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=-11.4 required=3.0 tests=DKIMWL_WL_MED,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,USER_IN_DEF_DKIM_WL autolearn=no 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 B0BE5C433E0 for ; Fri, 15 May 2020 17:49:37 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8D26D207D0 for ; Fri, 15 May 2020 17:49:37 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="ByCFc5Mi" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726407AbgEORth (ORCPT ); Fri, 15 May 2020 13:49:37 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38002 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-FAIL-OK-FAIL) by vger.kernel.org with ESMTP id S1726219AbgEORtg (ORCPT ); Fri, 15 May 2020 13:49:36 -0400 Received: from mail-lf1-x142.google.com (mail-lf1-x142.google.com [IPv6:2a00:1450:4864:20::142]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B55C4C061A0C for ; Fri, 15 May 2020 10:49:35 -0700 (PDT) Received: by mail-lf1-x142.google.com with SMTP id a4so2511664lfh.12 for ; Fri, 15 May 2020 10:49:35 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=UaYQTSaq9VZj8MAel6SAXsrwR35YF1SYRI2VQ4Ly89w=; b=ByCFc5MixtBs6AIiuvjXgLX8ciepEzcgMdBfkabVnYTEguZvFgct4xZxiY5o28FfEo /PBXJDk4bkppu1q48kIDeQC++nBqpVLtTif9qjyJETTP9M3e+pnxJkYmtcgOTkLsERga AA7gynk9XbG6MPGlazZvj+UUttrdRaeKr36ULZmiDOHwd8AoLTkl1NP4XRtXOPYsJMOz yvHiTJCMMd3aKBelaqHfPXPxzszh5IFHoBWdsGR2ucpZOzNa9FbE/xPsZSySmoAuRw0+ UUyYMytNomyz+jTuObIOKbCJwnuQ0PkBPQhWfvtoco6xTPkkLwCx1S4kJ2FTycMc7gFt FaZQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=UaYQTSaq9VZj8MAel6SAXsrwR35YF1SYRI2VQ4Ly89w=; b=dRWVa6av1ZYgmb7C0oI8NPzrt/iKAndxV7d3cQFjGRQArSO2bTuQdHnYl/aj5f+z56 uW1LW6Q0LthgqtUwBgJ0/ZU2Y1F9r/ff92deGPM6CaP69QJcFp1iOqxn0lpiGYA8K5Cg 9tLWMnd6R/4JrFEkfMN86rLVRYVUFswra8q0utBEr3w+0PEztp6Pe5OwoND9XOz2oT17 6Pk7ghBQWLqQsoXUwousUg6ZbvxFc19tWM0phXApgdRkEPqoTBEZC2z8ImKoQHVsHYJy m8MwqDrFFylLMvV49Jk+iGrGwkJIeQvoHfEwqHCgKEvI2yjrJvlPsjiVU4rzkDQv95j2 0d3A== X-Gm-Message-State: AOAM530QsB6jEQLiCIhqeBUFlVsj5qgTM7wBQKFpnf012Pcb/Qmsf2Ub rdbrHYqdqqUL52Am82j03H69Fol0DvBmUvZrzbXJhg== X-Google-Smtp-Source: ABdhPJxFY7EwOuokBF16WspLae1APA6Q7qnODWtV/Ot7OuSI12mNYEij3/T9g8jKQkXPrJRRbR3Y270E7KoyMm2RVWA= X-Received: by 2002:a19:c85:: with SMTP id 127mr3181989lfm.189.1589564973821; Fri, 15 May 2020 10:49:33 -0700 (PDT) MIME-Version: 1.0 References: <20200508170630.94406-1-shakeelb@google.com> <20200508214405.GA226164@cmpxchg.org> <20200515082955.GJ29153@dhcp22.suse.cz> <20200515132421.GC591266@cmpxchg.org> <20200515150026.GA94522@carbon.DHCP.thefacebook.com> In-Reply-To: <20200515150026.GA94522@carbon.DHCP.thefacebook.com> From: Shakeel Butt Date: Fri, 15 May 2020 10:49:22 -0700 Message-ID: Subject: Re: [PATCH] memcg: expose root cgroup's memory.stat To: Roman Gushchin Cc: Johannes Weiner , Michal Hocko , Mel Gorman , Andrew Morton , Yafang Shao , Linux MM , Cgroups , LKML Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, May 15, 2020 at 8:00 AM Roman Gushchin wrote: > > On Fri, May 15, 2020 at 06:44:44AM -0700, Shakeel Butt wrote: > > On Fri, May 15, 2020 at 6:24 AM Johannes Weiner wrote: > > > > > > On Fri, May 15, 2020 at 10:29:55AM +0200, Michal Hocko wrote: > > > > On Sat 09-05-20 07:06:38, Shakeel Butt wrote: > > > > > On Fri, May 8, 2020 at 2:44 PM Johannes Weiner wrote: > > > > > > > > > > > > On Fri, May 08, 2020 at 10:06:30AM -0700, Shakeel Butt wrote: > > > > > > > One way to measure the efficiency of memory reclaim is to look at the > > > > > > > ratio (pgscan+pfrefill)/pgsteal. However at the moment these stats are > > > > > > > not updated consistently at the system level and the ratio of these are > > > > > > > not very meaningful. The pgsteal and pgscan are updated for only global > > > > > > > reclaim while pgrefill gets updated for global as well as cgroup > > > > > > > reclaim. > > > > > > > > > > > > > > Please note that this difference is only for system level vmstats. The > > > > > > > cgroup stats returned by memory.stat are actually consistent. The > > > > > > > cgroup's pgsteal contains number of reclaimed pages for global as well > > > > > > > as cgroup reclaim. So, one way to get the system level stats is to get > > > > > > > these stats from root's memory.stat, so, expose memory.stat for the root > > > > > > > cgroup. > > > > > > > > > > > > > > from Johannes Weiner: > > > > > > > There are subtle differences between /proc/vmstat and > > > > > > > memory.stat, and cgroup-aware code that wants to watch the full > > > > > > > hierarchy currently has to know about these intricacies and > > > > > > > translate semantics back and forth. > > > > > > > > Can we have those subtle differences documented please? > > > > > > > > > > > > > > > > > > Generally having the fully recursive memory.stat at the root > > > > > > > level could help a broader range of usecases. > > > > > > > > > > > > The changelog begs the question why we don't just "fix" the > > > > > > system-level stats. It may be useful to include the conclusions from > > > > > > that discussion, and why there is value in keeping the stats this way. > > > > > > > > > > > > > > > > Right. Andrew, can you please add the following para to the changelog? > > > > > > > > > > Why not fix the stats by including both the global and cgroup reclaim > > > > > activity instead of exposing root cgroup's memory.stat? The reason is > > > > > the benefit of having metrics exposing the activity that happens > > > > > purely due to machine capacity rather than localized activity that > > > > > happens due to the limits throughout the cgroup tree. Additionally > > > > > there are userspace tools like sysstat(sar) which reads these stats to > > > > > inform about the system level reclaim activity. So, we should not > > > > > break such use-cases. > > > > > > > > > > > > Signed-off-by: Shakeel Butt > > > > > > > Suggested-by: Johannes Weiner > > > > > > > > > > > > Acked-by: Johannes Weiner > > > > > > > > > > Thanks a lot. > > > > > > > > I was quite surprised that the patch is so simple TBH. For some reason > > > > I've still had memories that we do not account for root memcg (likely > > > > because mem_cgroup_is_root(memcg) bail out in the try_charge. But stats > > > > are slightly different here. > > > > > > Yep, we skip the page_counter for root, but keep in mind that cgroup1 > > > *does* have a root-level memory.stat, so (for the most part) we've > > > been keeping consumer stats for the root level the whole time. > > > > > > > counters because they are not really all the same. E.g. > > > > - mem_cgroup_charge_statistics accounts for each memcg > > > > > > Yep, that's heritage from cgroup1. > > > > > > > - memcg_charge_kernel_stack relies on pages being associated with a > > > > memcg and that in turn relies on __memcg_kmem_charge_page which bails > > > > out on root memcg > > > > > > You're right. It should only bypass the page_counter, but still set > > > page->mem_cgroup = root_mem_cgroup, just like user pages. > > What about kernel threads? We consider them belonging to the root memory > cgroup. Should their memory consumption being considered in root-level stats? > > I'm not sure we really want it, but I guess we need to document how > kernel threads are handled. > What will be the cons of updating root-level stats for kthreads? > > > > > > This counter also doesn't get exported on cgroup1, so it would indeed > > > be a new bug. It needs to be fixed before this patch here. > > > > > > > - memcg_charge_slab (NR_SLAB*) skips over root memcg as well > > > > > > Same thing with these two. > > > > Yes, we skip page_counter for root but not the stats. I will go over > > all the stats and make sure we are not skipping the stats for root.