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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 72520C001DB for ; Tue, 15 Aug 2023 00:36:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233409AbjHOAfw (ORCPT ); Mon, 14 Aug 2023 20:35:52 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:48944 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233411AbjHOAf2 (ORCPT ); Mon, 14 Aug 2023 20:35:28 -0400 Received: from mail-pl1-x62d.google.com (mail-pl1-x62d.google.com [IPv6:2607:f8b0:4864:20::62d]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id BF4E093; Mon, 14 Aug 2023 17:35:27 -0700 (PDT) Received: by mail-pl1-x62d.google.com with SMTP id d9443c01a7336-1bdef6f5449so8233655ad.3; Mon, 14 Aug 2023 17:35:27 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20221208; t=1692059727; x=1692664527; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:sender:from:to:cc:subject:date:message-id :reply-to; bh=bfAmkgzB7vT7tpPqvlusS7lhE5MuyQY4MvJWqWEdMxw=; b=Q3FsTHwS9lFILlCZTL+VSW2aBgIikI37LyzDBlO0rg0yAWWl8WiLTTad1Bw93yvH2Z 6+H2tD04Xx312hOQjfVs27DFdL6BpDfUgH3bGOLnXfU6snGVTi+SuseJ4OifnvpGkxDr 7JxqVmNrBAZB0ief7FxYgn9iQKvIGAqQQi5CNmTYRSrxjBEfskBkDsspIzSrAK/aJg7x f2nsQFkayTk8wUfT/jQZzyxeA+3lz1jN9M8+8YF+8FCQmW5s/n9Zuni13dkFGyJoQPYv QmmmIL1K28eb5SymTU5DSEDwbxqSnpm9lF4RfGQ00GO5KnsBKtt5CZPR0kq/u1Kq9JmR OySw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20221208; t=1692059727; x=1692664527; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:sender:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=bfAmkgzB7vT7tpPqvlusS7lhE5MuyQY4MvJWqWEdMxw=; b=a/eIgEVsQVCK2jeuI6d7P2BrOjdS9rPGLob1BvH5OfyYv8kAeSvdistIysZtaGpH8s d/6AXPL3TwN5fvfaj5vZn89EZque8L4plmgAB9MLVcMWg3BBncipy8tU/4mH0JpjGW9q XkdaBdbZWbWfcK6CQmI2RMA0EuZD3i7zUj3v/vJwpBFpEJLcaletm0hMyX452wx/Fxdr IZ6tbTAFJp8HxtZEE3TOcTfD85W0qYLDfT/UwqxXopl3YfmmAoqmxOjujHGqM3E6xeu+ 3xSu8Hjbret7mes1lXkcchM07LS9v6tdAdSqwfklQZ/cKkNuMQXb/f1M64ibpEOXdqgj WHfQ== X-Gm-Message-State: AOJu0YwGwoUTUhEZGym9hJ9xUqLVpANwCTOZIvMya7gsBYYJFb9+bO9J +i9J7ksWOeIOBok200uESk7iItmiQW0= X-Google-Smtp-Source: AGHT+IHI7QnEnuIcrHvGdYK8SghKW3IFtA+uZj3a3P6LJs/SVdXPUY2rHy+foXwaqvyorqLt4HERmQ== X-Received: by 2002:a17:903:22cc:b0:1b6:797d:33fb with SMTP id y12-20020a17090322cc00b001b6797d33fbmr13717283plg.64.1692059727156; Mon, 14 Aug 2023 17:35:27 -0700 (PDT) Received: from localhost ([2620:10d:c090:400::5:93bd]) by smtp.gmail.com with ESMTPSA id bh9-20020a170902a98900b001b89a6164desm9972446plb.118.2023.08.14.17.35.26 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 14 Aug 2023 17:35:26 -0700 (PDT) Sender: Tejun Heo Date: Mon, 14 Aug 2023 14:35:25 -1000 From: Tejun Heo To: Yosry Ahmed Cc: Michal Hocko , Shakeel Butt , Johannes Weiner , Roman Gushchin , Andrew Morton , Muchun Song , cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm: memcg: provide accurate stats for userspace reads Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello, On Mon, Aug 14, 2023 at 05:28:22PM -0700, Yosry Ahmed wrote: > > So, the original design used mutex for synchronize flushing with the idea > > being that updates are high freq but reads are low freq and can be > > relatively slow. Using rstats for mm internal operations changed this > > assumption quite a bit and we ended up switching that mutex with a lock. > > Naive question, do mutexes handle thundering herd problems better than > spinlocks? I would assume so but I am not sure. I don't know. We can ask Waiman if that becomes a problem. > > * Flush-side, maybe we can break flushing into per-cpu or whatnot but > > there's no avoiding the fact that flushing can take quite a while if there > > are a lot to flush whether locks are split or not. I wonder whether it'd > > be possible to go back to mutex for flushing and update the users to > > either consume the cached values or operate in a sleepable context if > > synchronous read is necessary, which is the right thing to do anyway given > > how long flushes can take. > > Unfortunately it cannot be broken down into per-cpu as all flushers > update the same per-cgroup counters, so we need a bigger locking > scope. Switching to atomics really hurts performance. Breaking down > the lock to be per-cgroup is doable, but since we need to lock both > the parent and the cgroup, flushing top-level cgroups (which I assume > is most common) will lock the root anyway. Plus, there's not much point in flushing in parallel, so I don't feel too enthusiastic about splitting flush locking. > All flushers right now operate in sleepable context, so we can go > again to the mutex if you think this will make things better. The Yes, I think that'd be more sane. > slowness problem reported recently is in a sleepable context, it's > just too slow for userspace if I understand correctly. I mean, there's a certain amount of work to do. There's no way around it if you wanna read the counters synchronously. The only solution there would be using a cached value or having some sort of auto-flushing mechanism so that the amount to flush don't build up too much - e.g. keep a count of the number of entries to flush and trigger flush if it goes over some threshold. Thanks. -- tejun