From patchwork Fri Jun 26 14:28:01 2015 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: "Michael Kerrisk (man-pages)" X-Patchwork-Id: 488840 Return-Path: X-Original-To: incoming@patchwork.ozlabs.org Delivered-To: patchwork-incoming@bilbo.ozlabs.org Received: from sourceware.org (server1.sourceware.org [209.132.180.131]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ozlabs.org (Postfix) with ESMTPS id 8B701140285 for ; Sat, 27 Jun 2015 00:28:33 +1000 (AEST) Authentication-Results: ozlabs.org; dkim=pass (1024-bit key; unprotected) header.d=sourceware.org header.i=@sourceware.org header.b=y662qoxu; dkim-atps=neutral DomainKey-Signature: a=rsa-sha1; c=nofws; d=sourceware.org; h=list-id :list-unsubscribe:list-subscribe:list-archive:list-post :list-help:sender:message-id:date:from:reply-to:mime-version:to :cc:subject:references:in-reply-to:content-type :content-transfer-encoding; q=dns; s=default; b=cM4I47FqBrrESnET AHD2Jr9m0A22SgHfFrssO9UvxBoiBlyiFmOohvTESE9o7fK2HASbNRbc4UzvdHUl WhZbceINeftGK7IjRufHTfZpICEE7QS5Sog3ZvIe9aOar1KgNueEjIIufV1emrF2 pdxLMO5fOGylaRXqn5VkzDnwNpM= DKIM-Signature: v=1; a=rsa-sha1; c=relaxed; d=sourceware.org; h=list-id :list-unsubscribe:list-subscribe:list-archive:list-post :list-help:sender:message-id:date:from:reply-to:mime-version:to :cc:subject:references:in-reply-to:content-type :content-transfer-encoding; s=default; bh=ucrO6J8CxWrNFxHxXvy5+z ZyIxM=; b=y662qoxuFRjh9OtVHe7VE6Sgp2VHepBxZXlnI6GYrVGYYCBR8RhS7W pusmCaBDF48kjLtBdE4AxHTamICboIVUiDiJKAmyhk0Yu2zc+93X8KPr6BWd+90a VukLIHgEF/Lj4724VIwDOlKOmCZAam/VUOTOh8QvecZvzoaEQopTY= Received: (qmail 117817 invoked by alias); 26 Jun 2015 14:28:26 -0000 Mailing-List: contact libc-alpha-help@sourceware.org; run by ezmlm Precedence: bulk List-Id: List-Unsubscribe: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: libc-alpha-owner@sourceware.org Delivered-To: mailing list libc-alpha@sourceware.org Received: (qmail 117808 invoked by uid 89); 26 Jun 2015 14:28:26 -0000 Authentication-Results: sourceware.org; auth=none X-Virus-Found: No X-Spam-SWARE-Status: No, score=-1.6 required=5.0 tests=AWL, BAYES_00, FREEMAIL_FROM, FREEMAIL_REPLYTO, RCVD_IN_DNSWL_LOW, SPF_PASS autolearn=no version=3.3.2 X-HELO: mail-wi0-f170.google.com X-Received: by 10.180.91.100 with SMTP id cd4mr5315204wib.1.1435328892863; Fri, 26 Jun 2015 07:28:12 -0700 (PDT) Message-ID: <558D6171.1060901@gmail.com> Date: Fri, 26 Jun 2015 16:28:01 +0200 From: "Michael Kerrisk (man-pages)" Reply-To: mtk.manpages@gmail.com User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 MIME-Version: 1.0 To: Carlos O'Donell , Roland McGrath CC: mtk.manpages@gmail.com, KOSAKI Motohiro , libc-alpha , mtk.manpages@gmail.com, linux-man@vger.kernel.org Subject: Re: What *is* the API for sched_getaffinity? Should sched_getaffinity always succeed when using cpu_set_t? References: <51E42BFE.7000301@redhat.com> <51E4A0BB.2070802@gmail.com> <51E4A123.9070001@gmail.com> <51E6F3ED.8000502@redhat.com> <51E6F956.5050902@gmail.com> <51E714DE.6060802@redhat.com> <51E7B205.3060905@redhat.com> <20130722214335.D9AFF2C06F@topped-with-meat.com> <51EDB378.8070301@redhat.com> In-Reply-To: <51EDB378.8070301@redhat.com> Carlos, On 07/23/2013 12:34 AM, Carlos O'Donell wrote: > On 07/22/2013 05:43 PM, Roland McGrath wrote: >>> I can fix the glibc manual. A 'configured' CPU is one that the OS >>> can bring online. >> >> Where do you get this definition, in the absence of a standard that >> specifies _SC_NPROCESSORS_CONF? The only definition I've ever known for >> _SC_NPROCESSORS_CONF is a value that's constant for at least the life of >> the process (and probably until reboot) that is the upper bound for what >> _SC_NPROCESSORS_ONLN might ever report. If the implementation for Linux is >> inconsistent with that definition, then it's just a bug in the implementation. > > Let me reiterate my understanding such that you can help me clarify > exactly my interpretation of the glibc manual wording regarding the > two existing constants. > > The reality of the situation is that the linux kernel as an abstraction > presents the following: > > (a) The number of online cpus. > - Changes dynamically. > - Not constant for the life of the process, but pretty constant. > > (b) The number of configured cpus. > - The number of detected cpus that the OS could access. > - Some of them may be offline for various reasons. > - Changes dynamically with hotplug. > > (c) The number of possible CPUs the OS or hardware can support. > - The internal software infrastructure is designed to support at > most this many cpus. > - Constant for the uptime of the system. > - May be tied in some way to the hardware. > > On Linux, glibc currently maps _SC_NPROCESSORS_CONF to (b) via > /sys/devices/system/cpu/cpu*, and _SC_NPROCESSORS_ONLN to (a) via > /sys/devices/system/cpu/online. > > The problem is that sched_getaffinity and sched_setaffinity only cares > about (c) since the size of the kernel affinity mask is of size (c). > > What Motohiro-san was requesting was that the manual should make it clear > that _SC_NPROCESSORS_CONF is distinct from (c) which is an OS limit that > the user doesn't know. > > We need not expose (c) as a new _SC_* constant since it's not really > required, since glibc's sched_getaffinity and sched_setaffinity could > hide the fact that (c) exists from userspace (and that's what I suggest > should happen). > > Does that clarify my statement? It's a long time since the last activity in this discussion, and I see that https://sourceware.org/bugzilla/show_bug.cgi?id=15630 remains open, I propose to apply the patch below to the sched_setattr/sched_getattr man page. Seem okay? Cheers, Michael --- a/man2/sched_setaffinity.2 +++ b/man2/sched_setaffinity.2 @@ -333,6 +334,57 @@ main(int argc, char *argv[]) } } .fi +.SH BUGS +The glibc +.BR sched_setaffinity () +and +.BR sched_getaffinity () +wrapper functions do not handle systems with more than 1024 CPUs. +.\" FIXME . See https://sourceware.org/bugzilla/show_bug.cgi?id=15630 +.\" and https://sourceware.org/ml/libc-alpha/2013-07/msg00288.html +The +.I cpu_set_t +data type used by glibc has a fixed size of 128 bytes, +meaning that the the maximum CPU number that can be represented is 1023. +If the system has more than 1024 CPUs, then: +.IP * 3 +The +.BR sched_setaffinity () +.I mask +argument is not capable of representing the excess CPUs. +.IP * +Calls of the form: + + sched_getaffinity(pid, sizeof(cpu_set_t), &mask); + +will fail with error +.BR EINVAL , +the error produced by the underlying system call for the case where the +.I mask +size specified in +.I cpusetsize +is smaller than the size of the affinity mask used by the kernel. +.PP +The workaround for this problem is to fall back to the use of the +underlying system call (via +.BR syscall (2)), +passing +.I mask +arguments of a sufficient size. +Using a value based on the number of online CPUs: + + (sysconf(_SC_NPROCESSORS_CONF) / (sizeof(unsigned long) * 8) + 1) + * sizeof(unsigned long) + +is probably sufficient as the size of the mask, +although the value returned by the +.BR sysconf () +call can in theory change during the lifetime of the process. +Alternatively, one can probe for the size of the required mask using raw +.BR sched_getaffinity () +system calls with increasing mask sizes +until the call does not fail with the error +.BR EINVAL . .SH SEE ALSO .ad l .nh