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=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 D9886C433FF for ; Wed, 14 Aug 2019 11:32:46 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A43FC205F4 for ; Wed, 14 Aug 2019 11:32:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1565782366; bh=MtjEzRWbPiG6EVM0rtPP4w5o57pmiryRcKW53PcseX0=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=jHtG1fnyku8Ir+xNYqEfqD0LRflPYLBv570M4o/68LqlOvWayd+Mr1mWt+9Di3K+4 sCTdVfmaoQDNcr2LEjuNrS6/T0JDcYJ2Mlb6qVCYfVDDzjTAnMelyKVNwWafpKz5s2 yt5r8X9jM6c3gTj77FDqN6gpvtNhVyWWjUto43aU= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727775AbfHNLcp (ORCPT ); Wed, 14 Aug 2019 07:32:45 -0400 Received: from mx2.suse.de ([195.135.220.15]:40342 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727617AbfHNLcp (ORCPT ); Wed, 14 Aug 2019 07:32:45 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 6F9C7AEA5; Wed, 14 Aug 2019 11:32:43 +0000 (UTC) Date: Wed, 14 Aug 2019 13:32:42 +0200 From: Michal Hocko To: Roman Gushchin Cc: Andrew Morton , linux-mm@kvack.org, Johannes Weiner , linux-kernel@vger.kernel.org, kernel-team@fb.com Subject: Re: [PATCH 2/2] mm: memcontrol: flush percpu slab vmstats on kmem offlining Message-ID: <20190814113242.GV17933@dhcp22.suse.cz> References: <20190812222911.2364802-1-guro@fb.com> <20190812222911.2364802-3-guro@fb.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190812222911.2364802-3-guro@fb.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon 12-08-19 15:29:11, Roman Gushchin wrote: > I've noticed that the "slab" value in memory.stat is sometimes 0, > even if some children memory cgroups have a non-zero "slab" value. > The following investigation showed that this is the result > of the kmem_cache reparenting in combination with the per-cpu > batching of slab vmstats. > > At the offlining some vmstat value may leave in the percpu cache, > not being propagated upwards by the cgroup hierarchy. It means > that stats on ancestor levels are lower than actual. Later when > slab pages are released, the precise number of pages is substracted > on the parent level, making the value negative. We don't show negative > values, 0 is printed instead. So the difference with other counters is that slab ones are reparented and that's why we have treat them specially? I guess that is what the comment in the code suggest but being explicit in the changelog would be nice. [...] > -static void memcg_flush_percpu_vmstats(struct mem_cgroup *memcg) > +static void memcg_flush_percpu_vmstats(struct mem_cgroup *memcg, bool slab_only) > { > unsigned long stat[MEMCG_NR_STAT]; > struct mem_cgroup *mi; > int node, cpu, i; > + int min_idx, max_idx; > > - for (i = 0; i < MEMCG_NR_STAT; i++) > + if (slab_only) { > + min_idx = NR_SLAB_RECLAIMABLE; > + max_idx = NR_SLAB_UNRECLAIMABLE; > + } else { > + min_idx = 0; > + max_idx = MEMCG_NR_STAT; > + } This is just ugly has hell! I really detest how this implicitly makes counters value very special without any note in the node_stat_item definition. Is it such a big deal to have a per counter flush and do the loop over all counters resp. specific counters around it so much worse? This should be really a slow path to safe few instructions or cache misses, no? -- Michal Hocko SUSE Labs