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 D16E3C47095 for ; Tue, 6 Oct 2020 10:03:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9B5FC2080A for ; Tue, 6 Oct 2020 10:03:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726138AbgJFKDj (ORCPT ); Tue, 6 Oct 2020 06:03:39 -0400 Received: from outbound-smtp36.blacknight.com ([46.22.139.219]:58051 "EHLO outbound-smtp36.blacknight.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725943AbgJFKDj (ORCPT ); Tue, 6 Oct 2020 06:03:39 -0400 Received: from mail.blacknight.com (pemlinmail05.blacknight.ie [81.17.254.26]) by outbound-smtp36.blacknight.com (Postfix) with ESMTPS id 6B98116D0 for ; Tue, 6 Oct 2020 11:03:37 +0100 (IST) Received: (qmail 29118 invoked from network); 6 Oct 2020 10:03:37 -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); 6 Oct 2020 10:03:36 -0000 Date: Tue, 6 Oct 2020 11:03:34 +0100 From: Mel Gorman To: "Paul E. McKenney" Cc: Peter Zijlstra , Michal Hocko , Uladzislau Rezki , Vlastimil Babka , LKML , RCU , linux-mm@kvack.org, Andrew Morton , Thomas Gleixner , "Theodore Y . Ts'o" , Joel Fernandes , Sebastian Andrzej Siewior , Oleksiy Avramchenko Subject: Re: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func. Message-ID: <20201006100334.GK3227@techsingularity.net> References: <20200929220742.GB8768@pc636> <795d6aea-1846-6e08-ac1b-dbff82dd7133@suse.cz> <20201001192626.GA29606@pc636> <20201002071123.GB20872@dhcp22.suse.cz> <20201002085014.GC3227@techsingularity.net> <20201002090729.GU2628@hirez.programming.kicks-ass.net> <20201002094502.GD3227@techsingularity.net> <20201002095858.GN2611@hirez.programming.kicks-ass.net> <20201002101952.GE3227@techsingularity.net> <20201002144120.GI29330@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <20201002144120.GI29330@paulmck-ThinkPad-P72> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Oct 02, 2020 at 07:41:20AM -0700, Paul E. McKenney wrote: > On Fri, Oct 02, 2020 at 11:19:52AM +0100, Mel Gorman wrote: > > On Fri, Oct 02, 2020 at 11:58:58AM +0200, Peter Zijlstra wrote: > > > > It's enabled by default by enough distros that adding too many checks > > > > is potentially painful. Granted it would be missed by most benchmarking > > > > which tend to control allocations from userspace but a lot of performance > > > > problems I see are the "death by a thousand cuts" variety. > > > > > > Oh quite agreed, aka death by accounting. But if people are enabling > > > DEBUG options in production kernels, there's something wrong, no? > > > > > > > You'd think but historically I believe DEBUG_VM was enabled for some > > distributions because it made certain classes of problems easier to debug > > early. There is also a recent trend for enabling various DEBUG options for > > "hardening" even when they protect very specific corner cases or are for > > intended for kernel development. I've pushed back where I have an opinion > > that matters but it's generally corrosive. > > > > > Should we now go add CONFIG_REALLY_DEBUG_STAY_AWAY_ALREADY options? > > > > It's heading in that direction :( > > Given that you guys have just reiterated yet again that you are very > unhappy with either a GFP_ flag or a special function like the one that > Peter Zijlstra put together, it would be very helpful if you were to at > least voice some level of support for Thomas Gleixner's patchset, which, > if accepted, will allow me to solve at least 50% of the problem. > I read through the series and didn't find anything problematic that had not been covered already. Minimally, avoiding surprises about what preemptible() means in different contexts is nice. While I have not run it through a test grid to check, I'd be very surprised if this was problematic from a performance perspective on a preempt-disabled kernels. Last I checked, the difference between PREEMPT_NONE and PREEMPT_VOLUNTARY was less than 2% *at worst* and I don't think that was due to the preempt accounting. -- Mel Gorman SUSE Labs