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=-10.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, MAILING_LIST_MULTI,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 C4A2CC433E0 for ; Fri, 8 Jan 2021 13:25:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7D894235FF for ; Fri, 8 Jan 2021 13:25:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726901AbhAHNZC (ORCPT ); Fri, 8 Jan 2021 08:25:02 -0500 Received: from mx2.suse.de ([195.135.220.15]:53596 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726060AbhAHNZC (ORCPT ); Fri, 8 Jan 2021 08:25:02 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1610112255; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=kWajWUazhZoEFj2YC1/JZm5JagKSGgfHEeGYCRfJurE=; b=gs/586IHJv6CfKBEQW6uznolaSjSS7pyS5nCShXJcGLVgCkBmUTYiKM35HqVN6LdBhjcdl GSiu4KPjhiEhosFZ+Q5uNSk690PF1Oee5aOvtgSFQcJW6Ls9/9qCRSvcUHoRLguIVpVh2C Na5hi5eVTTxjyqSzml7eMuDkWWNgBtc= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 45D47AD11; Fri, 8 Jan 2021 13:24:15 +0000 (UTC) Date: Fri, 8 Jan 2021 14:24:14 +0100 From: Michal Hocko To: Shakeel Butt Cc: Johannes Weiner , Roman Gushchin , Yang Shi , Andrew Morton , Linux MM , Cgroups , LKML Subject: Re: [PATCH] mm: memcg: add swapcache stat for memcg v2 Message-ID: <20210108132414.GD13207@dhcp22.suse.cz> References: <20210101023955.250965-1-shakeelb@google.com> <20210106145349.GN13207@dhcp22.suse.cz> 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 On Wed 06-01-21 08:42:39, Shakeel Butt wrote: > On Wed, Jan 6, 2021 at 6:53 AM Michal Hocko wrote: > > > > On Thu 31-12-20 18:39:55, Shakeel Butt wrote: > > > This patch adds swapcache stat for the cgroup v2. The swapcache > > > represents the memory that is accounted against both the memory and the > > > swap limit of the cgroup. The main motivation behind exposing the > > > swapcache stat is for enabling users to gracefully migrate from cgroup > > > v1's memsw counter to cgroup v2's memory and swap counters. > > > > > > Cgroup v1's memsw limit allows users to limit the memory+swap usage of a > > > workload but without control on the exact proportion of memory and swap. > > > Cgroup v2 provides separate limits for memory and swap which enables > > > more control on the exact usage of memory and swap individually for the > > > workload. > > > > > > With some little subtleties, the v1's memsw limit can be switched with > > > the sum of the v2's memory and swap limits. However the alternative for > > > memsw usage is not yet available in cgroup v2. Exposing per-cgroup > > > swapcache stat enables that alternative. Adding the memory usage and > > > swap usage and subtracting the swapcache will approximate the memsw > > > usage. This will help in the transparent migration of the workloads > > > depending on memsw usage and limit to v2' memory and swap counters. > > > > Could you expand a bit more on why memsw usage is important even in > > cgroup v2 land? How are you going to use the approximated value? > > > > Two main benefits. First, it hides the underlying system's swap setup > from the applications. Applications with multiple instances running in > a datacenter with heterogeneous systems (some have swap and some > don't) will keep seeing a consistent view of their usage. > > Second, most of the applications (at least in our prod) are not really > interested in two separate memory and swap usage metrics. A single > usage metric is more simple to use and reason about for these > applications. OK fair enough. Thanks for the clarification. As I've said I do not see any problem with exporting the counter. Acked-by: Michal Hocko -- Michal Hocko SUSE Labs