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=-2.8 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 ABA1CC2D0F1 for ; Tue, 31 Mar 2020 17:49:07 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 7286C21473 for ; Tue, 31 Mar 2020 17:49:07 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="cYgWjwN1" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 7286C21473 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 11FAE6B000E; Tue, 31 Mar 2020 13:49:07 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 0AA086B0037; Tue, 31 Mar 2020 13:49:07 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id E8BFD6B006C; Tue, 31 Mar 2020 13:49:06 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0057.hostedemail.com [216.40.44.57]) by kanga.kvack.org (Postfix) with ESMTP id CCFBC6B000E for ; Tue, 31 Mar 2020 13:49:06 -0400 (EDT) Received: from smtpin09.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with ESMTP id 978F82C89 for ; Tue, 31 Mar 2020 17:49:06 +0000 (UTC) X-FDA: 76656393492.09.hat47_89ac4ca568a60 X-HE-Tag: hat47_89ac4ca568a60 X-Filterd-Recvd-Size: 4448 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by imf06.hostedemail.com (Postfix) with ESMTP for ; Tue, 31 Mar 2020 17:49:05 +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 04076212CC; Tue, 31 Mar 2020 17:49:05 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1585676945; bh=7YB6ND8mrDeGDIhLt1OHuowTw1wah86m08QhZkfLdrg=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=cYgWjwN1R88kTK3yT2gG1k3SpiZ9Vz0fYzWRtS56ooUUeRv4wJ+zDEmlu3P8LJOWP h8OEYSp4AikI9nTg/2BD2m4OiX2DEGbbakpNo75tOVvcmM9QGxkB+itppGyk5tKydr bh3+XF8WM7K0qL2ikOs7IigsYlsGE+HIoUSRWE8Q= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 72EED35226C3; Tue, 31 Mar 2020 10:49:04 -0700 (PDT) Date: Tue, 31 Mar 2020 10:49:04 -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: <20200331174904.GN19865@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> <20200331170232.GA28413@pc636> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200331170232.GA28413@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 Tue, Mar 31, 2020 at 07:02:32PM +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. > > > Just adding more thoughts about such concern. Even though in theory we > can run into something like that. But also please note, that under high > memory pressure it also does not mean that (X) will always succeed with > further infinite allocations, so memory pressure is something common. > As soon as the situation becomes slightly better we do our work much > efficient. > > Practically, i was trying to simulate memory pressure to hit synchronize_rcu() > on my test system. By just simulating head-less freeing(for any object) and > by always dynamic attaching path. So i could trigger it, but that was really > hard to achieve and it happened only few times. So that was not like a constant > hit. What i got constantly were: > > - System got recovered and proceed with "normal" path; > - The OOM hit as a final step, when the system is run out of memory fully. > > So, practically i have not seen massive synchronize_rcu() hit. Understood, but given the attractive properties of headless kfree_rcu(), it is not unreasonable to expect its usage to remain low. In addition, memory-pressure scenarios can be quite involved. Finally, as Joel pointed out offlist, the per-CPU cached structure acts as a small portion of kfree_rcu()-specific reserved memory, so you guys have at least partially addressed parts of my concerns already. I am not at all a fan of using GFP_MEMALLOC because kfree_rcu() is sufficiently low-level to be in the business of ensuring its own forward progress. Thanx, Paul