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 1AA90C433FE for ; Wed, 20 Apr 2022 15:32:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1380239AbiDTPfY (ORCPT ); Wed, 20 Apr 2022 11:35:24 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:36768 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1380153AbiDTPfW (ORCPT ); Wed, 20 Apr 2022 11:35:22 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 262B63EAB6; Wed, 20 Apr 2022 08:32:36 -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 D06C4B81EB2; Wed, 20 Apr 2022 15:32:34 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3E4C9C385AB; Wed, 20 Apr 2022 15:32:33 +0000 (UTC) Date: Wed, 20 Apr 2022 11:32:31 -0400 From: Steven Rostedt To: "Paul E. McKenney" Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com Subject: Re: [PATCH rcu 1/2] docs: Add documentation for rude and trace RCU flavors Message-ID: <20220420113231.6d3202e4@gandalf.local.home> In-Reply-To: <20220418225033.3944860-1-paulmck@kernel.org> References: <20220418225004.GA3944767@paulmck-ThinkPad-P17-Gen-1> <20220418225033.3944860-1-paulmck@kernel.org> X-Mailer: Claws Mail 3.17.8 (GTK+ 2.24.33; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 18 Apr 2022 15:50:32 -0700 "Paul E. McKenney" wrote: > This commit belatedly adds documentation of Tasks Rude RCU and Tasks > Trace RCU to RCU's requirements document. > > Signed-off-by: Paul E. McKenney > --- > .../RCU/Design/Requirements/Requirements.rst | 32 +++++++++++++++++++ > 1 file changed, 32 insertions(+) > > diff --git a/Documentation/RCU/Design/Requirements/Requirements.rst b/Documentation/RCU/Design/Requirements/Requirements.rst > index 45278e2974c0..ff2be1ac54c4 100644 > --- a/Documentation/RCU/Design/Requirements/Requirements.rst > +++ b/Documentation/RCU/Design/Requirements/Requirements.rst > @@ -2654,6 +2654,38 @@ synchronize_rcu(), and rcu_barrier(), respectively. In > three APIs are therefore implemented by separate functions that check > for voluntary context switches. > > +Tasks Rude RCU > +~~~~~~~~~~~~~~ > + > +Some forms of tracing need to wait for all preemption-disabled regions > +of code running on any online CPU, including those executed when RCU is > +not watching. This means that synchronize_rcu() is insufficient, and > +Tasks Rude RCU must be used instead. This flavor of RCU does its work by > +forcing a workqueue to be scheduled on each online CPU, hence the "Rude" > +moniker. And this operation is considered to be quite rude by real-time > +workloads that don't want their ``nohz_full`` CPUs receiving IPIs and > +by battery-powered systems that don't want their idle CPUs to be awakened. > + > +The tasks-rude-RCU API is also reader-marking-free and thus quite compact, > +consisting of call_rcu_tasks_rude(), synchronize_rcu_tasks_rude(), > +and rcu_barrier_tasks_rude(). Are we going to be able to get rid of the "rude" version once we have all tracing in a RCU visible section? > + > +Tasks Trace RCU > +~~~~~~~~~~~~~~~ > + > +Some forms of tracing need to sleep in readers, but cannot tolerate > +SRCU's read-side overhead, which includes a full memory barrier in both > +srcu_read_lock() and srcu_read_unlock(). This need is handled by a > +Tasks Trace RCU that uses scheduler locking and IPIs to synchronize with > +readers. Real-time systems that cannot tolerate IPIs may build their > +kernels with ``CONFIG_TASKS_TRACE_RCU_READ_MB=y``, which avoids the IPIs at > +the expense of adding full memory barriers to the read-side primitives. If NOHZ_FULL is enabled, is there a way to also be able to have this full mb on RT removed as well? > + > +The tasks-trace-RCU API is also reasonably compact, > +consisting of rcu_read_lock_trace(), rcu_read_unlock_trace(), > +rcu_read_lock_trace_held(), call_rcu_tasks_trace(), > +synchronize_rcu_tasks_trace(), and rcu_barrier_tasks_trace(). > + > Possible Future Changes > ----------------------- > For the documentation: Reviewed-by: Steven Rostedt (Google) -- Steve