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=-5.6 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 1C69DC433DF for ; Fri, 14 Aug 2020 18:01:49 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id BB0602078D for ; Fri, 14 Aug 2020 18:01:48 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="U9vNT1mS" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org BB0602078D Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 3E60B8D0003; Fri, 14 Aug 2020 14:01:48 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 397328D0002; Fri, 14 Aug 2020 14:01:48 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 284E98D0003; Fri, 14 Aug 2020 14:01:48 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0013.hostedemail.com [216.40.44.13]) by kanga.kvack.org (Postfix) with ESMTP id 1022D8D0002 for ; Fri, 14 Aug 2020 14:01:48 -0400 (EDT) Received: from smtpin13.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with ESMTP id B08384DDF for ; Fri, 14 Aug 2020 18:01:47 +0000 (UTC) X-FDA: 77149942254.13.sack44_4a02b0c26ffe Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin13.hostedemail.com (Postfix) with ESMTP id 4593E18140645 for ; Fri, 14 Aug 2020 18:01:43 +0000 (UTC) X-HE-Tag: sack44_4a02b0c26ffe X-Filterd-Recvd-Size: 7538 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by imf08.hostedemail.com (Postfix) with ESMTP for ; Fri, 14 Aug 2020 18:01:42 +0000 (UTC) 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 5889220771; Fri, 14 Aug 2020 18:01:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1597428101; bh=tlSBhpa4ToPHr0zR2hPhfIijVZq2J3X4X8TvGLSLkgU=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=U9vNT1mSKZy7w26/Fkt1gMXQdcR78Clo1DaaDjTx23XBJP544U/oZs0qT0S0/OczB eANl7ItRov1iQjT496z3+SF0LsTUlx0R5Zke9QLWzXvTfb4+J05zob5gC+DzYaZfFI j1YupVg3WG/EErHmAjFzlongRtzP3F+F1yePIsDQ= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 0FC073522A0E; Fri, 14 Aug 2020 11:01:41 -0700 (PDT) Date: Fri, 14 Aug 2020 11:01:41 -0700 From: "Paul E. McKenney" To: Michal Hocko Cc: Uladzislau Rezki , Thomas Gleixner , LKML , RCU , linux-mm@kvack.org, Andrew Morton , Vlastimil Babka , Matthew Wilcox , "Theodore Y . Ts'o" , Joel Fernandes , Sebastian Andrzej Siewior , Oleksiy Avramchenko , Peter Zijlstra Subject: Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag Message-ID: <20200814180141.GP4295@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <874kp6llzb.fsf@nanos.tec.linutronix.de> <20200813133308.GK9477@dhcp22.suse.cz> <87sgcqty0e.fsf@nanos.tec.linutronix.de> <20200813145335.GN9477@dhcp22.suse.cz> <87lfiitquu.fsf@nanos.tec.linutronix.de> <20200814071750.GZ9477@dhcp22.suse.cz> <20200814121544.GA32598@pc636> <20200814124832.GD9477@dhcp22.suse.cz> <20200814133450.GK4295@paulmck-ThinkPad-P72> <20200814140604.GE9477@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200814140604.GE9477@dhcp22.suse.cz> User-Agent: Mutt/1.9.4 (2018-02-28) X-Rspamd-Queue-Id: 4593E18140645 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam02 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: On Fri, Aug 14, 2020 at 04:06:04PM +0200, Michal Hocko wrote: > On Fri 14-08-20 06:34:50, Paul E. McKenney wrote: > > On Fri, Aug 14, 2020 at 02:48:32PM +0200, Michal Hocko wrote: > > > On Fri 14-08-20 14:15:44, Uladzislau Rezki wrote: > > > > > On Thu 13-08-20 19:09:29, Thomas Gleixner wrote: > > > > > > Michal Hocko writes: > > > > > [...] > > > > > > > Why should we limit the functionality of the allocator for something > > > > > > > that is not a real problem? > > > > > > > > > > > > We'd limit the allocator for exactly ONE new user which was aware of > > > > > > this problem _before_ the code hit mainline. And that ONE user is > > > > > > prepared to handle the fail. > > > > > > > > > > If we are to limit the functionality to this one particular user then > > > > > I would consider a dedicated gfp flag a huge overkill. It would be much > > > > > more easier to have a preallocated pool of pages and use those and > > > > > completely avoid the core allocator. That would certainly only shift the > > > > > complexity to the caller but if it is expected there would be only that > > > > > single user then it would be probably better than opening a can of worms > > > > > like allocator usable from raw spin locks. > > > > > > > > > Vlastimil raised same question earlier, i answered, but let me answer again: > > > > > > > > It is hard to achieve because the logic does not stick to certain static test > > > > case, i.e. it depends on how heavily kfree_rcu(single/double) are used. Based > > > > on that, "how heavily" - number of pages are formed, until the drain/reclaimer > > > > thread frees them. > > > > > > How many pages are talking about - ball park? 100s, 1000s? > > > > Under normal operation, a couple of pages per CPU, which would make > > preallocation entirely reasonable. Except that if someone does something > > that floods RCU callbacks (close(open) in a tight userspace loop, for but > > one example), then 2000 per CPU might not be enough, which on a 64-CPU > > system comes to about 500MB. This is beyond excessive for preallocation > > on the systems I am familiar with. > > > > And the flooding case is where you most want the reclamation to be > > efficient, and thus where you want the pages. > > I am not sure the page allocator would help you with this scenario > unless you are on very large machines. Pagesets scale with the available > memory and percpu_pagelist_fraction sysctl (have a look at > pageset_set_high_and_batch). It is roughly 1000th of the zone size for > each zone. You can check that in /proc/vmstat (my 8G machine) Small systems might have ~64G. The medium-sized systems might have ~250G. There are a few big ones that might have 1.5T. None of the /proc/vmstat files from those machines contain anything resembling the list below, though. > Node 0, zone DMA > Not interesting at all > Node 0, zone DMA32 > pagesets > cpu: 0 > count: 242 > high: 378 > batch: 63 > cpu: 1 > count: 355 > high: 378 > batch: 63 > cpu: 2 > count: 359 > high: 378 > batch: 63 > cpu: 3 > count: 366 > high: 378 > batch: 63 > Node 0, zone Normal > pagesets > cpu: 0 > count: 359 > high: 378 > batch: 63 > cpu: 1 > count: 241 > high: 378 > batch: 63 > cpu: 2 > count: 297 > high: 378 > batch: 63 > cpu: 3 > count: 227 > high: 378 > batch: 63 > > Besides that do you need to be per-cpu? Having 1000 pages available and > managed under your raw spinlock should be good enough already no? It needs to be almost entirely per-CPU for performance reasons. Plus a user could do a tight close(open()) loop on each CPU. > > This of course raises the question of how much memory the lockless caches > > contain, but fortunately these RCU callback flooding scenarios also > > involve process-context allocation of the memory that is being passed > > to kfree_rcu(). That allocation should keep the lockless caches from > > going empty in the common case, correct? > > Yes, those are refilled both on the allocation/free paths. But you > cannot really rely on that to happen early enough. So the really ugly scenarios with the tight loops normally allocate something and immediately either call_rcu() or kfree_rcu() it. But you are right, someone doing "rm -rf" on a large file tree with lots of small files might not be doing that many allocations. > Do you happen to have any numbers that would show the typical usage > and how often the slow path has to be taken becase pcp lists are > depleted? In other words even if we provide a functionality to give > completely lockless way to allocate memory how useful that is? Not yet, but let's see what we can do. Thanx, Paul