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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_2 autolearn=no 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 A191FC2BB1D for ; Mon, 16 Mar 2020 21:32:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 78DB120719 for ; Mon, 16 Mar 2020 21:32:23 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732697AbgCPVcX (ORCPT ); Mon, 16 Mar 2020 17:32:23 -0400 Received: from mail.kernel.org ([198.145.29.99]:42440 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1732672AbgCPVcX (ORCPT ); Mon, 16 Mar 2020 17:32:23 -0400 Received: from gandalf.local.home (cpe-66-24-58-225.stny.res.rr.com [66.24.58.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id E820020679; Mon, 16 Mar 2020 21:32:20 +0000 (UTC) Date: Mon, 16 Mar 2020 17:32:19 -0400 From: Steven Rostedt To: "Paul E. McKenney" Cc: Joel Fernandes , rcu , LKML , "kernel-team@fb.com," , Ingo Molnar , Lai Jiangshan , dipankar , Andrew Morton , Mathieu Desnoyers , Josh Triplett , Thomas Glexiner , Peter Zijlstra , David Howells , Eric Dumazet , Frederic Weisbecker , Oleg Nesterov Subject: Re: [PATCH RFC tip/core/rcu 09/16] rcu-tasks: Add an RCU-tasks rude variant Message-ID: <20200316173219.1f8b7443@gandalf.local.home> In-Reply-To: <20200316203241.GB3199@paulmck-ThinkPad-P72> References: <20200312181618.GA21271@paulmck-ThinkPad-P72> <20200312181702.8443-9-paulmck@kernel.org> <20200316194754.GA172196@google.com> <20200316203241.GB3199@paulmck-ThinkPad-P72> X-Mailer: Claws Mail 3.17.3 (GTK+ 2.24.32; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Mon, 16 Mar 2020 13:32:41 -0700 "Paul E. McKenney" wrote: > > Just curious, why is the "rude" version better than SRCU? Seems the > > schedule_on_each_cpu() would be much slower than SRCU especially if > > there are 1000s of CPUs involved. Is there any reason that is a better > > alternative? > > The rude version has much faster readers, and the story I hear is that > there are not expected to be all that many concurrent updaters. > > But to get more detail, why not ask Steven why he chose not to use SRCU? > (I know the story for the BPF guys, and it is because of SRCU's read-side > overhead.) Same for the function side (if not even more so). This would require adding a srcu_read_lock() to all functions that can be traced! That would be a huge kill in performance. Probably to the point no one would bother even using function tracer. -- Steve