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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 686DFC54EE9 for ; Wed, 7 Sep 2022 15:15:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230049AbiIGPO7 (ORCPT ); Wed, 7 Sep 2022 11:14:59 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52452 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229614AbiIGPOx (ORCPT ); Wed, 7 Sep 2022 11:14:53 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 224BF2528C; Wed, 7 Sep 2022 08:14:51 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id B0D0661987; Wed, 7 Sep 2022 15:14:50 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id BDDDFC433D7; Wed, 7 Sep 2022 15:14:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1662563690; bh=5AbubiYwxEPaLYyJ/ZkHFOYsZR8C3QQzlzKNYzwGstw=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=svJ/p1fBjlhUuC3NjkkkOnG0a5mN5dGvdkQVPSjacIqDu0UiuvDxvN6B5A/37TDUG M/e8UF/yY8zUN+v2oVpcOsXGBgszGMl1dXtpoQUelWvfuSZe29bB45/TIa5lnAMoyW VQujbw8IRuRNL0+KYgj0sGD+wu3OJx/NMuG16SyYedMgwVgS/hBO9rQLnO4m5BNegO huIPnOAR/si2YsCBjSWfTF8p9gJapGzpDGp+qDEA25qZ9FXL3q+JZaYCIphSL5J7Po Iz27/Mxawf9g+NXhPzFu5CmuuKtkt4O3yiAPSuQFvWkApsiAg4DtuJPN9CN14rLKPD sbkbAv8Qy2hSw== Date: Wed, 7 Sep 2022 17:14:47 +0200 From: Frederic Weisbecker To: "Paul E. McKenney" Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com, rostedt@goodmis.org, Zqiang Subject: Re: [PATCH rcu 03/10] rcu: Add QS check in rcu_exp_handler() for non-preemptible kernels Message-ID: <20220907151447.GA198228@lothringen> References: <20220831180759.GA2693289@paulmck-ThinkPad-P17-Gen-1> <20220831180805.2693546-3-paulmck@kernel.org> <20220907121010.GA196513@lothringen> <20220907145759.GJ4315@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220907145759.GJ4315@paulmck-ThinkPad-P17-Gen-1> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Sep 07, 2022 at 07:57:59AM -0700, Paul E. McKenney wrote: > On Wed, Sep 07, 2022 at 02:10:10PM +0200, Frederic Weisbecker wrote: > > On Wed, Aug 31, 2022 at 11:07:58AM -0700, Paul E. McKenney wrote: > > > From: Zqiang > > > > > > Kernels built with CONFIG_PREEMPTION=n and CONFIG_PREEMPT_COUNT=y maintain > > > preempt_count() state. Because such kernels map __rcu_read_lock() > > > and __rcu_read_unlock() to preempt_disable() and preempt_enable(), > > > respectively, this allows the expedited grace period's !CONFIG_PREEMPT_RCU > > > version of the rcu_exp_handler() IPI handler function to use > > > preempt_count() to detect quiescent states. > > > > > > This preempt_count() usage might seem to risk failures due to > > > use of implicit RCU readers in portions of the kernel under #ifndef > > > CONFIG_PREEMPTION, except that rcu_core() already disallows such implicit > > > RCU readers. The moral of this story is that you must use explicit > > > read-side markings such as rcu_read_lock() or preempt_disable() even if > > > the code knows that this kernel does not support preemption. > > > > > > This commit therefore adds a preempt_count()-based check for a quiescent > > > state in the !CONFIG_PREEMPT_RCU version of the rcu_exp_handler() > > > function for kernels built with CONFIG_PREEMPT_COUNT=y, reporting an > > > immediate quiescent state when the interrupted code had both preemption > > > and softirqs enabled. > > > > > > This change results in about a 2% reduction in expedited grace-period > > > latency in kernels built with both CONFIG_PREEMPT_RCU=n and > > > CONFIG_PREEMPT_COUNT=y. > > > > > > Signed-off-by: Zqiang > > > Signed-off-by: Paul E. McKenney > > > Link: https://lore.kernel.org/all/20220622103549.2840087-1-qiang1.zhang@intel.com/ > > > --- > > > kernel/rcu/tree_exp.h | 4 +++- > > > 1 file changed, 3 insertions(+), 1 deletion(-) > > > > > > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h > > > index be667583a5547..b07998159d1fa 100644 > > > --- a/kernel/rcu/tree_exp.h > > > +++ b/kernel/rcu/tree_exp.h > > > @@ -828,11 +828,13 @@ static void rcu_exp_handler(void *unused) > > > { > > > struct rcu_data *rdp = this_cpu_ptr(&rcu_data); > > > struct rcu_node *rnp = rdp->mynode; > > > + bool preempt_bh_enabled = !(preempt_count() & (PREEMPT_MASK | > > > SOFTIRQ_MASK)); > > > > I don't know if nested hardirqs still exist. I only heard old rumours > > about broken drivers. Should we take care of them? > > Last I checked, certain tracing scenarios from irq handlers looked > to RCU like nested irq handlers. Given that, does your more robust > approach below work correctly? I haven't observed that but in any case, the check I propose is more strict than the one on this patch. So in the worst case it's a QS not reported if a nested interrupt is detected. Thanks.