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=-5.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,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 23743C4741F for ; Fri, 25 Sep 2020 16:17:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D9BE32076B for ; Fri, 25 Sep 2020 16:17:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729437AbgIYQRS (ORCPT ); Fri, 25 Sep 2020 12:17:18 -0400 Received: from outbound-smtp30.blacknight.com ([81.17.249.61]:49877 "EHLO outbound-smtp30.blacknight.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728524AbgIYQRS (ORCPT ); Fri, 25 Sep 2020 12:17:18 -0400 Received: from mail.blacknight.com (pemlinmail01.blacknight.ie [81.17.254.10]) by outbound-smtp30.blacknight.com (Postfix) with ESMTPS id 52887BAC34 for ; Fri, 25 Sep 2020 17:17:15 +0100 (IST) Received: (qmail 28638 invoked from network); 25 Sep 2020 16:17:15 -0000 Received: from unknown (HELO techsingularity.net) (mgorman@techsingularity.net@[84.203.22.4]) by 81.17.254.9 with ESMTPSA (AES256-SHA encrypted, authenticated); 25 Sep 2020 16:17:15 -0000 Date: Fri, 25 Sep 2020 17:17:12 +0100 From: Mel Gorman To: Uladzislau Rezki Cc: Michal Hocko , "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: <20200925161712.GM3179@techsingularity.net> References: <20200921160318.GO12990@dhcp22.suse.cz> <20200921194819.GA24236@pc636> <20200922075002.GU12990@dhcp22.suse.cz> <20200922131257.GA29241@pc636> <20200923103706.GJ3179@techsingularity.net> <20200923154105.GO29330@paulmck-ThinkPad-P72> <20200923232251.GK3179@techsingularity.net> <20200924081614.GA14819@pc636> <20200925080503.GC3389@dhcp22.suse.cz> <20200925153129.GB25350@pc636> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <20200925153129.GB25350@pc636> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Sep 25, 2020 at 05:31:29PM +0200, Uladzislau Rezki wrote: > > > > > > > > > > All good points! > > > > > > > > > > On the other hand, duplicating a portion of the allocator functionality > > > > > within RCU increases the amount of reserved memory, and needlessly most > > > > > of the time. > > > > > > > > > > > > > But it's very similar to what mempools are for. > > > > > > > As for dynamic caching or mempools. It requires extra logic on top of RCU > > > to move things forward and it might be not efficient way. As a side > > > effect, maintaining of the bulk arrays in the separate worker thread > > > will introduce other drawbacks: > > > > This is true but it is also true that it is RCU to require this special > > logic and we can expect that we might need to fine tune this logic > > depending on the RCU usage. We definitely do not want to tune the > > generic page allocator for a very specific usecase, do we? > > > I look at it in scope of GFP_ATOMIC/GFP_NOWAIT issues, i.e. inability > to provide a memory service for contexts which are not allowed to > sleep, RCU is part of them. Both flags used to provide such ability > before but not anymore. > > Do you agree with it? > I was led to believe that hte problem was taking the zone lock while holding a raw spinlock that was specific to RCU. Are you saying that GFP_ATOMIC/GFP_NOWAIT users are also holding raw spinlocks at the same time on RT? -- Mel Gorman SUSE Labs