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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS 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 C88D2C433DB for ; Thu, 4 Feb 2021 16:44:12 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 6BF7164F43 for ; Thu, 4 Feb 2021 16:44:12 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 6BF7164F43 Authentication-Results: mail.kernel.org; dmarc=fail (p=quarantine dis=none) header.from=suse.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id C54CD6B006E; Thu, 4 Feb 2021 11:44:11 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id C05FC6B0072; Thu, 4 Feb 2021 11:44:11 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id AA6366B0075; Thu, 4 Feb 2021 11:44:11 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0018.hostedemail.com [216.40.44.18]) by kanga.kvack.org (Postfix) with ESMTP id 8E6AD6B006E for ; Thu, 4 Feb 2021 11:44:11 -0500 (EST) Received: from smtpin12.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with ESMTP id 399CE180AD811 for ; Thu, 4 Feb 2021 16:44:11 +0000 (UTC) X-FDA: 77781157902.12.mark73_300788a275dd Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin12.hostedemail.com (Postfix) with ESMTP id 8076918023AB6 for ; Thu, 4 Feb 2021 16:44:09 +0000 (UTC) X-HE-Tag: mark73_300788a275dd X-Filterd-Recvd-Size: 5249 Received: from mx2.suse.de (mx2.suse.de [195.135.220.15]) by imf42.hostedemail.com (Postfix) with ESMTP for ; Thu, 4 Feb 2021 16:44:08 +0000 (UTC) X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1612457047; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=uPMfnHJez5C0MYU9Bi3DY+Zzs1npEebnPNg9a6l1V9g=; b=OyUf2t2CyZUBOw3fyaefDcbgpMBcVsea+tJUuphQlBvBy9pQkyrcEsJBBG4GEajmW7vM6g H949fzmNE/GPXVwllufcLuwPy+hWIU2G05xDp8Yxm0U1lJsyNeCmeqSFogw1mxxO0JB1jW 4YsoerN08FU9CzXsrURvSYD6tYTNAaM= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id A54D5AD19; Thu, 4 Feb 2021 16:44:07 +0000 (UTC) Date: Thu, 4 Feb 2021 17:44:06 +0100 From: Michal Hocko To: Johannes Weiner Cc: Andrew Morton , Tejun Heo , Roman Gushchin , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com Subject: Re: [PATCH 6/7] mm: memcontrol: switch to rstat Message-ID: References: <20210202184746.119084-1-hannes@cmpxchg.org> <20210202184746.119084-7-hannes@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu 04-02-21 11:15:06, Johannes Weiner wrote: > Hello Michal, > > On Thu, Feb 04, 2021 at 03:19:17PM +0100, Michal Hocko wrote: > > On Tue 02-02-21 13:47:45, Johannes Weiner wrote: > > > Replace the memory controller's custom hierarchical stats code with > > > the generic rstat infrastructure provided by the cgroup core. > > > > > > The current implementation does batched upward propagation from the > > > write side (i.e. as stats change). The per-cpu batches introduce an > > > error, which is multiplied by the number of subgroups in a tree. In > > > systems with many CPUs and sizable cgroup trees, the error can be > > > large enough to confuse users (e.g. 32 batch pages * 32 CPUs * 32 > > > subgroups results in an error of up to 128M per stat item). This can > > > entirely swallow allocation bursts inside a workload that the user is > > > expecting to see reflected in the statistics. > > > > > > In the past, we've done read-side aggregation, where a memory.stat > > > read would have to walk the entire subtree and add up per-cpu > > > counts. This became problematic with lazily-freed cgroups: we could > > > have large subtrees where most cgroups were entirely idle. Hence the > > > switch to change-driven upward propagation. Unfortunately, it needed > > > to trade accuracy for speed due to the write side being so hot. > > > > > > Rstat combines the best of both worlds: from the write side, it > > > cheaply maintains a queue of cgroups that have pending changes, so > > > that the read side can do selective tree aggregation. This way the > > > reported stats will always be precise and recent as can be, while the > > > aggregation can skip over potentially large numbers of idle cgroups. > > > > > > This adds a second vmstats to struct mem_cgroup (MEMCG_NR_STAT + > > > NR_VM_EVENT_ITEMS) to track pending subtree deltas during upward > > > aggregation. It removes 3 words from the per-cpu data. It eliminates > > > memcg_exact_page_state(), since memcg_page_state() is now exact. > > > > I am still digesting details and need to look deeper into how rstat > > works but removing our own stats is definitely a good plan. Especially > > when there are existing limitations and problems that would need fixing. > > > > Just to check that my high level understanding is correct. The > > transition is effectivelly removing a need to manually sync counters up > > the hierarchy and partially outsources that decision to rstat core. The > > controller is responsible just to tell the core how that syncing is done > > (e.g. which specific counters etc). > > Yes, exactly. > > rstat implements a tree of cgroups that have local changes pending, > and a flush walk on that tree. But it's all driven by the controller. > > memcg needs to tell rstat 1) when stats in a local cgroup change > e.g. when we do mod_memcg_state() (cgroup_rstat_updated), 2) when to > flush, e.g. before a memory.stat read (cgroup_rstat_flush), and 3) how > to flush one cgroup's per-cpu state and propagate it upward to the > parent during rstat's flush walk (.css_rstat_flush). Can we have this short summary in a changelog please? > > Excplicit flushes are needed when you want an exact value (e.g. when > > values are presented to the userspace). I do not see any flushes to > > be done by the core pro-actively except for clean up on a release. > > > > Is the above correct understanding? > > Yes, that's correct. OK, thanks for the confirmation. I will have a closer look tomorrow but I do not see any problems now. -- Michal Hocko SUSE Labs