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 1E952E75434 for ; Tue, 3 Oct 2023 08:23:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S239265AbjJCIXi (ORCPT ); Tue, 3 Oct 2023 04:23:38 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59298 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S239473AbjJCIX1 (ORCPT ); Tue, 3 Oct 2023 04:23:27 -0400 Received: from smtp-out1.suse.de (smtp-out1.suse.de [195.135.220.28]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 8AE59173F; Tue, 3 Oct 2023 01:09:51 -0700 (PDT) Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out1.suse.de (Postfix) with ESMTPS id EEBB32188D; Tue, 3 Oct 2023 08:09:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1696320589; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=8zGAxBJBJ/nKdlR2cgsT4LA+cPc2AG65Q41ph8MLLtE=; b=N/cv9Xs5HY+XMUn5vcDhxDnfnmcLRn0BE1jaeLTWpl41lyhceJyEaLNmVwyGG9KKSf9cy1 ey0aR+GzSoh6XfWvi/n3DqWRLQcJJOZ0vl8H6qJhVXR2l+gBoyRdvLDc64U2c1P/JbLrgi YOR/Fv/5diZYz/Zfn2rG4dVmfu4NjHM= Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id 8805C132D4; Tue, 3 Oct 2023 08:09:49 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id V2XoH03MG2WuKQAAMHmgww (envelope-from ); Tue, 03 Oct 2023 08:09:49 +0000 Date: Tue, 3 Oct 2023 10:09:48 +0200 From: Michal Hocko To: Yosry Ahmed Cc: Andrew Morton , Shakeel Butt , Johannes Weiner , Roman Gushchin , Muchun Song , Michal =?iso-8859-1?Q?Koutn=FD?= , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 0/2] mm: memcg: fix tracking of pending stats updates values Message-ID: References: <20230922175741.635002-1-yosryahmed@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue 03-10-23 01:03:53, Yosry Ahmed wrote: > On Tue, Oct 3, 2023 at 12:57 AM Michal Hocko wrote: > > > > On Mon 25-09-23 10:11:05, Yosry Ahmed wrote: > > > On Mon, Sep 25, 2023 at 6:50 AM Michal Hocko wrote: > > > > > > > > On Fri 22-09-23 17:57:38, Yosry Ahmed wrote: > > > > > While working on adjacent code [1], I realized that the values passed > > > > > into memcg_rstat_updated() to keep track of the magnitude of pending > > > > > updates is consistent. It is mostly in pages, but sometimes it can be in > > > > > bytes or KBs. Fix that. > > > > > > > > What kind of practical difference does this change make? Is it worth > > > > additional code? > > > > > > As explained in patch 2's commit message, the value passed into > > > memcg_rstat_updated() is used for the "flush only if not worth it" > > > heuristic. As we have discussed in different threads in the past few > > > weeks, unnecessary flushes can cause increased global lock contention > > > and/or latency. > > > > > > Byte-sized paths (percpu, slab, zswap, ..) feed bytes into the > > > heuristic, but those are interpreted as pages, which means we will > > > flush earlier than we should. This was noticed by code inspection. How > > > much does this matter in practice? I would say it depends on the > > > workload: how many percpu/slab allocations are being made vs. how many > > > flushes are requested. > > > > > > On a system with 100 cpus, 25M of stat updates are needed for a flush > > > usually, but ~6K of slab/percpu updates will also (mistakenly) cause a > > > flush. > > > > This surely depends on workload and that is understandable. But it would > > be really nice to provide some numbers for typical workloads which > > exercise slab heavily. > > If you have a workload in mind I can run it and see how many flushes > we get with/without this patch. The first thing that pops into my head > is creating a bunch of empty files but I don't know if that's the best > thing to get numbers from. Let me remind you that you are proposing a performance optimization and such a change requires some numbers to actually show it is benefitial. There are cases where the resulting code is clearly an improvement and the performance benefit is just a nice side effect. I do not consider this to be the case. The whole thing is quite convoluted even without a better precision you are proposing. And let me be clear, I am not opposing your patch but I would rather see it based on more than just hand waving. -- Michal Hocko SUSE Labs