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=-19.0 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,URIBL_BLOCKED,USER_AGENT_GIT autolearn=unavailable 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 042BBC4360C for ; Thu, 4 Mar 2021 01:08:58 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E3C7B64EAE for ; Thu, 4 Mar 2021 01:08:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1356586AbhCDBE5 (ORCPT ); Wed, 3 Mar 2021 20:04:57 -0500 Received: from mail.kernel.org ([198.145.29.99]:59552 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1355337AbhCDAd1 (ORCPT ); Wed, 3 Mar 2021 19:33:27 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id D561D64EE4; Thu, 4 Mar 2021 00:32:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1614817966; bh=kTKBnDf6S5oQSLVubizC3D3zva8T+YonBRP+hWWdGXo=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=WgrxnXhgchztLRPFUzBr0yyTNHw3Xs+Q8Vi7IQKinIyRbCtyB4oIFPcAQ1Tt3tIXE JMKt1OytcF5xZQjgbIwwSulLaw37chNb1zR2ClbjBRqYJcdkBaGHV+uo4m4EcNAtoG SM5zbgFfTFyeq+TM3zsNiTWzaTUrWjuk3hZtiy8OXujwwJymJNXuMbFSqotr1LY4p/ mhQYiB4vngDFSKFeuWa+pn7vvOfNGRNGz/4Ubgzk6t71sqvDT+noMeoDTXsMCsi1dq A1rDtqlwOUucaIQPL+c8UCuEoPigoZz3OqOudf60a1iTwCFhqTqJNz1S787XiUIF1l Lx4/ye310oAvQ== From: paulmck@kernel.org 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, "Paul E. McKenney" Subject: [PATCH tip/core/rcu 2/2] rcu-tasks: Add block comment laying out RCU Tasks Trace design Date: Wed, 3 Mar 2021 16:32:45 -0800 Message-Id: <20210304003245.24327-2-paulmck@kernel.org> X-Mailer: git-send-email 2.9.5 In-Reply-To: <20210304003226.GA24261@paulmck-ThinkPad-P72> References: <20210304003226.GA24261@paulmck-ThinkPad-P72> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: "Paul E. McKenney" This commit adds a block comment that gives a high-level overview of how RCU tasks trace grace periods progress. It also adds a note about how exiting tasks are handled, plus it gives an overview of the memory ordering. Reported-by: Peter Zijlstra Reported-by: Mathieu Desnoyers [ paulmck: Fix commit log per Mathieu Desnoyers feedback. ] Signed-off-by: Paul E. McKenney --- kernel/rcu/tasks.h | 36 ++++++++++++++++++++++++++++++++++++ 1 file changed, 36 insertions(+) diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h index 17c8ebe..f818357 100644 --- a/kernel/rcu/tasks.h +++ b/kernel/rcu/tasks.h @@ -726,6 +726,42 @@ EXPORT_SYMBOL_GPL(show_rcu_tasks_rude_gp_kthread); // flavors, rcu_preempt and rcu_sched. The fact that RCU Tasks Trace // readers can operate from idle, offline, and exception entry/exit in no // way allows rcu_preempt and rcu_sched readers to also do so. +// +// The implementation uses rcu_tasks_wait_gp(), which relies on function +// pointers in the rcu_tasks structure. The rcu_spawn_tasks_trace_kthread() +// function sets these function pointers up so that rcu_tasks_wait_gp() +// invokes these functions in this order: +// +// rcu_tasks_trace_pregp_step(): +// Initialize the count of readers and block CPU-hotplug operations. +// rcu_tasks_trace_pertask(), invoked on every non-idle task: +// Initialize per-task state and attempt to identify an immediate +// quiescent state for that task, or, failing that, attempt to set +// that task's .need_qs flag so that that task's next outermost +// rcu_read_unlock_trace() will report the quiescent state (in which +// case the count of readers is incremented). If both attempts fail, +// the task is added to a "holdout" list. +// rcu_tasks_trace_postscan(): +// Initialize state and attempt to identify an immediate quiescent +// state as above (but only for idle tasks), unblock CPU-hotplug +// operations, and wait for an RCU grace period to avoid races with +// tasks that are in the process of exiting. +// check_all_holdout_tasks_trace(), repeatedly until holdout list is empty: +// Scans the holdout list, attempting to identify a quiescent state +// for each task on the list. If there is a quiescent state, the +// corresponding task is removed from the holdout list. +// rcu_tasks_trace_postgp(): +// Wait for the count of readers do drop to zero, reporting any stalls. +// Also execute full memory barriers to maintain ordering with code +// executing after the grace period. +// +// The exit_tasks_rcu_finish_trace() synchronizes with exiting tasks. +// +// Pre-grace-period update-side code is ordered before the grace +// period via the ->cbs_lock and barriers in rcu_tasks_kthread(). +// Pre-grace-period read-side code is ordered before the grace period by +// atomic_dec_and_test() of the count of readers (for IPIed readers) and by +// scheduler context-switch ordering (for locked-down non-running readers). // The lockdep state must be outside of #ifdef to be useful. #ifdef CONFIG_DEBUG_LOCK_ALLOC -- 2.9.5