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 824C3C54E8A for ; Sun, 10 May 2020 15:49:11 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5F6B220820 for ; Sun, 10 May 2020 15:49:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1589125751; bh=WUqz8SCxJRtX45w2ld++azi9BEtGeIHYbgoRCAw21rI=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=1vnKqhUmJidz/q8jxAeGqnMN8fgsHB5l3JuN+XI/Bbs+jocJRcS65A1VOxzirZQl+ 1tLzXFbX40PG6NI3LvAvTgXSDFswzfmnfI2jWQ3GaCpl1O21SS/GYpD1xdqYNSI6rg 8tiEmCLnnfcZ1Wpjhq2Rlec2kmG6oW++uGGi8UiE= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729058AbgEJPtK (ORCPT ); Sun, 10 May 2020 11:49:10 -0400 Received: from mail.kernel.org ([198.145.29.99]:52962 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726955AbgEJPtK (ORCPT ); Sun, 10 May 2020 11:49:10 -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 F0F7920735; Sun, 10 May 2020 15:49:08 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1589125749; bh=WUqz8SCxJRtX45w2ld++azi9BEtGeIHYbgoRCAw21rI=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=exZesUuZmVlROGKES3hDHZVxuDXJMkyhaNdMJIZPWMeWHhOg5LfOwaSwC8cqBrRqR Xr02om3heQemK1shc2YQ+LqSezuTAI5NidEQifCUoRRsxnc3DAG56htnAWvgMw13+5 ZFqu6m7YhNtoRlW8cWkJEiUYkxb5QMg0ZiUWr1JU= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id C6E5D35227BD; Sun, 10 May 2020 08:49:08 -0700 (PDT) Date: Sun, 10 May 2020 08:49:08 -0700 From: "Paul E. McKenney" To: Lai Jiangshan Cc: Steven Rostedt , Joel Fernandes , rcu , LKML , "kernel-team@fb.com," , Ingo Molnar , dipankar , Andrew Morton , Mathieu Desnoyers , Josh Triplett , Thomas Glexiner , Peter Zijlstra , David Howells , Eric Dumazet , Frederic Weisbecker , Oleg Nesterov , Masami Hiramatsu Subject: Re: [PATCH RFC tip/core/rcu 09/16] rcu-tasks: Add an RCU-tasks rude variant Message-ID: <20200510154908.GR2869@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200312181618.GA21271@paulmck-ThinkPad-P72> <20200312181702.8443-9-paulmck@kernel.org> <20200316194754.GA172196@google.com> <20200316203241.GB3199@paulmck-ThinkPad-P72> <20200316173219.1f8b7443@gandalf.local.home> <20200316180352.4816cb99@gandalf.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, May 10, 2020 at 05:59:27PM +0800, Lai Jiangshan wrote: > On Tue, Mar 17, 2020 at 6:03 AM Steven Rostedt wrote: > > > > On Mon, 16 Mar 2020 17:45:40 -0400 > > Joel Fernandes wrote: > > > > > > > > > > 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. > > > > > > Point well taken! Thanks, > > > > Actually, it's worse than that. (We talked about this on IRC but I wanted > > it documented here too). > > > > You can't use any type of locking, unless you insert it around all the > > callers of the nops (which is unreasonable). > > > > That is, we have gcc -pg -mfentry that creates at the start of all traced > > functions: > > > > : > > call __fentry__ > > [code for function here] > > > > At boot up (or even by the compiler itself) we convert that to: > > > > : > > nop > > [code for function here] > > > > > > When we want to trace this function we use text_poke (with current kernels) > > and convert it to this: > > > > : > > call trace_trampoline > > [code for function here] > > > > > > That trace_trampoline can be allocated, which means when its no longer > > needed, it must be freed. But when do we know it's safe to free it? Here's > > the issue. > > > > > > : > > call trace_trampoline <- interrupt happens just after the jump > > [code for function here] > > > > Now the task has just executed the call to the trace_trampoline. Which > > means the instruction pointer is set to the start of the trampoline. But it > > has yet executed that trampoline. > > > > Now if the task is preempted, and a real time hog is keeping it from > > running for minutes at a time (which is possible!). And in the mean time, > > we are done with that trampoline and free it. What happens when that task > > is scheduled back? There's no more trampoline to execute even though its > > instruction pointer is to execute the first operand on the trampoline! > > > > I used the analogy of jumping off the cliff expecting a magic carpet to be > > there to catch you, and just before you land, it disappears. That would be > > a very bad day indeed! > > > > We have no way to add a grace period between the start of a function (can > > be *any* function) and the start of the trampoline. > > Hello > > I think adding a small number of instructions to preempt_schedule_irq() > is sufficient to create the needed protected region between the start > of a function and the trampoline body. > > preempt_schedule_irq() { > + if (unlikely(is_trampoline_page(page_of(interrupted_ip)))) { > + return; // don't do preempt schedule > + > + } > preempt_schedule_irq() original body > } > > // generated on trampoline pages > trace_trampoline() { > preempt_disable(); > trace_trampoline body > jmp preempt_enable_traced(clobbers) > } > > asm(kernel text): > preempt_enable_traced: > preempt_enable_notrace(); > restore cobblers > return(the return ip on the stack is traced_function_start_code) > > > If the number of instructions added in preempt_schedule_irq() and > the complexity to make trampoline ip detectable(is_trampoline_page(), > or is_trampoline_range()) are small, and tasks_rcu is rendered useless, > I think it will be win-win. It certainly would provide a nice reduction in code size! This would provide a zero-instructions preempt_disable() at the beginning of the trampoline and a zero-instructions preempt_enable_no_resched() at the end, correct? If so, wouldn't this create a potentially long (though "weak") preempt-disable region extending to the next preempt_enable(), local_bh_enable(), schedule(), interrupt, transition to userspace, or similar? This could be quite some time. Note that cond_resched() wouldn't help, given that this is only in PREEMPT=y kernels. The "weak" refers to the fact that if a second resched IPI arrived in the meantime, preemption would then happen. But without that second IPI, the request for preemption could be ignored for quite some time. Or am I missing something here? Thanx, Paul > Thanks > > Lai > > > Since the problem is > > that the task was non-voluntarily preempted before it could execute the > > trampoline, and that trampolines are not allowed (suppose) to call > > schedule, then we have our quiescent state to track (voluntary scheduling). > > When all tasks have either voluntarily scheduled, or entered user space > > after disconnecting a trampoline from a function, we know that it is safe to > > free the trampoline. > > > > -- Steve