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 12663C433DF for ; Mon, 6 Jul 2020 21:06:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DE36B206BE for ; Mon, 6 Jul 2020 21:06:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1594069606; bh=8FQG3v8b4r+oJ69mw3ZRz9kF6YWmcxHtUuE/bUg9230=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=nWgTeq6YcOvICYmK2bDv1qei1eG/uy36ULtXKw0pRMrgr1JbHbAC+jqM3DyIsCWqQ EOXOPlNa1PZX3AFJmmsu6haxqQhoyynnHiGLha+sRdzyFRKLU63AWcsCysKvM3gXtX 6OgnKifqOwK8H+nvatbleEmPVEGn5xQqrRiTdblg= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726478AbgGFVGq (ORCPT ); Mon, 6 Jul 2020 17:06:46 -0400 Received: from mail.kernel.org ([198.145.29.99]:59432 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725860AbgGFVGq (ORCPT ); Mon, 6 Jul 2020 17:06:46 -0400 Received: from paulmck-ThinkPad-P72.home (50-39-111-31.bvtn.or.frontiernet.net [50.39.111.31]) (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 6B15F206B6; Mon, 6 Jul 2020 21:06:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1594069605; bh=8FQG3v8b4r+oJ69mw3ZRz9kF6YWmcxHtUuE/bUg9230=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=foRYrq9kQ8dcpUY0oJNtXHqyBIQDhSti8Zu24qyU8tMqrPo7ahmkAkAmccqfPDpI4 hBrQ+B0jTKbMlBQv+Fz6L+QKxuIukK9J3cWq8K+OzsFQ7QCQcAaiSEaRXs3QE//7KN oW6uKVLXAnnJJIXwvcMbiXxgIaQBw23ijiHqbfYE= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 549073522637; Mon, 6 Jul 2020 14:06:45 -0700 (PDT) Date: Mon, 6 Jul 2020 14:06:45 -0700 From: "Paul E. McKenney" To: Sebastian Andrzej Siewior Cc: joel@joelfernandes.org, 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, Uladzislau Rezki Subject: Re: [PATCH tip/core/rcu 03/17] rcu/tree: Skip entry into the page allocator for PREEMPT_RT Message-ID: <20200706210645.GJ9247@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200624201200.GA28901@paulmck-ThinkPad-P72> <20200624201226.21197-3-paulmck@kernel.org> <20200630164543.4mdcf6zb4zfclhln@linutronix.de> <20200630183534.GG9247@paulmck-ThinkPad-P72> <20200702141216.r4rbt5w3hjzafpgg@linutronix.de> <20200702164826.GQ9247@paulmck-ThinkPad-P72> <20200702201908.jfiacgvion6a4nmj@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20200702201908.jfiacgvion6a4nmj@linutronix.de> 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 Thu, Jul 02, 2020 at 10:19:08PM +0200, Sebastian Andrzej Siewior wrote: > On 2020-07-02 09:48:26 [-0700], Paul E. McKenney wrote: > > On Thu, Jul 02, 2020 at 04:12:16PM +0200, Sebastian Andrzej Siewior wrote: > > > On 2020-06-30 11:35:34 [-0700], Paul E. McKenney wrote: > > > > > This is not going to work together with the "wait context validator" > > > > > (CONFIG_PROVE_RAW_LOCK_NESTING). As of -rc3 it should complain about > > > > > printk() which is why it is still disabled by default. > > > > > > > > Fixing that should be "interesting". In particular, RCU CPU stall > > > > warnings rely on the raw spin lock to reduce false positives due > > > > to race conditions. Some thought will be required here. > > > > > > I don't get this part. Can you explain/give me an example where to look > > > at? > > > > Starting from the scheduler-clock interrupt's call into RCU, > > we have rcu_sched_clock_irq() which calls rcu_pending() which > > calls check_cpu_stall() which calls either print_cpu_stall() or > > print_other_cpu_stall(), depending on whether the stall is happening on > > the current CPU or on some other CPU, respectively. > > > > Both of these last functions acquire the rcu_node structure's raw ->lock > > and expect to do printk()s while holding it. > > … > > Thoughts? > > Okay. So in the RT queue there is a printk() rewrite which fixes this > kind of things. Upstream the printk() interface is still broken in this > regard and therefore CONFIG_PROVE_RAW_LOCK_NESTING is disabled. > [Earlier the workqueue would also trigger a warning but this has been > fixed as of v5.8-rc1.] > This was just me explaining why this bad, what debug function would > report it and why it is not enabled by default. Whew!!! ;-) > > > > > So assume that this is fixed and enabled then on !PREEMPT_RT it will > > > > > complain that you have a raw_spinlock_t acquired (the one from patch > > > > > 02/17) and attempt to acquire a spinlock_t in the memory allocator. > > > > > > > > Given that the slab allocator doesn't acquire any locks until it gets > > > > a fair way in, wouldn't it make sense to allow a "shallow" allocation > > > > while a raw spinlock is held? This would require yet another GFP_ flag, > > > > but that won't make all that much of a difference in the total. ;-) > > > > > > That would be one way of dealing with. But we could go back to > > > spinlock_t and keep the memory allocation even for RT as is. I don't see > > > a downside of this. And we would worry about kfree_rcu() from real > > > IRQ-off region once we get to it. > > > > Once we get to it, your thought would be to do per-CPU queuing of > > memory from IRQ-off kfree_rcu(), and have IRQ work or some such clean > > up after it? Or did you have some other trick in mind? > > So for now I would very much like to revert the raw_spinlock_t back to > the spinlock_t and add a migrate_disable() just avoid the tiny > possible migration between obtaining the CPU-ptr and acquiring the lock > (I think Joel was afraid of performance hit). Performance is indeed a concern here. > Should we get to a *real* use case where someone must invoke kfree_rcu() > from a hard-IRQ-off region then we can think what makes sense. per-CPU > queues and IRQ-work would be one way of dealing with it. It looks like workqueues can also be used, at least in their current form. And timers. Vlad, Joel, thoughts? Thanx, Paul