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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id AE112C433F5 for ; Fri, 28 Jan 2022 16:15:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1349965AbiA1QPv (ORCPT ); Fri, 28 Jan 2022 11:15:51 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39706 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1349962AbiA1QPu (ORCPT ); Fri, 28 Jan 2022 11:15:50 -0500 Received: from ams.source.kernel.org (ams.source.kernel.org [IPv6:2604:1380:4601:e00::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B0717C061714; Fri, 28 Jan 2022 08:15:50 -0800 (PST) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id 3F510B80D79; Fri, 28 Jan 2022 16:15:49 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id ECC36C340E0; Fri, 28 Jan 2022 16:15:47 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1643386548; bh=cDuSg+CrlslaXesh3qz8z39IshhsZtN6MoZi6F8K0xk=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=u7segef+gdncKpEQohgwHgIhGqvbnUFBifeTtUFV4ZygoDyYKVuVBB1xLa+u8dghY JG0few1oVaOtJlHk7++1ERnoM2DnjGmZy5yLLSfxuzjz9jSkNgVHAREodlMIrZhyMj 40yI84Andy6qnYfiG5JNiMiz6b0sHAh5LvOXxBwUQIQDmDbQMqoxyjZpH2V8hf6Ehe +h6YsLxwxK/F5wGMPFPJwEeWxnio35tF2EGaSPJHnDpjdqiD1L6MVFfvKh6AanrVvB 3w/AvnGsjDRCfsUW+dtDJAgXaPEtwzJrQDxAFFU0RHDxSG/Bvu8zexVuydM6m7I2Qz 8RODnJbK9Cq/A== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id A19145C098D; Fri, 28 Jan 2022 08:15:47 -0800 (PST) Date: Fri, 28 Jan 2022 08:15:47 -0800 From: "Paul E. McKenney" To: Mark Rutland Cc: Sven Schnelle , Steven Rostedt , LKML , Ingo Molnar , Andrew Morton , Yinan Liu , Ard Biesheuvel , Kees Cook , Sachin Sant , linuxppc-dev@lists.ozlabs.org, Russell King , linux-arm-kernel@lists.infradead.org, hca@linux.ibm.com, linux-s390@vger.kernel.org Subject: Re: ftrace hangs waiting for rcu Message-ID: <20220128161547.GL4285@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20220127114249.03b1b52b@gandalf.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-s390@vger.kernel.org On Fri, Jan 28, 2022 at 04:11:57PM +0000, Mark Rutland wrote: > On Fri, Jan 28, 2022 at 05:08:48PM +0100, Sven Schnelle wrote: > > Hi Mark, > > > > Mark Rutland writes: > > > > > On arm64 I bisected this down to: > > > > > > 7a30871b6a27de1a ("rcu-tasks: Introduce ->percpu_enqueue_shift for dynamic queue selection") > > > > > > Which was going wrong because ilog2() rounds down, and so the shift was wrong > > > for any nr_cpus that was not a power-of-two. Paul had already fixed that in > > > rcu-next, and just sent a pull request to Linus: > > > > > > https://lore.kernel.org/lkml/20220128143251.GA2398275@paulmck-ThinkPad-P17-Gen-1/ > > > > > > With that applied, I no longer see these hangs. > > > > > > Does your s390 test machine have a non-power-of-two nr_cpus, and does that fix > > > the issue for you? > > > > We noticed the PR from Paul and are currently testing the fix. So far > > it's looking good. The configuration where we have seen the hang is a > > bit unusual: > > > > - 16 physical CPUs on the kvm host > > - 248 logical CPUs inside kvm > > Aha! 248 is notably *NOT* a power of two, and in this case the shift would be > wrong (ilog2() would give 7, when we need a shift of 8). > > So I suspect you're hitting the same issue as I was. And apparently no one runs -next on systems having a non-power-of-two number of CPUs. ;-) Thanx, Paul > Thanks, > Mark. > > > - debug kernel both on the host and kvm guest > > > > So things are likely a bit slow in the kvm guest. Interesting is that > > the number of CPUs is even. But maybe RCU sees an odd number of CPUs > > and gets confused before all cpus are brought up. Have to read code/test > > to see whether that could be possible. > > > > Thanks for investigating! > > Sven