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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=ham 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 0B44CC282CE for ; Mon, 8 Apr 2019 07:49:41 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D59D420833 for ; Mon, 8 Apr 2019 07:49:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726431AbfDHHtj (ORCPT ); Mon, 8 Apr 2019 03:49:39 -0400 Received: from mx1.redhat.com ([209.132.183.28]:36862 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725877AbfDHHtj (ORCPT ); Mon, 8 Apr 2019 03:49:39 -0400 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 70B5381233; Mon, 8 Apr 2019 07:49:39 +0000 (UTC) Received: from oldenburg2.str.redhat.com (ovpn-116-33.ams2.redhat.com [10.36.116.33]) by smtp.corp.redhat.com (Postfix) with ESMTPS id F24DB6013C; Mon, 8 Apr 2019 07:49:37 +0000 (UTC) From: Florian Weimer To: Alexey Dobriyan Cc: Peter Zijlstra , mingo@redhat.com, linux-kernel@vger.kernel.org, linux-api@vger.kernel.org Subject: Re: [PATCH] sched/core: expand sched_getaffinity(2) to return number of CPUs References: <20190403200809.GA13876@avx2> <20190404084249.GS4038@hirez.programming.kicks-ass.net> <87wok83gfs.fsf@oldenburg2.str.redhat.com> <20190406194825.GA5106@avx2> Date: Mon, 08 Apr 2019 09:49:35 +0200 In-Reply-To: <20190406194825.GA5106@avx2> (Alexey Dobriyan's message of "Sat, 6 Apr 2019 22:48:25 +0300") Message-ID: <878swlszqo.fsf@oldenburg2.str.redhat.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.25]); Mon, 08 Apr 2019 07:49:39 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Alexey Dobriyan: >> >> Patch overloads sched_getaffinity(len=0) to simply return "nr_cpu_ids". >> >> This will make gettting CPU mask require at most 2 system calls >> >> and will eliminate unnecessary code. >> >> >> >> len=0 is chosen so that >> >> * passing zeroes is the simplest thing >> >> >> >> syscall(__NR_sched_getaffinity, 0, 0, NULL) >> >> >> >> will simply do the right thing, >> >> >> >> * old kernels returned -EINVAL unconditionally. >> >> >> >> Note: glibc segfaults upon exiting from system call because it tries to >> >> clear the rest of the buffer if return value is positive, so >> >> applications will have to use syscall(3). >> >> Good news is that it proves noone uses sched_getaffinity(pid, 0, NULL). >> >> Given that old kernels fail with EINVAL, that evidence is fairly >> restricted. >> >> I'm not sure if it's a good idea to overload this interface. I expect >> that users will want to call sched_getaffinity (the system call wrapper) >> with cpusetsize == 0 to query the value, so there will be pressure on >> glibc to remove the memset. At that point we have an API that obscurely >> fails with old glibc versions, but suceeds with newer ones, which isn't >> great. > > I can do "if (len == 536870912)" so that bit count overflows on old > kernels into EINVAL and is unlikely to be used ever. I don't see how this solves this particular issue. It will still result in a mysterious crash if programs use an updated system call wrapper. Thanks, Florian