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=-5.1 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,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 F1474C2D0E2 for ; Tue, 22 Sep 2020 13:13:05 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 6CDA82396D for ; Tue, 22 Sep 2020 13:13:05 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="g/uufrCp" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 6CDA82396D Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=gmail.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id BFD3290007E; Tue, 22 Sep 2020 09:13:04 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id B86AD900063; Tue, 22 Sep 2020 09:13:04 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id A263890007E; Tue, 22 Sep 2020 09:13:04 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0017.hostedemail.com [216.40.44.17]) by kanga.kvack.org (Postfix) with ESMTP id 80B5B900063 for ; Tue, 22 Sep 2020 09:13:04 -0400 (EDT) Received: from smtpin23.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay05.hostedemail.com (Postfix) with ESMTP id 17343181AC9BF for ; Tue, 22 Sep 2020 13:13:04 +0000 (UTC) X-FDA: 77290737888.23.loss57_5607e152714e Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin23.hostedemail.com (Postfix) with ESMTP id CEDA437606 for ; Tue, 22 Sep 2020 13:13:03 +0000 (UTC) X-HE-Tag: loss57_5607e152714e X-Filterd-Recvd-Size: 9995 Received: from mail-lj1-f194.google.com (mail-lj1-f194.google.com [209.85.208.194]) by imf50.hostedemail.com (Postfix) with ESMTP for ; Tue, 22 Sep 2020 13:13:01 +0000 (UTC) Received: by mail-lj1-f194.google.com with SMTP id b19so14062571lji.11 for ; Tue, 22 Sep 2020 06:13:01 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=from:date:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=2z8KLYcV2WyIOYkugsYC62B54jH3ZqWmai+zjoiHURM=; b=g/uufrCp4Tieb1/bP3Sk+L6niHHhoLetIn719xh8g8W3aR+ShzNF+eI0YiqyuwjztJ lxyPlfmXWIUwg7FR8tFYeiyWvGtJ/G85fUPGe9y4AuTlcLO+Hj+xTmzEr0K5zEDipXbg v6TlSBvwk3T7CCc7GECiGJ0V/WgZGWIgr7FejBbqX9kiQG65CcX3obzHIvZYFIt88IwX /VuONgmxdlA+9utpKa6wzIjyJnEBeWMe3jgpnI3lzbNS2lcHNJjQEALgj9zD2KdoGVoy v8JKrvSiMWWC1EPk0Clq2i9Unfxpe39ZItLJ/TIMywKgXbLTY9HH9I6/NF9uR75pckYW KOYw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:date:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=2z8KLYcV2WyIOYkugsYC62B54jH3ZqWmai+zjoiHURM=; b=pnrnMm0+p3xmXkXRO8Dljtae9+69SsvzvOjTGqYVIiG0C3I5BlR2QZwwU5mPvPgPWw YRviN2DJqubWDY3yCtP75mFwjSfLz1VCl3Jw3326k4P2QT+ZPU5lsy1q755H0YCEtRkB vyiwkrnJzvZou+aA4UotIOW5RyjUAd6Sl+IF6QlVynRmD74isrdWqPUgPx20aJBPn8KD GNxxmu6GZIqmen9OVE6WpMMu7/I83PH4sJ6HFSVNoBKFU38UwsWuKoH+/RNVsXmIT09+ 8zyNwb63PcDXbmN0zbQL2Eqd7/wdZlVew+klMveisHN3nT5+qfRJBmged08mWdSTROVq Pgzg== X-Gm-Message-State: AOAM531z+scMt84jjlC4mvsb1Nhn0WCpiHWvXdZ8juH6Qy3vHIX6POw4 xPS2r7iGb2gbGcQ2AsulAcs= X-Google-Smtp-Source: ABdhPJynL2GGAC64a28puyBSgBYDy+TPN8QQij3z4tz/mhOyog30o8cnGg8b25izSCXatkCgDS7C7w== X-Received: by 2002:a2e:964e:: with SMTP id z14mr1450241ljh.86.1600780380316; Tue, 22 Sep 2020 06:13:00 -0700 (PDT) Received: from pc636 (h5ef52e31.seluork.dyn.perspektivbredband.net. [94.245.46.49]) by smtp.gmail.com with ESMTPSA id y10sm3472646lfj.271.2020.09.22.06.12.59 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 22 Sep 2020 06:12:59 -0700 (PDT) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Tue, 22 Sep 2020 15:12:57 +0200 To: Michal Hocko Cc: Uladzislau Rezki , "Paul E. McKenney" , 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: <20200922131257.GA29241@pc636> 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/1.10.1 (2018-07-13) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: > > > > 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. > We rely on it, indeed. If the pcp-cache is depleted our special work is triggered to charge our local cache(few pages) such way will also initiate the process of pre-featching pages from the buddy allocator populating the depleted pcp-cache. I do not have any concern here. > > 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. > I see your point Michal. As i mentioned before, it is important to avoid of having such own pools, because the aim is not to waste memory resources. A page will be returned back to "page allocator" as soon as a scheduler place our reclaim thread on a CPU and grace period is passed. So, the resource can be used for other needs. What is important. Otherwise a memory footprint is increased what is bad for low memory conditions when OOM is involved. Just in case, it is a big issue for mobile devices. > > 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? > It might be an issue for embedded devices when such devices run into a low memory condition resulting in OOM or slow allocations due to mentioned condition. For servers and big system it will not be visible. > > 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... > I mentioned above. We can not rely on static model. We would like to have a mechanism that gives back ASAP used pages to page allocator for other needs. > > 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 see your concern. pcplist can be monopolized by already existing API: while (i < 100) __get_free_page(GFP_NOWAIT | __GFP_NOWARN); > > > 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. > Hmm.. I think it is. Sorry if i missed that but i hope i mentioned about it. > > > > Single-argument details is here: https://lkml.org/lkml/2020/4/28/1626 > > Error 501 > Could you please elaborate? Do not want to speculate :) > > > > > 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. > I covered it above. > > 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. > OK. Thank you for your comments, Michal! -- Vlad Rezki