From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1163262AbdEXJiY (ORCPT ); Wed, 24 May 2017 05:38:24 -0400 Received: from mail.kernel.org ([198.145.29.99]:55434 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1163202AbdEXJhk (ORCPT ); Wed, 24 May 2017 05:37:40 -0400 DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org BD969239F3 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=mhiramat@kernel.org Date: Wed, 24 May 2017 18:37:35 +0900 From: Masami Hiramatsu To: Steven Rostedt Cc: "Paul E. McKenney" , Ingo Molnar , linux-kernel@vger.kernel.org, Masami Hiramatsu Subject: Re: Use case for TASKS_RCU Message-Id: <20170524183735.2ce2c117a1249f8082549fda@kernel.org> In-Reply-To: <20170516090708.35fd8a12@gandalf.local.home> References: <20170515182354.GA25440@linux.vnet.ibm.com> <20170516062233.tyz7ze7ilmbkxtjc@gmail.com> <20170516122354.GB3956@linux.vnet.ibm.com> <20170516090708.35fd8a12@gandalf.local.home> X-Mailer: Sylpheed 3.5.0 (GTK+ 2.24.30; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Sorry, I missed this thread, On Tue, 16 May 2017 09:07:08 -0400 Steven Rostedt wrote: > On Tue, 16 May 2017 05:23:54 -0700 > "Paul E. McKenney" wrote: > > > On Tue, May 16, 2017 at 08:22:33AM +0200, Ingo Molnar wrote: > > > > > > * Paul E. McKenney wrote: > > > > > > > Hello! > > > > > > > > The question of the use case for TASKS_RCU came up, and here is my > > > > understanding. Steve will not be shy about correcting any misconceptions > > > > I might have. ;-) > > > > > > > > The use case is to support freeing of trampolines used in tracing/probing > > > > in CONFIG_PREEMPT=y kernels. It is necessary to wait until any task > > > > executing in the trampoline in question has left it, taking into account > > > > that the trampoline's code might be interrupted and preempted. However, > > > > the code in the trampolines is guaranteed never to context switch. > > > > > > > > Note that in CONFIG_PREEMPT=n kernels, synchronize_sched() suffices. > > > > It is therefore tempting to think in terms of disabling preemption across > > > > the trampolines, but there is apparently not enough room to accommodate > > > > the needed preempt_disable() and preempt_enable() in the code invoking > > > > the trampoline, and putting the preempt_disable() and preempt_enable() > > > > in the trampoline itself fails because of the possibility of preemption > > > > just before the preempt_disable() and just after the preempt_enable(). > > > > Similar reasoning rules out use of rcu_read_lock() and rcu_read_unlock(). > > > > > > So how was this solved before TASKS_RCU? Also, nothing uses call_rcu_tasks() at > > > the moment, so it's hard for me to review its users. What am I missing? > > > > Before TASKS_RCU, the trampolines were just leaked when CONFIG_PREEMPT=y. > > Actually, things simply were not implemented. This is why optimized > kprobes is dependent on !CONFIG_PREEMPT. In fact, we can now optimize > kprobes on CONFIG_PREEMPT with this utility. Right Masami? Yes, I just haven't implemented it. OK, I'll use synchronize_rcu_tasks. > With ftrace, perf and other "dynamic" users (where the ftrace_ops was > created via a kmalloc), would not get the benefit of being called > directly. They all needed to have their mcount/fentry's call a static > trampoline that disabled preemption before calling the callback. This > static trampoline is shared by all, so even if perf was the only > callback for the function, it had to call this trampoline that iterated > through all registered ftrace_ops to see which one had a callback for > the given function. For the optimized kprobes, it always jumps into dynamically allocated trampoline, so we have no chance to disable preemption. Thank you, > > With this utility, perf not only gets the benefit of not having to use > that static loop trampoline, it can even have its own trampoline > created that doesn't even need to do the check if perf wants this > function or not, as the only way the trampoline is called, is if perf > wanted it. > > > > > Current mainline kernel/trace/ftrace.c uses synchronize_rcu_tasks(). > > So yes, currently one user. > > > > And the kpatch folks want to use it too. > > -- Steve -- Masami Hiramatsu