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=-5.0 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=ham 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 67A40C43387 for ; Thu, 3 Jan 2019 18:53:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 36C20217F5 for ; Thu, 3 Jan 2019 18:53:47 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1546541627; bh=DQDF3jKF8fdNGBlQpm3U7kPBpjQHeZRdpKrbZzma2Io=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=Ig/z9G7G92N4NAVEDc7wkPQv3voEYHxxjLWFUNl5FOIvTSyXoWlaD9+qWB1FaLuJ/ ZZTd67ItGczE1uGa4Lcme83u83avDYuU/uX63wlPd9U1bv8791vkC0pN5tNnzzXqbx 4VqHU0dz+DjJQAyBZePLZpZxi1CAWqz+JOc/rlIQ= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726563AbfACSxi (ORCPT ); Thu, 3 Jan 2019 13:53:38 -0500 Received: from mx2.suse.de ([195.135.220.15]:37586 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726154AbfACSxh (ORCPT ); Thu, 3 Jan 2019 13:53:37 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 9226CAF8D; Thu, 3 Jan 2019 18:53:34 +0000 (UTC) Date: Thu, 3 Jan 2019 19:53:33 +0100 From: Michal Hocko To: Yang Shi Cc: hannes@cmpxchg.org, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH 0/3] mm: memcontrol: delayed force empty Message-ID: <20190103185333.GX31793@dhcp22.suse.cz> References: <1546459533-36247-1-git-send-email-yang.shi@linux.alibaba.com> <20190103101215.GH31793@dhcp22.suse.cz> <20190103181329.GW31793@dhcp22.suse.cz> <6f43e926-3bb5-20d1-2e39-1d30bf7ad375@linux.alibaba.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <6f43e926-3bb5-20d1-2e39-1d30bf7ad375@linux.alibaba.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu 03-01-19 10:40:54, Yang Shi wrote: > > > On 1/3/19 10:13 AM, Michal Hocko wrote: > > On Thu 03-01-19 09:33:14, Yang Shi wrote: > > > > > > On 1/3/19 2:12 AM, Michal Hocko wrote: > > > > On Thu 03-01-19 04:05:30, Yang Shi wrote: > > > > > Currently, force empty reclaims memory synchronously when writing to > > > > > memory.force_empty. It may take some time to return and the afterwards > > > > > operations are blocked by it. Although it can be interrupted by signal, > > > > > it still seems suboptimal. > > > > Why it is suboptimal? We are doing that operation on behalf of the > > > > process requesting it. What should anybody else pay for it? In other > > > > words why should we hide the overhead? > > > Please see the below explanation. > > > > > > > > Now css offline is handled by worker, and the typical usecase of force > > > > > empty is before memcg offline. So, handling force empty in css offline > > > > > sounds reasonable. > > > > Hmm, so I guess you are talking about > > > > echo 1 > $MEMCG/force_empty > > > > rmdir $MEMCG > > > > > > > > and you are complaining that the operation takes too long. Right? Why do > > > > you care actually? > > > We have some usecases which create and remove memcgs very frequently, and > > > the tasks in the memcg may just access the files which are unlikely accessed > > > by anyone else. So, we prefer force_empty the memcg before rmdir'ing it to > > > reclaim the page cache so that they don't get accumulated to incur > > > unnecessary memory pressure. Since the memory pressure may incur direct > > > reclaim to harm some latency sensitive applications. > > Yes, this makes sense to me. > > > > > And, the create/remove might be run in a script sequentially (there might be > > > a lot scripts or applications are run in parallel to do this), i.e. > > > mkdir cg1 > > > do something > > > echo 0 > cg1/memory.force_empty > > > rmdir cg1 > > > > > > mkdir cg2 > > > ... > > > > > > The creation of the afterwards memcg might be blocked by the force_empty for > > > long time if there are a lot page caches, so the overall throughput of the > > > system may get hurt. > > Is there any reason for your scripts to be strictly sequential here? In > > other words why cannot you offload those expensive operations to a > > detached context in _userspace_? > > I would say it has not to be strictly sequential. The above script is just > an example to illustrate the pattern. But, sometimes it may hit such pattern > due to the complicated cluster scheduling and container scheduling in the > production environment, for example the creation process might be scheduled > to the same CPU which is doing force_empty. I have to say I don't know too > much about the internals of the container scheduling. In that case I do not see a strong reason to implement the offloding into the kernel. It is an additional code and semantic to maintain. I think it is more important to discuss whether we want to introduce force_empty in cgroup v2. -- Michal Hocko SUSE Labs