From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf1-f198.google.com (mail-pf1-f198.google.com [209.85.210.198]) by kanga.kvack.org (Postfix) with ESMTP id 899D06B7512 for ; Wed, 5 Sep 2018 16:51:55 -0400 (EDT) Received: by mail-pf1-f198.google.com with SMTP id b69-v6so4572119pfc.20 for ; Wed, 05 Sep 2018 13:51:55 -0700 (PDT) Received: from mail.linuxfoundation.org (mail.linuxfoundation.org. [140.211.169.12]) by mx.google.com with ESMTPS id w2-v6si3173483pgh.182.2018.09.05.13.51.54 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 05 Sep 2018 13:51:54 -0700 (PDT) Date: Wed, 5 Sep 2018 13:51:52 -0700 From: Andrew Morton Subject: Re: [PATCH v2] mm: slowly shrink slabs with a relatively small number of objects Message-Id: <20180905135152.1238c7103b2ecd6da206733c@linux-foundation.org> In-Reply-To: <20180904224707.10356-1-guro@fb.com> References: <20180904224707.10356-1-guro@fb.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Roman Gushchin Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@fb.com, Rik van Riel , Josef Bacik , Johannes Weiner On Tue, 4 Sep 2018 15:47:07 -0700 Roman Gushchin wrote: > Commit 9092c71bb724 ("mm: use sc->priority for slab shrink targets") > changed the way how the target slab pressure is calculated and > made it priority-based: > > delta = freeable >> priority; > delta *= 4; > do_div(delta, shrinker->seeks); > > The problem is that on a default priority (which is 12) no pressure > is applied at all, if the number of potentially reclaimable objects > is less than 4096 (1<<12). > > This causes the last objects on slab caches of no longer used cgroups > to never get reclaimed, resulting in dead cgroups staying around forever. But this problem pertains to all types of objects, not just the cgroup cache, yes? > Slab LRU lists are reparented on memcg offlining, but corresponding > objects are still holding a reference to the dying cgroup. > If we don't scan them at all, the dying cgroup can't go away. > Most likely, the parent cgroup hasn't any directly associated objects, > only remaining objects from dying children cgroups. So it can easily > hold a reference to hundreds of dying cgroups. > > If there are no big spikes in memory pressure, and new memory cgroups > are created and destroyed periodically, this causes the number of > dying cgroups grow steadily, causing a slow-ish and hard-to-detect > memory "leak". It's not a real leak, as the memory can be eventually > reclaimed, but it could not happen in a real life at all. I've seen > hosts with a steadily climbing number of dying cgroups, which doesn't > show any signs of a decline in months, despite the host is loaded > with a production workload. > > It is an obvious waste of memory, and to prevent it, let's apply > a minimal pressure even on small shrinker lists. E.g. if there are > freeable objects, let's scan at least min(freeable, scan_batch) > objects. > > This fix significantly improves a chance of a dying cgroup to be > reclaimed, and together with some previous patches stops the steady > growth of the dying cgroups number on some of our hosts. > > ... > > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -476,6 +476,17 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl, > delta = freeable >> priority; > delta *= 4; > do_div(delta, shrinker->seeks); > + > + /* > + * Make sure we apply some minimal pressure even on > + * small cgroups. This is necessary because some of > + * belonging objects can hold a reference to a dying > + * child cgroup. If we don't scan them, the dying > + * cgroup can't go away unless the memory pressure > + * (and the scanning priority) raise significantly. > + */ > + delta = max(delta, min(freeable, batch_size)); > + If so I think the comment should be cast in more general terms. Maybe with a final sentence "the cgroup cache is one such case". Also, please use all 80 columns in block comments to save a few display lines. And `delta' has type ULL whereas the other two are longs. We'll presumably hit warnings here, preventable with max_t.