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 A1038C433DF for ; Thu, 13 Aug 2020 18:52:58 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8342A20838 for ; Thu, 13 Aug 2020 18:52:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1597344778; bh=2mtKqGaeDkG8Ee6GYdHtGerH4iVD6LZfV/g1lZcfGsg=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=s2lrrLwFSj2noU2CtYZ25esLbc9h53g5xJzRKcpwO2/M0J0Or3Ym/0klqeSPR9dQh A7aswacqfIOvuaCIpAT7+ikGr1mUtgE6hv7ux1aHhN0RM2VKVUxOOSj9LqnKOke8s2 TVilZhq2zvYiZhvWpU1YCVDSeqeWxvyVSypgpzeQ= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726253AbgHMSw6 (ORCPT ); Thu, 13 Aug 2020 14:52:58 -0400 Received: from mail.kernel.org ([198.145.29.99]:50332 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726167AbgHMSw6 (ORCPT ); Thu, 13 Aug 2020 14:52:58 -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 5A2EF20774; Thu, 13 Aug 2020 18:52:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1597344777; bh=2mtKqGaeDkG8Ee6GYdHtGerH4iVD6LZfV/g1lZcfGsg=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=uTMvGRFrpS2IgaqAEMO9B9XA/NuHEQtdGsjW9Rgdx6F0TcGUpampmvjFvu+JF2O3G jL8RAe3HF19kd2xITEDQWGruPNzzD+XJp0B6/Dxe0LtTGFb3ZvXedFrgjHw8wONwzM Z1+mI5lj4ZRC3S4R2W7gSIyCiDH52Klb7HMs87as= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 2F969352279C; Thu, 13 Aug 2020 11:52:57 -0700 (PDT) Date: Thu, 13 Aug 2020 11:52:57 -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: <20200813185257.GF4295@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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200813182618.GX2674@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Thu, Aug 13, 2020 at 08:26:18PM +0200, peterz@infradead.org wrote: > On Thu, Aug 13, 2020 at 04:34:57PM +0200, Thomas Gleixner wrote: > > Michal Hocko writes: > > > On Thu 13-08-20 15:22:00, Thomas Gleixner wrote: > > >> It basically requires to convert the wait queue to something else. Is > > >> the waitqueue strict single waiter? > > > > > > I would have to double check. From what I remember only kswapd should > > > ever sleep on it. > > > > That would make it trivial as we could simply switch it over to rcu_wait. > > > > >> So that should be: > > >> > > >> if (!preemptible() && gfp == GFP_RT_NOWAIT) > > >> > > >> which is limiting the damage to those callers which hand in > > >> GFP_RT_NOWAIT. > > >> > > >> lockdep will yell at invocations with gfp != GFP_RT_NOWAIT when it hits > > >> zone->lock in the wrong context. And we want to know about that so we > > >> can look at the caller and figure out how to solve it. > > > > > > Yes, that would have to somehow need to annotate the zone_lock to be ok > > > in those paths so that lockdep doesn't complain. > > > > That opens the worst of all cans of worms. If we start this here then > > Joe programmer and his dog will use these lockdep annotation to evade > > warnings and when exposed to RT it will fall apart in pieces. Just that > > at that point Joe programmer moved on to something else and the usual > > suspects can mop up the pieces. We've seen that all over the place and > > some people even disable lockdep temporarily because annotations don't > > help. > > > > PeterZ might have opinions about that too I suspect. > > PeterZ is mightily confused by all of this -- also heat induced brain > melt. > > 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. If the allocation fails, for example, during OOM events, we fall back to the linked-list approach. So, as with much of the rest of the kernel, under OOM we just run a bit slower. Thanx, Paul