linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Roman Gushchin <guro@fb.com>
To: Shakeel Butt <shakeelb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linux MM <linux-mm@kvack.org>, Cgroups <cgroups@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Kernel Team <Kernel-team@fb.com>
Subject: Re: [PATCH 3/4] mm: memcontrol: fix recursive statistics correctness & scalabilty
Date: Fri, 12 Apr 2019 20:15:38 +0000	[thread overview]
Message-ID: <20190412201534.GB24377@tower.DHCP.thefacebook.com> (raw)
In-Reply-To: <CALvZod4xu10+E41YyaamigysZAnDcdA09f5m-hGd72LeJ9VmEg@mail.gmail.com>

On Fri, Apr 12, 2019 at 12:55:10PM -0700, Shakeel Butt wrote:
> On Fri, Apr 12, 2019 at 8:15 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
> >
> > Right now, when somebody needs to know the recursive memory statistics
> > and events of a cgroup subtree, they need to walk the entire subtree
> > and sum up the counters manually.
> >
> > There are two issues with this:
> >
> > 1. When a cgroup gets deleted, its stats are lost. The state counters
> > should all be 0 at that point, of course, but the events are not. When
> > this happens, the event counters, which are supposed to be monotonic,
> > can go backwards in the parent cgroups.
> >
> 
> We also faced this exact same issue as well and had the similar solution.
> 
> > 2. During regular operation, we always have a certain number of lazily
> > freed cgroups sitting around that have been deleted, have no tasks,
> > but have a few cache pages remaining. These groups' statistics do not
> > change until we eventually hit memory pressure, but somebody watching,
> > say, memory.stat on an ancestor has to iterate those every time.
> >
> > This patch addresses both issues by introducing recursive counters at
> > each level that are propagated from the write side when stats change.
> >
> > Upward propagation happens when the per-cpu caches spill over into the
> > local atomic counter. This is the same thing we do during charge and
> > uncharge, except that the latter uses atomic RMWs, which are more
> > expensive; stat changes happen at around the same rate. In a sparse
> > file test (page faults and reclaim at maximum CPU speed) with 5 cgroup
> > nesting levels, perf shows __mod_memcg_page state at ~1%.
> >
> 
> (Unrelated to this patchset) I think there should also a way to get
> the exact memcg stats. As the machines are getting bigger (more cpus
> and larger basic page size) the accuracy of stats are getting worse.
> Internally we have an additional interface memory.stat_exact for that.
> However I am not sure in the upstream kernel will an additional
> interface is better or something like /proc/sys/vm/stat_refresh which
> sync all per-cpu stats.

I was thinking about eventually consistent counters: sync them periodically
from a worker thread. It should keep the cost of reading small, but
should increase the accuracy. Will it work for you?

  parent reply	other threads:[~2019-04-12 20:16 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-04-12 15:15 [PATCH 0/4] mm: memcontrol: memory.stat cost & correctness Johannes Weiner
2019-04-12 15:15 ` [PATCH 1/4] mm: memcontrol: make cgroup stats and events query API explicitly local Johannes Weiner
2019-04-12 15:15 ` [PATCH 2/4] mm: memcontrol: move stat/event counting functions out-of-line Johannes Weiner
2019-04-12 15:15 ` [PATCH 3/4] mm: memcontrol: fix recursive statistics correctness & scalabilty Johannes Weiner
2019-04-12 19:55   ` Shakeel Butt
2019-04-12 20:10     ` Johannes Weiner
2019-04-12 20:38       ` Shakeel Butt
2019-04-12 20:15     ` Roman Gushchin [this message]
2019-04-12 20:50       ` Shakeel Butt
2019-04-12 15:15 ` [PATCH 4/4] mm: memcontrol: fix NUMA round-robin reclaim at intermediate level Johannes Weiner
2019-04-12 20:07 ` [PATCH 0/4] mm: memcontrol: memory.stat cost & correctness Shakeel Butt
2019-04-12 22:04 ` Roman Gushchin

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20190412201534.GB24377@tower.DHCP.thefacebook.com \
    --to=guro@fb.com \
    --cc=Kernel-team@fb.com \
    --cc=akpm@linux-foundation.org \
    --cc=cgroups@vger.kernel.org \
    --cc=hannes@cmpxchg.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=shakeelb@google.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).