From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-f70.google.com (mail-ed1-f70.google.com [209.85.208.70]) by kanga.kvack.org (Postfix) with ESMTP id 036F88E0002 for ; Tue, 15 Jan 2019 02:25:56 -0500 (EST) Received: by mail-ed1-f70.google.com with SMTP id o21so770652edq.4 for ; Mon, 14 Jan 2019 23:25:55 -0800 (PST) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id h2-v6si229880ejq.203.2019.01.14.23.25.53 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 14 Jan 2019 23:25:53 -0800 (PST) Date: Tue, 15 Jan 2019 08:25:51 +0100 From: Michal Hocko Subject: Re: [PATCH v3] memcg: schedule high reclaim for remote memcgs on high_work Message-ID: <20190115072551.GO21345@dhcp22.suse.cz> References: <20190110174432.82064-1-shakeelb@google.com> <20190111205948.GA4591@cmpxchg.org> <20190113183402.GD1578@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: Shakeel Butt Cc: Johannes Weiner , Andrew Morton , Vladimir Davydov , Cgroups , Linux MM , LKML On Mon 14-01-19 12:18:07, Shakeel Butt wrote: > On Sun, Jan 13, 2019 at 10:34 AM Michal Hocko wrote: > > > > On Fri 11-01-19 14:54:32, Shakeel Butt wrote: > > > Hi Johannes, > > > > > > On Fri, Jan 11, 2019 at 12:59 PM Johannes Weiner wrote: > > > > > > > > Hi Shakeel, > > > > > > > > On Thu, Jan 10, 2019 at 09:44:32AM -0800, Shakeel Butt wrote: > > > > > If a memcg is over high limit, memory reclaim is scheduled to run on > > > > > return-to-userland. However it is assumed that the memcg is the current > > > > > process's memcg. With remote memcg charging for kmem or swapping in a > > > > > page charged to remote memcg, current process can trigger reclaim on > > > > > remote memcg. So, schduling reclaim on return-to-userland for remote > > > > > memcgs will ignore the high reclaim altogether. So, record the memcg > > > > > needing high reclaim and trigger high reclaim for that memcg on > > > > > return-to-userland. However if the memcg is already recorded for high > > > > > reclaim and the recorded memcg is not the descendant of the the memcg > > > > > needing high reclaim, punt the high reclaim to the work queue. > > > > > > > > The idea behind remote charging is that the thread allocating the > > > > memory is not responsible for that memory, but a different cgroup > > > > is. Why would the same thread then have to work off any high excess > > > > this could produce in that unrelated group? > > > > > > > > Say you have a inotify/dnotify listener that is restricted in its > > > > memory use - now everybody sending notification events from outside > > > > that listener's group would get throttled on a cgroup over which it > > > > has no control. That sounds like a recipe for priority inversions. > > > > > > > > It seems to me we should only do reclaim-on-return when current is in > > > > the ill-behaved cgroup, and punt everything else - interrupts and > > > > remote charges - to the workqueue. > > > > > > This is what v1 of this patch was doing but Michal suggested to do > > > what this version is doing. Michal's argument was that the current is > > > already charging and maybe reclaiming a remote memcg then why not do > > > the high excess reclaim as well. > > > > Johannes has a good point about the priority inversion problems which I > > haven't thought about. > > > > > Personally I don't have any strong opinion either way. What I actually > > > wanted was to punt this high reclaim to some process in that remote > > > memcg. However I didn't explore much on that direction thinking if > > > that complexity is worth it. Maybe I should at least explore it, so, > > > we can compare the solutions. What do you think? > > > > My question would be whether we really care all that much. Do we know of > > workloads which would generate a large high limit excess? > > > > The current semantics of memory.high is that it can be breached under > extreme conditions. However any workload where memory.high is used and > a lot of remote memcg charging happens (inotify/dnotify example given > by Johannes or swapping in tmpfs file or shared memory region) the > memory.high breach will become common. This is exactly what I am asking about. Is this something that can happen easily? Remote charges on themselves should be rare, no? -- Michal Hocko SUSE Labs