shrink 1GB memory once shrink scan is called. > Now, we have a 8GB guest, and we balloon out 7GB. When shrink scan is > called, the balloon driver will get back 1GB memory and give them back to > mm, then the ballooned memory becomes 6GB. > > When the shrinker scan is called the second time, another 1GB will be given > back to mm. So the ballooned pages are given back to mm gradually. I think what's being asked here is a description of tests that were run. Which workloads see improved behaviour? Our behaviour under memory pressure isn't great, in particular it is not clear when it's safe to re-inflate the balloon, if host attempts to re-inflate it too soon then we still get OOM. It would be better if VIRTIO_BALLOON_F_DEFLATE_ON_OOM would somehow mean "it's ok to ask for almost all of memory, if guest needs memory from balloon for apps to function it can take it from the balloon". -- MST