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=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 60E69C0044D for ; Mon, 16 Mar 2020 15:39:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 3624620679 for ; Mon, 16 Mar 2020 15:39:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1584373184; bh=BF/Y6VdwrEGYZgDYRo8+Zp93Zdq85GyGk4o7+hsg5UU=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=bja85iHiHrqCAIADL4fu3oXok/oWR6fN8tcS/btd+cfD3Wqd2f9j8n+8n/01p16ZL fBMMG4qUIOxU/GigMCPaDNxJRfEa4d/9fJ6O9sTPxJvZlsBVnfSFtRfiCcd6NqRGYU iBgaLmm8UxBQo0+FPg1l3kT9A+RiKu2s9mlON8RY= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731703AbgCPPjn (ORCPT ); Mon, 16 Mar 2020 11:39:43 -0400 Received: from mail.kernel.org ([198.145.29.99]:43726 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731780AbgCPPjn (ORCPT ); Mon, 16 Mar 2020 11:39:43 -0400 Received: from paulmck-ThinkPad-P72.home (50-39-105-78.bvtn.or.frontiernet.net [50.39.105.78]) (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 D5F4720719; Mon, 16 Mar 2020 15:39:42 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1584373182; bh=BF/Y6VdwrEGYZgDYRo8+Zp93Zdq85GyGk4o7+hsg5UU=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=MJoPs6o6i35Vcgl3lZ4NI4X5LKNVqBMQ3tRpe8xHgHtE1SjxVdrCSDGhAaLOspQOF giOWYDYJY3iVb6Jy3Xj0RVEAa+xjmudZAbQBWPQMGGqTK5+oWifGNV5/4sMM6PEUoC KATXZRnN+BV7IOI2iBEW4TyTEle8VaLvG8uU39oo= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 7C3323522DE1; Mon, 16 Mar 2020 08:39:42 -0700 (PDT) Date: Mon, 16 Mar 2020 08:39:42 -0700 From: "Paul E. McKenney" To: Frederic Weisbecker Cc: mutt@paulmck-ThinkPad-P72, rcu@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.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 Subject: Re: [PATCH RFC tip/core/rcu 0/16] Prototype RCU usable from idle, exception, offline Message-ID: <20200316153942.GW3199@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200312181618.GA21271@paulmck-ThinkPad-P72> <20200313144145.GA31604@lenoir> <20200313154243.GU3199@paulmck-ThinkPad-P72> <20200316144535.GA501@lenoir> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200316144535.GA501@lenoir> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Mon, Mar 16, 2020 at 03:45:36PM +0100, Frederic Weisbecker wrote: > On Fri, Mar 13, 2020 at 08:42:43AM -0700, Paul E. McKenney wrote: > > On Fri, Mar 13, 2020 at 03:41:46PM +0100, Frederic Weisbecker wrote: > > > On Thu, Mar 12, 2020 at 11:16:18AM -0700, Paul E. McKenney wrote: > > > > Hello! > > > > > > > > This series provides two variants of Tasks RCU, a rude variant inspired > > > > by Steven Rostedt's use of schedule_on_each_cpu(), and a tracing variant > > > > requested by the BPF folks and perhaps also of use for other tracing > > > > use cases. > > > > > > > > The tracing variant has explicit read-side markers to permit finite grace > > > > periods even given in-kernel loops in PREEMPT=n builds It also protects > > > > code in the idle loop, on exception entry/exit paths, and on the various > > > > CPU-hotplug online/offline code paths, thus having protection properties > > > > similar to SRCU. However, unlike SRCU, this variant avoids expensive > > > > instructions in the read-side primitives, thus having read-side overhead > > > > similar to that of preemptible RCU. > > > > > > > > There are of course downsides. The grace-period code can send IPIs to > > > > CPUs, even when those CPUs are in the idle loop or in nohz_full userspace. > > > > It is necessary to scan the full tasklist, much as for Tasks RCU. There > > > > is a single callback queue guarded by a single lock, again, much as for > > > > Tasks RCU. If needed, these downsides can be at least partially remedied > > > > > > So what we trade to fix the issues we are having with tracing against extended > > > grace periods, we lose in CPU isolation. That worries me a bit as tracing can > > > be thoroughly used with nohz_full and CPU isolation. > > > > First, disturbing nohz_full CPUs can be avoided by the sysadm simply > > refusing to remove tracepoints while sensitive applications are running > > on nohz_full CPUs. > > So, in that case we'll need to modify the tools such as perf tools to avoid > releasing the related buffers until we are ready to do so. > > That's possible but it's kindof an ABI breakage. Also what if there is a > long running service on that nohz full CPU polling on the networking card... In the near term, I do admit that Mathieu's point about using smp_mb() in readers but only on nohz_full CPUs is attractive. I have some other ideas, but simplicity has its advantages, and if no one complains, perhaps those advantages are also good for the long term. Thanx, Paul