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=-6.6 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 DDA52C32750 for ; Tue, 13 Aug 2019 15:59:58 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A4BFF2084D for ; Tue, 13 Aug 2019 15:59:58 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="adgbz1vo" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728699AbfHMP75 (ORCPT ); Tue, 13 Aug 2019 11:59:57 -0400 Received: from mail-qk1-f196.google.com ([209.85.222.196]:45516 "EHLO mail-qk1-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726900AbfHMP75 (ORCPT ); Tue, 13 Aug 2019 11:59:57 -0400 Received: by mail-qk1-f196.google.com with SMTP id m2so6213327qki.12; Tue, 13 Aug 2019 08:59:56 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=5wYbeJt062YGni4JJRrpw5Er0ip3eb9YlimfJPWTmeY=; b=adgbz1vohNtcXb218auKPIEJMAA4N/AhL+EgCkoIrZTw4sJjkmuk6yITXcyaAjlHGw QYOYISgAtqVQixxeBPadxlyzgMpyeEPsuR8kJwvdRnfD45cB1MuXD1OEJisG6QWMPOSM yvtMTES6gK4c0TbvpJvooOZiXhLo2EzMXVlEXafTmZ0biTJfI9ngTmFRxSj57OEbWpCX l3wuA8gr7t5CLHPM90fc362Exw0vmfxq2tYr7PSRghSq+ayFEqpBI2aOl6f4nIHWAIlx kb9mwCTQRoPazUD9WX/3aKbiOvUVKLSPn0NMp0tCvKTkw3/Guhy52q1lbEfqKtIEBd3F /NYg== 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=5wYbeJt062YGni4JJRrpw5Er0ip3eb9YlimfJPWTmeY=; b=Qq8v0DI9zs2ZhKqcTOB1tEzssMBs5Ws+FBWF4lqr9J3qTo7j1Y0RhGOnVReFROEHSV Ru4R5SvT5L3QYDdJwiT4t47vClJafrFpWdJV3kdayNKai9rordjplMvtKGasUymW5Lrm vBRddCPu6Y8JucC1vK/6LMs5JuBQ7AT/HjxnGaDEnYlnEhuy2DzFEsWFsKdlPpDJAzJT 14zIrJYMmS2z7ABp2Ej2r2JOk+dj82AdBjHpZcu5Ts6FI6aCEdHbaxARNTQtn7qyRXue 2+1IlMcFPsUOBeZ4CjtFYvSQd5/tb457HjpvdZQDmGZa1kuM8gq4CBNU78xVHhEVI+FT EmBg== X-Gm-Message-State: APjAAAVWMETKRtgienDalLCeSaZbR+8fgfWGiACmenATyiYhNFLiL1Y7 HgcNK2voKx0eSQOm8MIDKKJv1oq6JlNx4MhXql4= X-Google-Smtp-Source: APXvYqwedf7ItGfvmp28v2dVpPWQXYz4bLyvLzDni/9I/2kDnDxkLY1j9Sty4jAaWxCMpgXPjNhgwikLK39hOOpxNtE= X-Received: by 2002:a37:9d94:: with SMTP id g142mr33317931qke.209.1565711995789; Tue, 13 Aug 2019 08:59:55 -0700 (PDT) MIME-Version: 1.0 References: <20190812192316.13615-1-hannes@cmpxchg.org> <20190813132938.GJ17933@dhcp22.suse.cz> In-Reply-To: <20190813132938.GJ17933@dhcp22.suse.cz> From: Yang Shi Date: Tue, 13 Aug 2019 08:59:38 -0700 Message-ID: Subject: Re: [PATCH] mm: vmscan: do not share cgroup iteration between reclaimers To: Michal Hocko Cc: Johannes Weiner , Andrew Morton , Linux MM , cgroups@vger.kernel.org, Linux Kernel Mailing List , kernel-team@fb.com 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 Tue, Aug 13, 2019 at 6:29 AM Michal Hocko wrote: > > On Mon 12-08-19 15:23:16, Johannes Weiner wrote: > > One of our services observed a high rate of cgroup OOM kills in the > > presence of large amounts of clean cache. Debugging showed that the > > culprit is the shared cgroup iteration in page reclaim. > > > > Under high allocation concurrency, multiple threads enter reclaim at > > the same time. Fearing overreclaim when we first switched from the > > single global LRU to cgrouped LRU lists, we introduced a shared > > iteration state for reclaim invocations - whether 1 or 20 reclaimers > > are active concurrently, we only walk the cgroup tree once: the 1st > > reclaimer reclaims the first cgroup, the second the second one etc. > > With more reclaimers than cgroups, we start another walk from the top. > > > > This sounded reasonable at the time, but the problem is that reclaim > > concurrency doesn't scale with allocation concurrency. As reclaim > > concurrency increases, the amount of memory individual reclaimers get > > to scan gets smaller and smaller. Individual reclaimers may only see > > one cgroup per cycle, and that may not have much reclaimable > > memory. We see individual reclaimers declare OOM when there is plenty > > of reclaimable memory available in cgroups they didn't visit. > > > > This patch does away with the shared iterator, and every reclaimer is > > allowed to scan the full cgroup tree and see all of reclaimable > > memory, just like it would on a non-cgrouped system. This way, when > > OOM is declared, we know that the reclaimer actually had a chance. > > > > To still maintain fairness in reclaim pressure, disallow cgroup > > reclaim from bailing out of the tree walk early. Kswapd and regular > > direct reclaim already don't bail, so it's not clear why limit reclaim > > would have to, especially since it only walks subtrees to begin with. > > The code does bail out on any direct reclaim - be it limit or page > allocator triggered. Check the !current_is_kswapd part of the condition. Yes, please see commit 2bb0f34fe3c1 ("mm: vmscan: do not iterate all mem cgroups for global direct reclaim") > > > This change completely eliminates the OOM kills on our service, while > > showing no signs of overreclaim - no increased scan rates, %sys time, > > or abrupt free memory spikes. I tested across 100 machines that have > > 64G of RAM and host about 300 cgroups each. > > What is the usual direct reclaim involvement on those machines? > > > [ It's possible overreclaim never was a *practical* issue to begin > > with - it was simply a concern we had on the mailing lists at the > > time, with no real data to back it up. But we have also added more > > bail-out conditions deeper inside reclaim (e.g. the proportional > > exit in shrink_node_memcg) since. Regardless, now we have data that > > suggests full walks are more reliable and scale just fine. ] > > I do not see how shrink_node_memcg bail out helps here. We do scan up-to > SWAP_CLUSTER_MAX pages for each LRU at least once. So we are getting to > nr_memcgs_with_pages multiplier with the patch applied in the worst case. > > How much that matters is another question and it depends on the > number of cgroups and the rate the direct reclaim happens. I do not > remember exact numbers but even walking a very large memcg tree was > noticeable. I'm concerned by this too. It might be ok to cgroup v2, but v1 still dominates. And, considering offline memcgs it might be not unusual to have quite large memcg tree. > > For the over reclaim part SWAP_CLUSTER_MAX is a relatively small number > so even hundreds of memcgs on a "reasonably" sized system shouldn't be > really observable (we are talking about 7MB per reclaim per reclaimer on > 1k memcgs with pages). This would get worse with many reclaimers. Maybe > we will need something like the regular direct reclaim throttling of > mmemcg limit reclaim as well in the future. > > That being said, I do agree that the oom side of the coin is causing > real troubles and it is a real problem to be addressed first. Especially with > cgroup v2 where we have likely more memcgs without any pages because > inner nodes do not have any tasks and direct charges which makes some > reclaimers hit memcgs without pages more likely. > > Let's see whether we see regression due to over-reclaim. > > > Signed-off-by: Johannes Weiner > > With the direct reclaim bail out reference fixed - unless I am wrong > there of course > > Acked-by: Michal Hocko > > It is sad to see this piece of fun not being used after that many years > of bugs here and there and all the lockless fun but this is the life > ;) > > > --- > > mm/vmscan.c | 22 ++-------------------- > > 1 file changed, 2 insertions(+), 20 deletions(-) > > > > diff --git a/mm/vmscan.c b/mm/vmscan.c > > index dbdc46a84f63..b2f10fa49c88 100644 > > --- a/mm/vmscan.c > > +++ b/mm/vmscan.c > > @@ -2667,10 +2667,6 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc) > > > > do { > > struct mem_cgroup *root = sc->target_mem_cgroup; > > - struct mem_cgroup_reclaim_cookie reclaim = { > > - .pgdat = pgdat, > > - .priority = sc->priority, > > - }; > > unsigned long node_lru_pages = 0; > > struct mem_cgroup *memcg; > > > > @@ -2679,7 +2675,7 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc) > > nr_reclaimed = sc->nr_reclaimed; > > nr_scanned = sc->nr_scanned; > > > > - memcg = mem_cgroup_iter(root, NULL, &reclaim); > > + memcg = mem_cgroup_iter(root, NULL, NULL); > > do { > > unsigned long lru_pages; > > unsigned long reclaimed; > > @@ -2724,21 +2720,7 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc) > > sc->nr_scanned - scanned, > > sc->nr_reclaimed - reclaimed); > > > > - /* > > - * Kswapd have to scan all memory cgroups to fulfill > > - * the overall scan target for the node. > > - * > > - * Limit reclaim, on the other hand, only cares about > > - * nr_to_reclaim pages to be reclaimed and it will > > - * retry with decreasing priority if one round over the > > - * whole hierarchy is not sufficient. > > - */ > > - if (!current_is_kswapd() && > > - sc->nr_reclaimed >= sc->nr_to_reclaim) { > > - mem_cgroup_iter_break(root, memcg); > > - break; > > - } > > - } while ((memcg = mem_cgroup_iter(root, memcg, &reclaim))); > > + } while ((memcg = mem_cgroup_iter(root, memcg, NULL))); > > > > if (reclaim_state) { > > sc->nr_reclaimed += reclaim_state->reclaimed_slab; > > -- > > 2.22.0 > > -- > Michal Hocko > SUSE Labs >