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 4EE2CC433DF for ; Thu, 13 Aug 2020 23:23:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 27CE820838 for ; Thu, 13 Aug 2020 23:23:15 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1597360995; bh=EF/BQjTohXyx9VE5DQ/ITuXE4RPxFZdWjxUAUMPQgis=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=UIKiQC6yTk61MQEDnG0A2KtX3WCbec9GaB8FD8fJP+UA6csD5dcixwUzmg08ZSAeC 3sPL5l0CI/usHl9tdtpnCCykFJMoEn8AicsQoyXQ8ytqDdUC2DzqBjmbKPmqz+0NFk 3gXZZn3KFFCTDmNGGldyi0t+FqxjP7rwsg2Kizsk= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726623AbgHMXXN (ORCPT ); Thu, 13 Aug 2020 19:23:13 -0400 Received: from mail.kernel.org ([198.145.29.99]:49204 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726205AbgHMXXM (ORCPT ); Thu, 13 Aug 2020 19:23:12 -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 0CE6F2078B; Thu, 13 Aug 2020 23:23:12 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1597360992; bh=EF/BQjTohXyx9VE5DQ/ITuXE4RPxFZdWjxUAUMPQgis=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=OaolggrC6srwXS8tGt+JS42uCRCpec88GQCLh5QhHajIEVXDFMShzGP7TW190sPDr pE+gnLxrebTbmEzgL+JxQ/PdE7R008T4SWLfTEVxp0xpcNg+0fiBEUqmrspPhUKZ3v OoXglsjuwb9N5DsG9S6DlUNTRtGdWbr5XqFwVD8w= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id E3E15352279C; Thu, 13 Aug 2020 16:23:11 -0700 (PDT) Date: Thu, 13 Aug 2020 16:23:11 -0700 From: "Paul E. McKenney" To: peterz@infradead.org Cc: Thomas Gleixner , Michal Hocko , 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 Subject: Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag Message-ID: <20200813232311.GJ4295@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200811210931.GZ4295@paulmck-ThinkPad-P72> <874kp87mca.fsf@nanos.tec.linutronix.de> <20200813075027.GD9477@dhcp22.suse.cz> <20200813095840.GA25268@pc636> <874kp6llzb.fsf@nanos.tec.linutronix.de> <20200813133308.GK9477@dhcp22.suse.cz> <87sgcqty0e.fsf@nanos.tec.linutronix.de> <20200813182618.GX2674@hirez.programming.kicks-ass.net> <20200813185257.GF4295@paulmck-ThinkPad-P72> <20200813220619.GA2674@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200813220619.GA2674@hirez.programming.kicks-ass.net> 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 Fri, Aug 14, 2020 at 12:06:19AM +0200, peterz@infradead.org wrote: > On Thu, Aug 13, 2020 at 11:52:57AM -0700, Paul E. McKenney wrote: > > On Thu, Aug 13, 2020 at 08:26:18PM +0200, peterz@infradead.org wrote: > > > > I thought the rule was: > > > > > > - No allocators (alloc/free) inside raw_spinlock_t, full-stop. > > > > > > Why are we trying to craft an exception? > > > > So that we can reduce post-grace-period cache misses by a factor of > > eight when invoking RCU callbacks. This reduction in cache misses also > > makes it more difficult to overrun RCU with floods of either call_rcu() > > or kfree_rcu() invocations. > > > > The idea is to allocate page-sized arrays of pointers so that the callback > > invocation can sequence through the array instead of walking a linked > > list, hence the reduction in cache misses. > > I'm still not getting it, how do we end up trying to allocate memory > from under raw spinlocks if you're not allowed to use kfree_rcu() under > one ? You are indeed not allowed to use kfree() under a raw spinlock, given that it can acquire a non-raw spinlock. But kfree_rcu() was just a wrapper around call_rcu(), which can be and is called from raw atomic context. > Can someone please spell out the actual problem? And as noted above, reducing the kfree()-time cache misses would be a good thing. Thanx, Paul