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=-11.6 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_PASS,USER_IN_DEF_DKIM_WL autolearn=unavailable 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 C4426C10F05 for ; Fri, 29 Mar 2019 17:47:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 93E3C2184D for ; Fri, 29 Mar 2019 17:47:51 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="q1iL4joG" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729924AbfC2Rru (ORCPT ); Fri, 29 Mar 2019 13:47:50 -0400 Received: from mail-pl1-f201.google.com ([209.85.214.201]:51239 "EHLO mail-pl1-f201.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729646AbfC2Rrt (ORCPT ); Fri, 29 Mar 2019 13:47:49 -0400 Received: by mail-pl1-f201.google.com with SMTP id t17so3107plj.18 for ; Fri, 29 Mar 2019 10:47:49 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=date:in-reply-to:message-id:mime-version:references:subject:from:to :cc; bh=KkbObhazcqZn021nZHNNPAhwea543VJsHp1ie7WsQS4=; b=q1iL4joGvSC0SWToSbwIk7h6Qw0Wb7TsIpPlSjnGJ5khNWGgoW6HoIP4C4lYwbmThG E1aFKEsCc4VKFp2/QV7UANYkePMHJTvOhIpjHSTTOc4wi3+B9lTrqCf+8V1E6ODwIbWT ih9i2SwjFA5nj9xkVXZRJ7/2iypFpTJZb+ULj26lcaW/fh25fO2pReE+UhaFQq73PV/E zk051W9ZXHLYHg/f818uqCJN/C8ZzYbQ2Y7pVeFXgZyjqSUT9Q8mm8z81JCeH35RqrTI p8ybWL2qwqcck/fugZG9mwJPQ+zKo1pR4FNVc1iKkLFp2CPLDruCK3gnUpxTqIMvc5I8 yoAg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:in-reply-to:message-id:mime-version :references:subject:from:to:cc; bh=KkbObhazcqZn021nZHNNPAhwea543VJsHp1ie7WsQS4=; b=Nq2RXa1VxH8BJM/Hi8I6jMBtkg1vt30Qzr2qtChajYX6d8UDPh8Qto8thX53dx73zE 0APEqJe+atVozt2YwaDWJ7fmr5Ll9y3sq1I4CDiqD6HXDN++5rduHU+N64eudnYmG7bZ pSAj2Z5ncmtWlFLo/w51zk/hAcWOA56VAc141SJYlFeYorPBffX7R6K8IjRLZIxm7jxJ yaYHLFXlk+52J/EiN5lfymQpwJEP6+WbfkhiG+LVY+qJhiuCJJIkUxDf9LpoE15q+/WY 3ZkT+XVAq/QL007VxAzHzndNmG0Fj9eT7xUG4gnEh64TCTZJrAXrhCFvWFC77lP4Q8si vtaw== X-Gm-Message-State: APjAAAX/9tBbIf/MlSyohaFX0QGvQbM3e892cb3pNGGCcMxkToMXOO9t z/89WKMAcrggnfSBt0fpHVD8bvoAyHZu X-Google-Smtp-Source: APXvYqxXhjZZFeoHf4Hk+3gXZNWGmuUCXzlgNhD2c84zWNsCYlOZQnhIri7FEmZ36ZU3YB0RStIcvl2gjIeB X-Received: by 2002:a62:3444:: with SMTP id b65mr634398pfa.27.1553881668537; Fri, 29 Mar 2019 10:47:48 -0700 (PDT) Date: Fri, 29 Mar 2019 10:47:46 -0700 In-Reply-To: <20190321164453.46143c8bf2dd8bfd0f91d71c@linux-foundation.org> Message-Id: Mime-Version: 1.0 References: <20190307165632.35810-1-gthelen@google.com> <20190321164453.46143c8bf2dd8bfd0f91d71c@linux-foundation.org> Subject: Re: [PATCH] writeback: sum memcg dirty counters as needed From: Greg Thelen To: Andrew Morton Cc: Johannes Weiner , Michal Hocko , Vladimir Davydov , Tejun Heo , linux-mm@kvack.org, linux-kernel@vger.kernel.org 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 Andrew Morton wrote: > On Thu, 7 Mar 2019 08:56:32 -0800 Greg Thelen wrote: > >> Since commit a983b5ebee57 ("mm: memcontrol: fix excessive complexity in >> memory.stat reporting") memcg dirty and writeback counters are managed >> as: >> 1) per-memcg per-cpu values in range of [-32..32] >> 2) per-memcg atomic counter >> When a per-cpu counter cannot fit in [-32..32] it's flushed to the >> atomic. Stat readers only check the atomic. >> Thus readers such as balance_dirty_pages() may see a nontrivial error >> margin: 32 pages per cpu. >> Assuming 100 cpus: >> 4k x86 page_size: 13 MiB error per memcg >> 64k ppc page_size: 200 MiB error per memcg >> Considering that dirty+writeback are used together for some decisions >> the errors double. >> >> This inaccuracy can lead to undeserved oom kills. One nasty case is >> when all per-cpu counters hold positive values offsetting an atomic >> negative value (i.e. per_cpu[*]=32, atomic=n_cpu*-32). >> balance_dirty_pages() only consults the atomic and does not consider >> throttling the next n_cpu*32 dirty pages. If the file_lru is in the >> 13..200 MiB range then there's absolutely no dirty throttling, which >> burdens vmscan with only dirty+writeback pages thus resorting to oom >> kill. >> >> It could be argued that tiny containers are not supported, but it's more >> subtle. It's the amount the space available for file lru that matters. >> If a container has memory.max-200MiB of non reclaimable memory, then it >> will also suffer such oom kills on a 100 cpu machine. >> >> ... >> >> Make balance_dirty_pages() and wb_over_bg_thresh() work harder to >> collect exact per memcg counters when a memcg is close to the >> throttling/writeback threshold. This avoids the aforementioned oom >> kills. >> >> This does not affect the overhead of memory.stat, which still reads the >> single atomic counter. >> >> Why not use percpu_counter? memcg already handles cpus going offline, >> so no need for that overhead from percpu_counter. And the >> percpu_counter spinlocks are more heavyweight than is required. >> >> It probably also makes sense to include exact dirty and writeback >> counters in memcg oom reports. But that is saved for later. > > Nice changelog, thanks. > >> Signed-off-by: Greg Thelen > > Did you consider cc:stable for this? We may as well - the stablebots > backport everything which might look slightly like a fix anyway :( Good idea. Done in -v2 of the patch. >> --- a/include/linux/memcontrol.h >> +++ b/include/linux/memcontrol.h >> @@ -573,6 +573,22 @@ static inline unsigned long memcg_page_state(struct mem_cgroup *memcg, >> return x; >> } >> >> +/* idx can be of type enum memcg_stat_item or node_stat_item */ >> +static inline unsigned long >> +memcg_exact_page_state(struct mem_cgroup *memcg, int idx) >> +{ >> + long x = atomic_long_read(&memcg->stat[idx]); >> +#ifdef CONFIG_SMP >> + int cpu; >> + >> + for_each_online_cpu(cpu) >> + x += per_cpu_ptr(memcg->stat_cpu, cpu)->count[idx]; >> + if (x < 0) >> + x = 0; >> +#endif >> + return x; >> +} > > This looks awfully heavyweight for an inline function. Why not make it > a regular function and avoid the bloat and i-cache consumption? Done in -v2. > Also, did you instead consider making this spill the percpu counters > into memcg->stat[idx]? That might be more useful for potential future > callers. It would become a little more expensive though. I looked at that approach, but couldn't convince myself it was safe. I kept staring at "Remote [...] Write accesses can cause unique problems due to the relaxed synchronization requirements for this_cpu operations." from this_cpu_ops.txt. So I'd like to delay this possible optimization for a later patch.