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=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, 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 71DCFC2D0F0 for ; Wed, 1 Apr 2020 13:47:48 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 3248920787 for ; Wed, 1 Apr 2020 13:47:48 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="n9dQjVEJ" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 3248920787 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 C697A8E0006; Wed, 1 Apr 2020 09:47:47 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id C19B88E0001; Wed, 1 Apr 2020 09:47:47 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id B30798E0006; Wed, 1 Apr 2020 09:47:47 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0082.hostedemail.com [216.40.44.82]) by kanga.kvack.org (Postfix) with ESMTP id 9B70B8E0001 for ; Wed, 1 Apr 2020 09:47:47 -0400 (EDT) Received: from smtpin11.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with ESMTP id 5F59A180AD80F for ; Wed, 1 Apr 2020 13:47:47 +0000 (UTC) X-FDA: 76659414174.11.lunch45_7e28a6a0d2427 X-HE-Tag: lunch45_7e28a6a0d2427 X-Filterd-Recvd-Size: 4828 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by imf05.hostedemail.com (Postfix) with ESMTP for ; Wed, 1 Apr 2020 13:47:46 +0000 (UTC) Received: from paulmck-ThinkPad-P72.home (50-39-105-78.bvtn.or.frontiernet.net [50.39.105.78]) (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 C8DB9206F5; Wed, 1 Apr 2020 13:47:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1585748865; bh=CvwldOZwrOCHacHE+jmnj9rjAqgJ9cJeX4YmgaNFGyQ=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=n9dQjVEJndzYqA/D9qyvlmeRzljwKA0hKpPcwuwEarzVa+LiZrEsSpVfsP+S8HIDJ mzfj1ojIB7cIlLeirIEAO9Ok5YuWlmxTGha+MxAf7z2o9S93ZZtW4SUNWAVDsTM65t 3ijG+Q8QhIylR5xd7VP0XDa0zmWTzp02vPvydJRw= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 9645535226AC; Wed, 1 Apr 2020 06:47:45 -0700 (PDT) Date: Wed, 1 Apr 2020 06:47:45 -0700 From: "Paul E. McKenney" To: Uladzislau Rezki Cc: Joel Fernandes , linux-kernel@vger.kernel.org, linux-mm@kvack.org, rcu@vger.kernel.org, willy@infradead.org, peterz@infradead.org, neilb@suse.com, vbabka@suse.cz, mgorman@suse.de, Andrew Morton , Josh Triplett , Lai Jiangshan , Mathieu Desnoyers , Steven Rostedt Subject: Re: [PATCH RFC] rcu/tree: Use GFP_MEMALLOC for alloc memory to free memory pattern Message-ID: <20200401134745.GV19865@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200331131628.153118-1-joel@joelfernandes.org> <20200331140433.GA26498@pc636> <20200331150911.GC236678@google.com> <20200331160119.GA27614@pc636> <20200331183000.GD236678@google.com> <20200401122550.GA32593@pc636> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200401122550.GA32593@pc636> User-Agent: Mutt/1.9.4 (2018-02-28) 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 Wed, Apr 01, 2020 at 02:25:50PM +0200, Uladzislau Rezki wrote: [ . . . ] > > > > Paul was concerned about following scenario with hitting synchronize_rcu(): > > > > 1. Consider a system under memory pressure. > > > > 2. Consider some other subsystem X depending on another system Y which uses > > > > kfree_rcu(). If Y doesn't complete the operation in time, X accumulates > > > > more memory. > > > > 3. Since kfree_rcu() on Y hits synchronize_rcu() a lot, it slows it down. > > > > This causes X to further allocate memory, further causing a chain > > > > reaction. > > > > Paul, please correct me if I'm wrong. > > > > > > > I see your point and agree that in theory it can happen. So, we should > > > make it more tight when it comes to rcu_head attachment logic. > > > > Right. Per discussion with Paul, we discussed that it is better if we > > pre-allocate N number of array blocks per-CPU and use it for the cache. > > Default for N being 1 and tunable with a boot parameter. I agree with this. > > > As discussed before, we can make use of memory pool API for such > purpose. But i am not sure if it should be one pool per CPU or > one pool per NR_CPUS, that would contain NR_CPUS * N pre-allocated > blocks. There are advantages and disadvantages either way. The advantage of the per-CPU pool is that you don't have to worry about something like lock contention causing even more pain during an OOM event. One potential problem wtih the per-CPU pool can happen when callbacks are offloaded, in which case the CPUs needing the memory might never be getting it, because in the offloaded case (RCU_NOCB_CPU=y) the CPU posting callbacks might never be invoking them. But from what I know now, systems built with CONFIG_RCU_NOCB_CPU=y either don't have heavy callback loads (HPC systems) or are carefully configured (real-time systems). Plus large systems would probably end up needing something pretty close to a slab allocator to keep from dying from lock contention, and it is hard to justify that level of complexity at this point. Or is there some way to mark a specific slab allocator instance as being able to keep some amount of memory no matter what the OOM conditions are? If not, the current per-CPU pre-allocated cache is a better choice in the near term. Thanx, Paul > > In current code, we have 1 cache page per CPU, but this is allocated only on > > the first kvfree_rcu() request. So we could change this behavior as well to > > make it pre-allocated. > > > > Does this all sound good to you? > > > I think that makes sense :) > > -- > Vlad Rezki