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 9B634C54E8B for ; Mon, 11 May 2020 02:44:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 731E8208E4 for ; Mon, 11 May 2020 02:44:29 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1589165069; bh=8L9FDF4no7+/puiAJxaaBkaIAsM7CjahfcFOlftQHgc=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=tO7ctqvxc617l8LZn/0Tr2Qh95Fk2emYsLDWy38U9ByeVi65CZxwy7zYPxnz5W1yP CndEIuZOlzObTwrFc/F41JemAc8VYRg1xuCrfROMoutZ+U9Z+3EZ47H1BX/DiY5ynw DTihS0ziLaEW1umTL4YhOH13bPIootMpZj4FPAvg= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728260AbgEKCo2 (ORCPT ); Sun, 10 May 2020 22:44:28 -0400 Received: from mail.kernel.org ([198.145.29.99]:48874 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726013AbgEKCo2 (ORCPT ); Sun, 10 May 2020 22:44:28 -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 4116920820; Mon, 11 May 2020 02:44:27 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1589165067; bh=8L9FDF4no7+/puiAJxaaBkaIAsM7CjahfcFOlftQHgc=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=ev2EyPPf/NnjFauStDQEsb4T5AgD+bMqxYlm/LPj/HZcINHp1hYnFMbh4NqKhU+CX se5sO+i1hhq6OOKDJdLuMThOGSjV3HC9ROA+cUMMJaNHYWGXeYnFbgrrXFNmXzs1QE wh+vrMfowggESChHpbWmmygsKO2XiixQZZVKuUAY= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 233BC3523303; Sun, 10 May 2020 19:44:27 -0700 (PDT) Date: Sun, 10 May 2020 19:44:27 -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: <20200511024427.GT2869@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <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> <20200510154908.GR2869@paulmck-ThinkPad-P72> 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 Mon, May 11, 2020 at 08:06:29AM +0800, Lai Jiangshan wrote: > On Sun, May 10, 2020 at 11:49 PM Paul E. McKenney wrote: > > > > 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? > > Hello, > > I'm sorry to note that preempt_enable_traced() is in *kernel text*, it > is *not* in trace_trampoline_protected region. So preempt_enable_traced() > can be preempted. And preempt_enable_notrace() in it checks any previous > resched requested during the trampoline. So no resched request is lost. > > The idea is that "semi-automatically preempt-disable-protecting" > the trampoline. "semi" means the trampoline still needs > preempt_disable() and the beginning, and preempt_enable() at > the end after leaving trace_trampoline_preempt_protected region. > "automatically" means the region between the start ip of > trampoline and the first preempt_disable() is also protected. > This automatically protected region is IP based, which means the > code should be put in "trace_trampoline_preempt_protected". Good point, and I did fail to connect the dots in your earlier email. > In my previous email, "trace_trampoline_preempt_protected" is detected > by information in the "struct page". But the trampolines are often > created in module_alloc() region, if so, 13-page bitmap is sufficient > to store "is this virtual-page-frame-number in trace_trampoline?" info > for all vpfn in 1520 MB module_alloc() region. If the bitmap > is too big for some cases, we might need to use bloom filter > for the fast path. I still don't know what is the best way to control > the ip of trace_trampoline or attach info to it and to fast detect it. Or perhaps use a different approach to trampoline allocation as Masami suggests. Thanx, Paul > Thanks, > Lai > > > > > 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