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=-1.1 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS 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 B1177C433DF for ; Thu, 2 Jul 2020 20:19:13 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 84E3D20772 for ; Thu, 2 Jul 2020 20:19:13 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="dsHAlvNM"; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="+9j4rmT8" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726003AbgGBUTN (ORCPT ); Thu, 2 Jul 2020 16:19:13 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:45116 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725994AbgGBUTN (ORCPT ); Thu, 2 Jul 2020 16:19:13 -0400 Received: from galois.linutronix.de (Galois.linutronix.de [IPv6:2a0a:51c0:0:12e:550::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 588B1C08C5C1; Thu, 2 Jul 2020 13:19:11 -0700 (PDT) Date: Thu, 2 Jul 2020 22:19:08 +0200 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1593721150; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=NHcHHOfu1jSeEknaYtdDpWniCj0/u4/s6M6KwnTpcsc=; b=dsHAlvNM0vIrsjkOgti9Yzdx7SDL5bCd4q+aSP7Q0hSY/PJZQKuM1r2/uIiCvYr3gDG+Gh YUBaDl8bAPd3SJpIgW8tujcUMixvm4ojCa2AWkc95/vTpEcKnqB2YTedrCJXNDSkmtu6n3 mSSrkou2HNtIDfBJj8eBFlHKx2UJfL1xm9PWJI7s9Zlz0KJw0LoHP98yfWXyWHB0VDT1ip a8PZW5MRPvgnE+RSNV/U4gPNhN5m1L6NrHra5c94hVIqH90lrJdcjhxNaiuJlA3fCmDqAi 0aKl73CRYqea5lcvNHQShEpNNbYbXXCOboYgUxBVpwEXsTT28xadxRfX2pZuRw== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1593721150; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=NHcHHOfu1jSeEknaYtdDpWniCj0/u4/s6M6KwnTpcsc=; b=+9j4rmT8q/2x/Qm3/hEbQCNEhekY1t3FqLlhPDfvACust2St4iEcP7o69y7TwgZtOtM67X e/klwLVav4ghqbDA== From: Sebastian Andrzej Siewior To: "Paul E. McKenney" 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: <20200702201908.jfiacgvion6a4nmj@linutronix.de> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable In-Reply-To: <20200702164826.GQ9247@paulmck-ThinkPad-P72> Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org 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. > > >=20 > > > 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. > >=20 > > I don't get this part. Can you explain/give me an example where to look > > at? >=20 > 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. >=20 > Both of these last functions acquire the rcu_node structure's raw ->lock > and expect to do printk()s while holding it. =E2=80=A6 > 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. > > > > 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. > > >=20 > > > 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_ fl= ag, > > > but that won't make all that much of a difference in the total. ;-) > >=20 > > 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. >=20 > 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). Should we get to a *real* use case where someone must invoke kfree_rcu() =66rom 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. > Thanx, Paul >=20 Sebastian