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=-20.5 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_GIT 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 42E30C6379B for ; Wed, 21 Jul 2021 20:21:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 29CB86127C for ; Wed, 21 Jul 2021 20:21:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240437AbhGUTlA (ORCPT ); Wed, 21 Jul 2021 15:41:00 -0400 Received: from mail.kernel.org ([198.145.29.99]:50102 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S240399AbhGUTky (ORCPT ); Wed, 21 Jul 2021 15:40:54 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id F02016139A; Wed, 21 Jul 2021 20:21:29 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1626898890; bh=MwdcEzyqxvGGf9rRbzj9W8+01WNtdWk5AM4dfPjKIEU=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=K13FYWMXVDFnYgHMWDqShTRRCby/S3XEDN1NsAWSgfRe4Lln9ZUG+EX8GJe/yrWFI 0iljXuShFyFQvWaDPRRyOfLnH0j4ySizE0uC9EFjH/zJr/5V9Pmb8KZOGh7lxamGj8 NLmRU/BpiJfVD3ST19GG9RqsOQpsbX1cckPppr75+BN6bsV39xCSVwEeFdlfXlybJm xEg9ZADcb2Gs+iUH+m0wwZsRadf/fy5InpNrXnzUmCGH+ciRhqCPdq1pXV3PQCUEvi 3sP16LZc5C2NI+jlyRQzJ5WQEYo3tkEgcY8FSoB4MczliifLAovLvJanXFi6E6F4qB C0CaNQc/qgg8A== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 4CA415C0F00; Wed, 21 Jul 2021 13:21:29 -0700 (PDT) From: "Paul E. McKenney" To: rcu@vger.kernel.org Cc: linux-kernel@vger.kernel.org, kernel-team@fb.com, mingo@kernel.org, jiangshanlai@gmail.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org, Frederic Weisbecker , Neeraj Upadhyay , Uladzislau Rezki , Boqun Feng , "Paul E . McKenney" Subject: [PATCH rcu 17/18] rcu: Explain why rcu_all_qs() is a stub in preemptible TREE RCU Date: Wed, 21 Jul 2021 13:21:25 -0700 Message-Id: <20210721202127.2129660-17-paulmck@kernel.org> X-Mailer: git-send-email 2.31.1.189.g2e36527f23 In-Reply-To: <20210721202042.GA1472052@paulmck-ThinkPad-P17-Gen-1> References: <20210721202042.GA1472052@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org From: Frederic Weisbecker The cond_resched() function reports an RCU quiescent state only in non-preemptible TREE RCU implementation. This commit therefore adds a comment explaining why cond_resched() does nothing in preemptible kernels. Signed-off-by: Frederic Weisbecker Cc: Neeraj Upadhyay Cc: Joel Fernandes Cc: Uladzislau Rezki Cc: Boqun Feng Cc: Peter Zijlstra (Intel) Cc: Ingo Molnar Signed-off-by: Paul E. McKenney --- kernel/sched/core.c | 11 +++++++++++ 1 file changed, 11 insertions(+) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 2d9ff40f46619..6a03c3fac55cc 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -7781,6 +7781,17 @@ int __sched __cond_resched(void) preempt_schedule_common(); return 1; } + /* + * In preemptible kernels, ->rcu_read_lock_nesting tells the tick + * whether the current CPU is in an RCU read-side critical section, + * so the tick can report quiescent states even for CPUs looping + * in kernel context. In contrast, in non-preemptible kernels, + * RCU readers leave no in-memory hints, which means that CPU-bound + * processes executing in kernel context might never report an + * RCU quiescent state. Therefore, the following code causes + * cond_resched() to report a quiescent state, but only when RCU + * is in urgent need of one. + */ #ifndef CONFIG_PREEMPT_RCU rcu_all_qs(); #endif -- 2.31.1.189.g2e36527f23