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=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=ham 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 9B919C433DF for ; Wed, 27 May 2020 15:57:05 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 764DD20776 for ; Wed, 27 May 2020 15:57:05 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1590595025; bh=Ik6t3kw79TqNezTHS+OiIuNo2tlFgr4eybIwx61pe/8=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=OGG2ImT04+thmgAiBXdsec7LPyFukVopm6Ni0UserDj3pgTf5a125O8SWnW/TyjP1 Qs9ODHN/gVp3FGt0NNFYeUjaKWbeEx3+2rgvRefnepP2sJu575bwxFXvE+Df/qHQiM gJ4S0wSPdvOfZcORKOoQl96uxmbWjCLWu4OyOODs= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730886AbgE0P5E (ORCPT ); Wed, 27 May 2020 11:57:04 -0400 Received: from mail.kernel.org ([198.145.29.99]:41252 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730870AbgE0P45 (ORCPT ); Wed, 27 May 2020 11:56:57 -0400 Received: from paulmck-ThinkPad-P72.home (50-39-105-78.bvtn.or.frontiernet.net [50.39.105.78]) (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 1682320776; Wed, 27 May 2020 15:56:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1590595017; bh=Ik6t3kw79TqNezTHS+OiIuNo2tlFgr4eybIwx61pe/8=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=ooaILInhRblQrCFPRyWa/J6/8OQCLqzd3B+W0MH4F3bJu7mCY5F7RVJzR++paYS+6 mAxqnYdkYb5zDA5FlQYjrOE4JounVdJ88UBifHr/S6Vx9GUKBZRRx3P8fvR0Ae02QS /TR5lXbcf2etddptyx08Pq6Bbad0r8xzRHhD8R40= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id DA74F3522839; Wed, 27 May 2020 08:56:56 -0700 (PDT) Date: Wed, 27 May 2020 08:56:56 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: tglx@linutronix.de, frederic@kernel.org, linux-kernel@vger.kernel.org, x86@kernel.org, cai@lca.pw, mgorman@techsingularity.net, joel@joelfernandes.org Subject: Re: [RFC][PATCH 4/7] smp: Optimize send_call_function_single_ipi() Message-ID: <20200527155656.GU2869@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200526161057.531933155@infradead.org> <20200526161907.953304789@infradead.org> <20200527095645.GH325280@hirez.programming.kicks-ass.net> <20200527101513.GJ325303@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200527101513.GJ325303@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 Wed, May 27, 2020 at 12:15:13PM +0200, Peter Zijlstra wrote: > On Wed, May 27, 2020 at 11:56:45AM +0200, Peter Zijlstra wrote: > > > This is rcu_is_cpu_rrupt_from_idle()'s lockdep_assert_in_irq() tripping > > up (it's comment is obviously a bit antiquated). > > > > Now, if I read that code correctly, it actually relies on > > rcu_irq_enter() and thus really wants to be in an interrupt. Is there > > any way this code can be made to work from the new context too? > > > > After all, all that really is different is not having gone throught he > > bother of setting up the IRQ context, nothing else changed -- it just so > > happens you actually relied on that ;/ > > At first glance, something like the below could work. But obviously I > might have overlooked something more subtle than a brick :-) This can work, but only if the call from the idle loop is a place where either RCU isn't watching on the one hand or that cannot be in an RCU read-side critical section on the other. Because rcu_exp_handler() assumes that if this function returns true, we are not in an RCU read-side critical section. (I would expect this to be the case, but I figured that I should make it explicit.) > --- > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 90c8be22d57a..0792c032a972 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -426,8 +426,11 @@ EXPORT_SYMBOL_GPL(rcu_momentary_dyntick_idle); > */ Could we please have a comment noting the change in semantics and the reason? > static int rcu_is_cpu_rrupt_from_idle(void) > { > - /* Called only from within the scheduling-clock interrupt */ > - lockdep_assert_in_irq(); > + /* > + * Usually called from the tick; but also used from smp_call_function() > + * for expedited grace periods. > + */ > + lockdep_assert_irqs_disabled(); > > /* Check for counter underflows */ > RCU_LOCKDEP_WARN(__this_cpu_read(rcu_data.dynticks_nesting) < 0, > @@ -435,8 +438,11 @@ static int rcu_is_cpu_rrupt_from_idle(void) > RCU_LOCKDEP_WARN(__this_cpu_read(rcu_data.dynticks_nmi_nesting) <= 0, > "RCU dynticks_nmi_nesting counter underflow/zero!"); > > - /* Are we at first interrupt nesting level? */ > - if (__this_cpu_read(rcu_data.dynticks_nmi_nesting) != 1) > + /* > + * Are we at first interrupt nesting level? -- or below, when running > + * directly from the idle loop itself. > + */ > + if (__this_cpu_read(rcu_data.dynticks_nmi_nesting) > 1) Wouldn't it also be a good idea to check that we are in the context of an idle thread? Just in case some idiot like me drops a call to this function in the wrong place, for example, if I were to mistakenly remember the old semantics where it would return false from process context? Maybe something like this? nesting = __this_cpu_read(rcu_data.dynticks_nmi_nesting; if (nesting > 1) return false; WARN_ON_ONCE(!nesting && !is_idle_task(current)); > return false; > > /* Does CPU appear to be idle from an RCU standpoint? */ And let's check the other callers: rcu_sched_clock_irq(): This will always be called from IRQ (right?), so no problem. rcu_pending(): Only called from rcu_sched_clock_irq(), so still no problem. rcu_flavor_sched_clock_irq(): Ditto for both definitions. Thanx, Paul