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=-17.5 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,URIBL_BLOCKED,USER_AGENT_SANE_1 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 AB1A0C433DB for ; Thu, 25 Feb 2021 21:08:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 763D964E83 for ; Thu, 25 Feb 2021 21:08:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235017AbhBYVHt (ORCPT ); Thu, 25 Feb 2021 16:07:49 -0500 Received: from mail.kernel.org ([198.145.29.99]:56508 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234612AbhBYU42 (ORCPT ); Thu, 25 Feb 2021 15:56:28 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id 5BEBF64E28; Thu, 25 Feb 2021 20:55:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1614286544; bh=xp+Ya1f7eA0CAwrMV+RyvxCVmV65qyl6ZQeqq3b5zNI=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=FN8t790iSgUqSVAwJ5xFvuQfXmE8LFx3NH8isZ7HAShyGbefJENPRs63HLBoiS9rX xlVF41e9KHpMnw/PpHKjAz4v6+PKbLi6KAS9B6t9WWQOwtBG/QJjIGSVrRASijHvID CB3LOETUstUTBnrrYXDR6J+E+1HndkENtoawB2H3eExnrECE009ambM6Q3HS2FJyRV 1Q+Q7Lbwln/G2YTDgdOAtG6W5veLOC877MiGQUSGw4I+h70EMQc1Em/bf2UiJtCdCu /nUjz6z+EpF3XARZQ+0sy0lLDziumw1s5D24r6ESlhnGCvdH0CpNJMYD27fB4Zawjk K/Bm8z3Sfnphg== Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id F39EE35227D5; Thu, 25 Feb 2021 12:55:43 -0800 (PST) Date: Thu, 25 Feb 2021 12:55:43 -0800 From: "Paul E. McKenney" To: Mathieu Desnoyers Cc: linux-kernel , rcu , Peter Zijlstra , Josh Triplett , rostedt , Lai Jiangshan , "Joel Fernandes, Google" Subject: Re: tasks-trace RCU: question about grace period forward progress Message-ID: <20210225205543.GV2743@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <354598689.4868.1614262968890.JavaMail.zimbra@efficios.com> <20210225153656.GQ2743@paulmck-ThinkPad-P72> <47558398.5024.1614268052985.JavaMail.zimbra@efficios.com> <20210225183321.GT2743@paulmck-ThinkPad-P72> <2096901411.5697.1614284434230.JavaMail.zimbra@efficios.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <2096901411.5697.1614284434230.JavaMail.zimbra@efficios.com> User-Agent: Mutt/1.9.4 (2018-02-28) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Feb 25, 2021 at 03:20:34PM -0500, Mathieu Desnoyers wrote: > ----- On Feb 25, 2021, at 1:33 PM, paulmck paulmck@kernel.org wrote: > [...] > > commit 581f79546b6be406a9c7280b2d3511b60821efe0 > > Author: Paul E. McKenney > > Date: Thu Feb 25 10:26:00 2021 -0800 > > > > rcu-tasks: Add block comment laying out RCU Tasks Trace design > > > > 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 handles, plus it gives an overview of the memory > > handles -> handled Good eyes, fixed! > > ordering. > > > > Reported-by: Peter Zijlstra > > Reported-by: Mathieu Desnoyers > > Signed-off-by: Paul E. McKenney > > > > 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 rest looks good, thanks! Thank you for looking it over! Thanx, Paul