From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751913AbbJCIP5 (ORCPT ); Sat, 3 Oct 2015 04:15:57 -0400 Received: from mail-wi0-f181.google.com ([209.85.212.181]:36367 "EHLO mail-wi0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750925AbbJCIPw (ORCPT ); Sat, 3 Oct 2015 04:15:52 -0400 Message-ID: <1443860148.3786.5.camel@gmail.com> Subject: Re: 4.2.2: NR_CPUS effectively being 1 bug From: Mike Galbraith To: Ingo Molnar Cc: Peter Zijlstra , =?ISO-8859-1?Q?Fr=E9d=E9ric?= Weisbecker , Andrew Morton , Alexey Dobriyan , linux-kernel@vger.kernel.org, Ingo Molnar Date: Sat, 03 Oct 2015 10:15:48 +0200 In-Reply-To: <20151003073812.GA24548@gmail.com> References: <20151002204658.GA1970@p183.telecom.by> <20151002140033.ef62cd83936263311eb210d8@linux-foundation.org> <20151003065320.GE3816@twins.programming.kicks-ass.net> <20151003073812.GA24548@gmail.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.12.11 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, 2015-10-03 at 09:38 +0200, Ingo Molnar wrote: > * Peter Zijlstra wrote: > > > On Fri, Oct 02, 2015 at 02:00:33PM -0700, Andrew Morton wrote: > > > On Fri, 2 Oct 2015 23:46:59 +0300 Alexey Dobriyan wrote: > > > > > > > FYI, I've upgraded from 4.1.7 to 4.2.1 (and retested with 4.2.2) and > > > > everything is scheduled on 1 CPU out of 4 (i5 760). > > > > > > > > $ sudo cat /proc/1/status | grep cpu -i > > > > Cpus_allowed: 1 > > > > Cpus_allowed_list: 0 > > > > > > > > Every process inherits this tiny cpumask. > > > > > > Sell the other CPUs on ebay? > > > > > > I haven't seen such a report before - maybe it rings a bell with Peter > > > & Ingo? > > > > I think this is related to some NO_HZ_FULL quackery. People seem to have > > enabled stuff they've really no sane reason for. > > So the question is, is CONFIG_NO_HZ_FULL_ALL=y enabled? If yes then please disable > it. > > Frederic, is there a fix for that? The Kconfig help text for CONFIG_NO_HZ_FULL_ALL > says:: > > CONFIG_NO_HZ_FULL_ALL: > > If the user doesn't pass the nohz_full boot option to > define the range of full dynticks CPUs, consider that all > CPUs in the system are full dynticks by default. > Note the boot CPU will still be kept outside the range to > handle the timekeeping duty. > > I can see people enabling that. Why are all CPUs lost if it's done? Simple. Rik made it such that cpu_isolated_map is immune to cpusets, and Chris made tick_nohz_full_mask automatically set cpu_isolated_map, so now if nohz_full is ever turned on, that CPU is gone from the generic pool forever, with obvious consequences for CONFIG_NO_HZ_FULL_ALL. Lovely eh? -Mike