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=-7.1 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no 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 88F43C2D0E2 for ; Tue, 22 Sep 2020 15:49:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3835323A1B for ; Tue, 22 Sep 2020 15:49:54 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1600789794; bh=k8lQswR/ur1/wuPRVh5gbCd394H2XZkoHnw+6GQw8FU=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=H7iZyHdWIMyKZKV79c7yOLUlywrbkk1JUZB4pW6qQfZE6vKEsREAnZepEQizv2hzU 0Pd5A8JBjUEY5b391uzTBk4KfiFtiaNKk82PUXt2YNwJlw5dBQF8Xmma5/jrqIttdH QpeMMq/F+QC37VNTausTtbatCG+u9Gx5WzEyuUxA= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726719AbgIVPtx (ORCPT ); Tue, 22 Sep 2020 11:49:53 -0400 Received: from mail.kernel.org ([198.145.29.99]:37834 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726589AbgIVPtw (ORCPT ); Tue, 22 Sep 2020 11:49:52 -0400 Received: from paulmck-ThinkPad-P72.home (unknown [50.45.173.55]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id EB2702399A; Tue, 22 Sep 2020 15:49:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1600789792; bh=k8lQswR/ur1/wuPRVh5gbCd394H2XZkoHnw+6GQw8FU=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=K/CfZDPvToBo2iV6+CtBqFBAdcthZDngvNYyI9CcDd+DuMk7OwDKf5n4gXuah9DTJ uiPh7It0WasVrKHkqUFF1PxtDL52ZoX336L8T8boxKrdWQwf2ntpVBGQb5ld463Ke0 pFe4r7+ABm2TaCSUtf3Qd5LlK4yvLlykf6eb26mA= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id B537535226AA; Tue, 22 Sep 2020 08:49:51 -0700 (PDT) Date: Tue, 22 Sep 2020 08:49:51 -0700 From: "Paul E. McKenney" To: Michal Hocko Cc: Uladzislau Rezki , LKML , RCU , linux-mm@kvack.org, Andrew Morton , Peter Zijlstra , Vlastimil Babka , Thomas Gleixner , "Theodore Y . Ts'o" , Joel Fernandes , Sebastian Andrzej Siewior , Oleksiy Avramchenko , Mel Gorman Subject: Re: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func. Message-ID: <20200922154951.GX29330@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200918194817.48921-1-urezki@gmail.com> <20200918194817.48921-3-urezki@gmail.com> <20200921074716.GC12990@dhcp22.suse.cz> <20200921154558.GD29330@paulmck-ThinkPad-P72> <20200921160318.GO12990@dhcp22.suse.cz> <20200921194819.GA24236@pc636> <20200922075002.GU12990@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200922075002.GU12990@dhcp22.suse.cz> User-Agent: Mutt/1.9.4 (2018-02-28) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Sep 22, 2020 at 09:50:02AM +0200, Michal Hocko wrote: > [Cc Mel - the thread starts http://lkml.kernel.org/r/20200918194817.48921-1-urezki@gmail.com] > > On Mon 21-09-20 21:48:19, Uladzislau Rezki wrote: > > Hello, Michal. > > > > > > > > > > Yes, I do well remember that you are unhappy with this approach. > > > > Unfortunately, thus far, there is no solution that makes all developers > > > > happy. You might be glad to hear that we are also looking into other > > > > solutions, each of which makes some other developers unhappy. So we > > > > are at least not picking on you alone. :-/ > > > > > > No worries I do not feel like a whipping boy here. But do expect me to > > > argue against the approach. I would also appreciate it if there was some > > > more information on other attempts, why they have failed. E.g. why > > > pre-allocation is not an option that works well enough in most > > > reasonable workloads. > > Pre-allocating has some drawbacks: > > > > a) It is impossible to predict how many pages will be required to > > cover a demand that is controlled by different workloads on > > various systems. > > Yes, this is not trivial but not a rocket science either. Remember that > you are relying on a very dumb watermark based pcp pool from the > allocator. Mimicing a similar implementation shouldn't be all that hard > and you will get your own pool which doesn't affect other page allocator > users as much as a bonus. > > > b) Memory overhead since we do not know how much pages should be > > preloaded: 100, 200 or 300 > > Does anybody who really needs this optimization actually cares about 300 > pages? That would be 100-300 (maybe more) pages -per- -CPU-, so yes, some people will care quite deeply about this. Thanx, Paul > > As for memory overhead, it is important to reduce it because of > > embedded devices like phones, where a low memory condition is a > > big issue. In that sense pre-allocating is something that we strongly > > would like to avoid. > > How big "machines" are we talking about here? I would expect that really > tiny machines would have hard times to really fill up thousands of pages > with pointers to free... > > Would a similar scaling as the page allocator feasible. Really I mostly > do care about shared nature of the pcp allocator list that one user can > easily monopolize with this API. > > > > I would also appreciate some more thoughts why we > > > need to optimize for heavy abusers of RCU (like close(open) extremes). > > > > > I think here is a small misunderstanding. Please note, that is not only > > about performance and corner cases. There is a single argument support > > of the kvfree_rcu(ptr), where maintaining an array in time is needed. > > The fallback of the single argument case is extrimely slow. > > This should be part of the changelog. > > > > Single-argument details is here: https://lkml.org/lkml/2020/4/28/1626 > > Error 501 > > > > > > I strongly agree with Thomas http://lkml.kernel.org/r/87tux4kefm.fsf@nanos.tec.linutronix.de > > > > > that this optimization is not aiming at reasonable workloads. Really, go > > > > > with pre-allocated buffer and fallback to whatever slow path you have > > > > > already. Exposing more internals of the allocator is not going to do any > > > > > good for long term maintainability. > > > > > > > > I suggest that you carefully re-read the thread following that email. > > > > > > I clearly remember Thomas not being particularly happy that you optimize > > > for a corner case. I do not remember there being a consensus that this > > > is the right approach. There was some consensus that this is better than > > > a gfp flag. Still quite bad though if you ask me. > > > > > > > Given a choice between making users unhappy and making developers > > > > unhappy, I will side with the users each and every time. > > > > > > Well, let me rephrase. It is not only about me (as a developer) being > > > unhappy but also all the side effects this would have for users when > > > performance of their favorite workload declines for no apparent reason > > > just because pcp caches are depleted by an unrelated process. > > > > > If depleted, we have a special worker that charge it. From the other hand, > > the pcplist can be depleted by its nature, what _is_ not wrong. But just > > in case we secure it since you had a concern about it. > > pcp free lists should ever get empty when we run out of memory and need > to reclaim. Otherwise they are constantly refilled/rebalanced on demand. > The fact that you are refilling them from outside just suggest that you > are operating on a wrong layer. Really, create your own pool of pages > and rebalance them based on the workload. > > > Could you please specify a real test case or workload you are talking about? > > I am not a performance expert but essentially any memory allocator heavy > workload might notice. I am pretty sure Mel would tell you more. > > -- > Michal Hocko > SUSE Labs