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,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 23387C433E4 for ; Thu, 13 Aug 2020 17:27:59 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id EBC6320781 for ; Thu, 13 Aug 2020 17:27:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1597339679; bh=1J9DGzv1tvKPIFHOskjX9pySZrD3/jFzJAYWaDPvh3Y=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=F2qVfbCOgWehzHEBga0xhE4ET8JvdFLkX8TU7IxUoEhuSUxmN4MEJiC9Tqu6XpSI5 MAD+8DxIsY+ciOR/K3Bivjx0Mmjk/UJRcXQHwsjNCSRMn3GQK3JjlRjvzMds3rWWzH dt3KSXQLvzbwpIuQzrqWyBlFHPfcq8LqBO404eVU= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726622AbgHMR15 (ORCPT ); Thu, 13 Aug 2020 13:27:57 -0400 Received: from mail.kernel.org ([198.145.29.99]:48390 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726522AbgHMR14 (ORCPT ); Thu, 13 Aug 2020 13:27:56 -0400 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 027BC20774; Thu, 13 Aug 2020 17:27:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1597339675; bh=1J9DGzv1tvKPIFHOskjX9pySZrD3/jFzJAYWaDPvh3Y=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=Bc/A3/sr8cu5ewCaN5zT7oIyt4YTgCBU/BIJi7D/mJcv0NDb5xv/dNEVPnFUAWpHt h5nBGRlU5PeRUpqAXuh20MC9QZSTQ88JNRDnJxR0yg2ZGpGXbZzDAg87p4v2Wi1lwr bVonXjFFXd+IzTtnH01nD1vNEaFm+L3DpNpK4zAA= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id B8A30352279C; Thu, 13 Aug 2020 10:27:54 -0700 (PDT) Date: Thu, 13 Aug 2020 10:27:54 -0700 From: "Paul E. McKenney" To: Michal Hocko Cc: Thomas Gleixner , Uladzislau Rezki , 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: <20200813172754.GA4295@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> <20200813154159.GR4295@paulmck-ThinkPad-P72> <20200813155412.GP9477@dhcp22.suse.cz> <20200813160442.GV4295@paulmck-ThinkPad-P72> <20200813161357.GQ9477@dhcp22.suse.cz> <20200813162904.GX4295@paulmck-ThinkPad-P72> <20200813171211.GT9477@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200813171211.GT9477@dhcp22.suse.cz> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Aug 13, 2020 at 07:12:11PM +0200, Michal Hocko wrote: > On Thu 13-08-20 09:29:04, Paul E. McKenney wrote: > > On Thu, Aug 13, 2020 at 06:13:57PM +0200, Michal Hocko wrote: > > > On Thu 13-08-20 09:04:42, Paul E. McKenney wrote: > > > > On Thu, Aug 13, 2020 at 05:54:12PM +0200, Michal Hocko wrote: > > > [...] > > > > > If the whole bailout is guarded by CONFIG_PREEMPT_RT specific atomicity > > > > > check then there is no functional problem - GFP_RT_SAFE would still be > > > > > GFP_NOWAIT so functional wise the allocator will still do the right > > > > > thing. > > > > > > > > Perhaps it was just me getting confused, early hour Pacific Time and > > > > whatever other excuses might apply. But I thought that you still had > > > > an objection to GFP_RT_SAFE based on changes in allocator semantics for > > > > other users. > > > > > > There is still that problem with lockdep complaining about raw->regular > > > spinlock on !PREEMPT_RT that would need to get resolved somehow. Thomas > > > is not really keen on adding some lockdep annotation mechanism and > > > unfortunatelly I do not have a different idea how to get rid of those. > > > > OK. So the current situation requires a choice between these these > > alternatives, each of which has shortcomings that have been mentioned > > earlier in this thread: > > > > 1. Prohibit invoking allocators from raw atomic context, such > > as when holding a raw spinlock. > > > > 2. Adding a GFP_ flag. > > Which would implemente a completely new level atomic allocation for all > preemption models > > > > > 3. Reusing existing GFP_ flags/values/whatever to communicate > > the raw-context information that was to be communicated by > > the new GFP_ flag. > > this would have to be RT specific to not change the semantic for > existing users. In other words make NOWAIT semantic working for > RT atomic contexts. > > > > > 4. Making lockdep forgive acquiring spinlocks while holding > > raw spinlocks, but only in CONFIG_PREEMPT_NONE=y kernels. > > and this would have to go along with 3 to remove false positives on !RT. OK, let's fill in the issues, then: 1. Prohibit invoking allocators from raw atomic context, such as when holding a raw spinlock. o This would prevent an important cache-locality optimization. 2. Adding a GFP_ flag. o Requires a new level atomic allocation for all preemption models, namely, confined to the allocator's lockless caches. 3. Reusing existing GFP_ flags/values/whatever to communicate the raw-context information that was to be communicated by the new GFP_ flag. o There are existing users of all combinations that might be unhappy with a change of semantics. o But Michal is OK with this if usage is restricted to RT. Except that this requires #4 below: 4. Making lockdep forgive acquiring spinlocks while holding raw spinlocks, but only in CONFIG_PREEMPT_NONE=y kernels. o This would allow latency degradation and other bad coding practices to creep in, per Thomas's recent email. Again, am I missing anything? Thanx, Paul