From nobody Fri Oct 17 10:32:12 2025 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 EE040C433FE for ; Wed, 19 Oct 2022 10:58:21 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231714AbiJSK6T (ORCPT ); Wed, 19 Oct 2022 06:58:19 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49718 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234748AbiJSK4p (ORCPT ); Wed, 19 Oct 2022 06:56:45 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B79A7BB2; Wed, 19 Oct 2022 03:27:51 -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 ams.source.kernel.org (Postfix) with ESMTPS id 615E9B82400; Wed, 19 Oct 2022 09:07:38 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id CA2C5C433D6; Wed, 19 Oct 2022 09:07:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1666170457; bh=EpH8F25ftH4qXMt5E1/aEv6zxIBXRJCjac9DWYRJ0Jc=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=ahEE6/0ubT+ZsrDMqXgYRehTmoCwCckBwG+Drr5loJ2osrBiXwEm9tZTHut123VOH DhQUGUGMF+k0YGBOU6W+gzTdTfueRAXSSqupZrOQzevA7+dpLu5DL3QqOpnVkWhcNK ySiQbpUkBuIGCOR20GR5SVjNUxTIgx88fjWf/Jzo= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Zqiang , "Paul E. McKenney" , Sasha Levin Subject: [PATCH 6.0 662/862] rcu: Avoid triggering strict-GP irq-work when RCU is idle Date: Wed, 19 Oct 2022 10:32:29 +0200 Message-Id: <20221019083319.221155015@linuxfoundation.org> X-Mailer: git-send-email 2.38.0 In-Reply-To: <20221019083249.951566199@linuxfoundation.org> References: <20221019083249.951566199@linuxfoundation.org> User-Agent: quilt/0.67 MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" From: Zqiang [ Upstream commit 621189a1fe93cb2b34d62c5cdb9e258bca044813 ] Kernels built with PREEMPT_RCU=3Dy and RCU_STRICT_GRACE_PERIOD=3Dy trigger irq-work from rcu_read_unlock(), and the resulting irq-work handler invokes rcu_preempt_deferred_qs_handle(). The point of this triggering is to force grace periods to end quickly in order to give tools like KASAN a better chance of detecting RCU usage bugs such as leaking RCU-protected pointers out of an RCU read-side critical section. However, this irq-work triggering is unconditional. This works, but there is no point in doing this irq-work unless the current grace period is waiting on the running CPU or task, which is not the common case. After all, in the common case there are many rcu_read_unlock() calls per CPU per grace period. This commit therefore triggers the irq-work only when the current grace period is waiting on the running CPU or task. This change was tested as follows on a four-CPU system: echo rcu_preempt_deferred_qs_handler > /sys/kernel/debug/tracing/set_ftrac= e_filter echo 1 > /sys/kernel/debug/tracing/function_profile_enabled insmod rcutorture.ko sleep 20 rmmod rcutorture.ko echo 0 > /sys/kernel/debug/tracing/function_profile_enabled echo > /sys/kernel/debug/tracing/set_ftrace_filter This procedure produces results in this per-CPU set of files: /sys/kernel/debug/tracing/trace_stat/function* Sample output from one of these files is as follows: Function Hit Time Avg = s^2 -------- --- ---- --- = --- rcu_preempt_deferred_qs_handle 838746 182650.3 us 0.217 us = 0.004 us The baseline sum of the "Hit" values (the number of calls to this function) was 3,319,015. With this commit, that sum was 1,140,359, for a 2.9x reduction. The worst-case variance across the CPUs was less than 25%, so this large effect size is statistically significant. The raw data is available in the Link: URL. Link: https://lore.kernel.org/all/20220808022626.12825-1-qiang1.zhang@intel= .com/ Signed-off-by: Zqiang Signed-off-by: Paul E. McKenney Signed-off-by: Sasha Levin --- kernel/rcu/tree_plugin.h | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index 438ecae6bd7e..49468b4d1b43 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -641,7 +641,8 @@ static void rcu_read_unlock_special(struct task_struct = *t) =20 expboost =3D (t->rcu_blocked_node && READ_ONCE(t->rcu_blocked_node->exp_= tasks)) || (rdp->grpmask & READ_ONCE(rnp->expmask)) || - IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) || + (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) && + ((rdp->grpmask & READ_ONCE(rnp->qsmask)) || t->rcu_blocked_node)) || (IS_ENABLED(CONFIG_RCU_BOOST) && irqs_were_disabled && t->rcu_blocked_node); // Need to defer quiescent state until everything is enabled. --=20 2.35.1