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 0C708C38147 for ; Wed, 18 Jan 2023 18:08:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231327AbjARSIW (ORCPT ); Wed, 18 Jan 2023 13:08:22 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60888 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230171AbjARSHm (ORCPT ); Wed, 18 Jan 2023 13:07:42 -0500 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 6E92956EEC; Wed, 18 Jan 2023 10:07:17 -0800 (PST) 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 ams.source.kernel.org (Postfix) with ESMTPS id 207A4B81C66; Wed, 18 Jan 2023 18:07:16 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id C1BB8C433D2; Wed, 18 Jan 2023 18:07:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1674065234; bh=ASgMJN11LRkZcvhH2vNd49vPSeWr47r6GOPKrQfkOMg=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=mE687v8Zu+Uw01zoSDV+x0Nf0erR6F+JaF83WrhBGZu6qHHb0O2IDsV0yDUNoDkXg Q5Zo0/SMOdGD4O6XTpUodtnWMAWTiIdsdssPghBC4UOKhdRpjFXlxYjg1wsPiFoIIL 453JlqpAV5Rh5PZEMQGj8JtWiS+aExwaGff9N3UBCDlcv7G001nawxo+g8IyUQo9TJ JpAgzlRl8sHyExpXBaEnNx8RdlZ0KzRu0BpTCVqMf/rjHQsBDIlnyP0WZKuCCjS8oH fyR4x3HkSJyUdi29EhFhf5oiYTFFooGymHpVYqk6neT9lVIYxWUTC7kE9fuNGeclhj AKsY1ibf+wxzQ== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 587BD5C0920; Wed, 18 Jan 2023 10:07:14 -0800 (PST) Date: Wed, 18 Jan 2023 10:07:14 -0800 From: "Paul E. McKenney" To: Zqiang Cc: frederic@kernel.org, quic_neeraju@quicinc.com, joel@joelfernandes.org, rcu@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v3] rcu: Remove impossible wakeup rcu GP kthread action from rcu_report_qs_rdp() Message-ID: <20230118180714.GD2948950@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20230118073014.2020743-1-qiang1.zhang@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230118073014.2020743-1-qiang1.zhang@intel.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jan 18, 2023 at 03:30:14PM +0800, Zqiang wrote: > When inovke rcu_report_qs_rdp(), if current CPU's rcu_data structure's -> > grpmask has not been cleared from the corresponding rcu_node structure's > ->qsmask, after that will clear and report quiescent state, but in this > time, this also means that current grace period is not end, the current > grace period is ongoing, because the rcu_gp_in_progress() currently return > true, so for non-offloaded rdp, invoke rcu_accelerate_cbs() is impossible > to return true. > > This commit therefore remove impossible rcu_gp_kthread_wake() calling. > > Signed-off-by: Zqiang > Reviewed-by: Frederic Weisbecker Queued (wordsmithed as shown below, as always, please check) for further testing and review, thank you both! Thanx, Paul ------------------------------------------------------------------------ commit fbe3e300ec8b3edd2b8f84dab4dc98947cf71eb8 Author: Zqiang Date: Wed Jan 18 15:30:14 2023 +0800 rcu: Remove never-set needwake assignment from rcu_report_qs_rdp() The rcu_accelerate_cbs() function is invoked by rcu_report_qs_rdp() only if there is a grace period in progress that is still blocked by at least one CPU on this rcu_node structure. This means that rcu_accelerate_cbs() should never return the value true, and thus that this function should never set the needwake variable and in turn never invoke rcu_gp_kthread_wake(). This commit therefore removes the needwake variable and the invocation of rcu_gp_kthread_wake() in favor of a WARN_ON_ONCE() on the call to rcu_accelerate_cbs(). The purpose of this new WARN_ON_ONCE() is to detect situations where the system's opinion differs from ours. Signed-off-by: Zqiang Reviewed-by: Frederic Weisbecker Signed-off-by: Paul E. McKenney diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index b2c2045294780..7a3085ad0a7df 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -1956,7 +1956,6 @@ rcu_report_qs_rdp(struct rcu_data *rdp) { unsigned long flags; unsigned long mask; - bool needwake = false; bool needacc = false; struct rcu_node *rnp; @@ -1988,7 +1987,12 @@ rcu_report_qs_rdp(struct rcu_data *rdp) * NOCB kthreads have their own way to deal with that... */ if (!rcu_rdp_is_offloaded(rdp)) { - needwake = rcu_accelerate_cbs(rnp, rdp); + /* + * The current GP has not yet ended, so it + * should not be possible for rcu_accelerate_cbs() + * to return true. So complain, but don't awaken. + */ + WARN_ON_ONCE(rcu_accelerate_cbs(rnp, rdp)); } else if (!rcu_segcblist_completely_offloaded(&rdp->cblist)) { /* * ...but NOCB kthreads may miss or delay callbacks acceleration @@ -2000,8 +2004,6 @@ rcu_report_qs_rdp(struct rcu_data *rdp) rcu_disable_urgency_upon_qs(rdp); rcu_report_qs_rnp(mask, rnp, rnp->gp_seq, flags); /* ^^^ Released rnp->lock */ - if (needwake) - rcu_gp_kthread_wake(); if (needacc) { rcu_nocb_lock_irqsave(rdp, flags);