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=-19.7 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_GIT autolearn=unavailable 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 77CE4C433B4 for ; Tue, 11 May 2021 22:53:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4DC4B61421 for ; Tue, 11 May 2021 22:53:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231222AbhEKWy5 (ORCPT ); Tue, 11 May 2021 18:54:57 -0400 Received: from mail.kernel.org ([198.145.29.99]:33906 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230338AbhEKWyP (ORCPT ); Tue, 11 May 2021 18:54:15 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 582236194D; Tue, 11 May 2021 22:53:07 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1620773587; bh=KKDkezvfsvDBcdHebQoHjhjJnMglnYNTHv+gz9t1HBs=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=Wv5J72k9xCDolJsIJoZ+aeuYiSqTTOeKIhECnyRex1vgQD5y/3AKdg2fZnv4X2LI7 FwH1eZZs19Nzl/oPoVFcvYrEV1ioTFcnChYFXqZ9Dks2RTBDS3wz0EWDSH2Igc1KLj N2+v5hIjRPffpT+GHr6pQTw/JgN78vQTtx41QOr40+PsoJI0CSAisxp20ps3s73UoQ s5Gwr5t6ZP2l5xLr2HhNebW1TMtLhO43k8rWw6waflWsciyw+UyXPFqi8OQk93s6xR Qq2grU5mZc65jEAEoMFT4l6t37r8bSt+dRhxsY+7YTkjR0dM3uOyfmrHS3zHSfYxdm eSvMSHwrJ0qcA== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id A3EEC5C0E3F; Tue, 11 May 2021 15:53:06 -0700 (PDT) From: "Paul E. McKenney" To: rcu@vger.kernel.org Cc: linux-kernel@vger.kernel.org, kernel-team@fb.com, mingo@kernel.org, jiangshanlai@gmail.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, joel@joelfernandes.org, Paul Gortmaker , Ingo Molnar , "Paul E . McKenney" , Frederic Weisbecker Subject: [PATCH tip/core/rcu 16/19] sched/isolation: reconcile rcu_nocbs= and nohz_full= Date: Tue, 11 May 2021 15:53:01 -0700 Message-Id: <20210511225304.2893154-16-paulmck@kernel.org> X-Mailer: git-send-email 2.31.1.189.g2e36527f23 In-Reply-To: <20210511225241.GA2893003@paulmck-ThinkPad-P17-Gen-1> References: <20210511225241.GA2893003@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Paul Gortmaker We have a mismatch between RCU and isolation -- in relation to what is considered the maximum valid CPU number. This matters because nohz_full= and rcu_nocbs= are joined at the hip; in fact the former will enforce the latter. So we don't want a CPU mask to be valid for one and denied for the other. The difference 1st appeared as of v4.15; further details are below. As it is confusing to anyone who isn't looking at the code regularly, a reminder is in order; three values exist here: CONFIG_NR_CPUS - compiled in maximum cap on number of CPUs supported. nr_cpu_ids - possible # of CPUs (typically reflects what ACPI says) cpus_present - actual number of present/detected/installed CPUs. For this example, I'll refer to NR_CPUS=64 from "make defconfig" and nr_cpu_ids=6 for ACPI reporting on a board that could run a six core, and present=4 for a quad that is physically in the socket. From dmesg: smpboot: Allowing 6 CPUs, 2 hotplug CPUs setup_percpu: NR_CPUS:64 nr_cpumask_bits:64 nr_cpu_ids:6 nr_node_ids:1 rcu: RCU restricting CPUs from NR_CPUS=64 to nr_cpu_ids=6. smp: Brought up 1 node, 4 CPUs And from userspace, see: paul@trash:/sys/devices/system/cpu$ cat present 0-3 paul@trash:/sys/devices/system/cpu$ cat possible 0-5 paul@trash:/sys/devices/system/cpu$ cat kernel_max 63 Everything is fine if we boot 5x5 for rcu/nohz: Command line: BOOT_IMAGE=/boot/bzImage nohz_full=2-5 rcu_nocbs=2-5 root=/dev/sda1 ro NO_HZ: Full dynticks CPUs: 2-5. rcu: Offload RCU callbacks from CPUs: 2-5. ..even though there is no CPU 4 or 5. Both RCU and nohz_full are OK. Now we push that > 6 but less than NR_CPU and with 15x15 we get: Command line: BOOT_IMAGE=/boot/bzImage rcu_nocbs=2-15 nohz_full=2-15 root=/dev/sda1 ro rcu: Note: kernel parameter 'rcu_nocbs=', 'nohz_full', or 'isolcpus=' contains nonexistent CPUs. rcu: Offload RCU callbacks from CPUs: 2-5. These are both functionally equivalent, as we are only changing flags on phantom CPUs that don't exist, but note the kernel interpretation changes. And worse, it only changes for one of the two - which is the problem. RCU doesn't care if you want to restrict the flags on phantom CPUs but clearly nohz_full does after this change from v4.15 (edb9382175c3): - if (cpulist_parse(str, non_housekeeping_mask) < 0) { - pr_warn("Housekeeping: Incorrect nohz_full cpumask\n"); + err = cpulist_parse(str, non_housekeeping_mask); + if (err < 0 || cpumask_last(non_housekeeping_mask) >= nr_cpu_ids) { + pr_warn("Housekeeping: nohz_full= or isolcpus= incorrect CPU range\n"); To be clear, the sanity check on "possible" (nr_cpu_ids) is new here. The goal was reasonable ; not wanting housekeeping to land on a not-possible CPU, but note two things: 1) this is an exclusion list, not an inclusion list; we are tracking non_housekeeping CPUs; not ones who are explicitly assigned housekeeping 2) we went one further in 9219565aa890 - ensuring that housekeeping was sanity checking against present and not just possible CPUs. To be clear, this means the check added in v4.15 is doubly redundant. And more importantly, overly strict/restrictive. We care now, because the bitmap boot arg parsing now knows that a value of "N" is NR_CPUS; the size of the bitmap, but the bitmap code doesn't know anything about the subtleties of our max/possible/present CPU specifics as outlined above. So drop the check added in v4.15 (edb9382175c3) and make RCU and nohz_full both in alignment again on NR_CPUS so "N" works for both, and then they can fall back to nr_cpu_ids internally just as before. Command line: BOOT_IMAGE=/boot/bzImage nohz_full=2-N rcu_nocbs=2-N root=/dev/sda1 ro NO_HZ: Full dynticks CPUs: 2-5. rcu: Offload RCU callbacks from CPUs: 2-5. As shown above, with this change, RCU and nohz_full are in sync, even with the use of the "N" placeholder. Same result is achieved with "15". Cc: Ingo Molnar Cc: Peter Zijlstra Cc: Paul E. McKenney Cc: Frederic Weisbecker Signed-off-by: Paul Gortmaker Signed-off-by: Paul E. McKenney --- kernel/sched/isolation.c | 4 +--- 1 file changed, 1 insertion(+), 3 deletions(-) diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c index 5a6ea03f9882..7f06eaf12818 100644 --- a/kernel/sched/isolation.c +++ b/kernel/sched/isolation.c @@ -81,11 +81,9 @@ static int __init housekeeping_setup(char *str, enum hk_flags flags) { cpumask_var_t non_housekeeping_mask; cpumask_var_t tmp; - int err; alloc_bootmem_cpumask_var(&non_housekeeping_mask); - err = cpulist_parse(str, non_housekeeping_mask); - if (err < 0 || cpumask_last(non_housekeeping_mask) >= nr_cpu_ids) { + if (cpulist_parse(str, non_housekeeping_mask) < 0) { pr_warn("Housekeeping: nohz_full= or isolcpus= incorrect CPU range\n"); free_bootmem_cpumask_var(non_housekeeping_mask); return 0; -- 2.31.1.189.g2e36527f23