From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757274Ab3K0ON7 (ORCPT ); Wed, 27 Nov 2013 09:13:59 -0500 Received: from merlin.infradead.org ([205.233.59.134]:51410 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750907Ab3K0ON4 (ORCPT ); Wed, 27 Nov 2013 09:13:56 -0500 Date: Wed, 27 Nov 2013 15:13:34 +0100 From: Peter Zijlstra To: Ingo Molnar Cc: Juri Lelli , tglx@linutronix.de, mingo@redhat.com, rostedt@goodmis.org, oleg@redhat.com, fweisbec@gmail.com, darren@dvhart.com, johan.eker@ericsson.com, p.faure@akatech.ch, linux-kernel@vger.kernel.org, claudio@evidence.eu.com, michael@amarulasolutions.com, fchecconi@gmail.com, tommaso.cucinotta@sssup.it, nicola.manica@disi.unitn.it, luca.abeni@unitn.it, dhaval.giani@gmail.com, hgu1972@gmail.com, paulmck@linux.vnet.ibm.com, raistlin@linux.it, insop.song@gmail.com, liming.wang@windriver.com, jkacur@redhat.com, harald.gustafsson@ericsson.com, vincent.guittot@linaro.org, bruce.ashfield@windriver.com, Andrew Morton , Linus Torvalds Subject: Re: [PATCH 02/14] sched: add extended scheduling interface. (new ABI) Message-ID: <20131127141334.GA13532@twins.programming.kicks-ass.net> References: <1383831828-15501-1-git-send-email-juri.lelli@gmail.com> <1383831828-15501-3-git-send-email-juri.lelli@gmail.com> <20131127132354.GA18422@gmail.com> <20131127133027.GE10022@twins.programming.kicks-ass.net> <20131127140143.GB24403@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20131127140143.GB24403@gmail.com> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Nov 27, 2013 at 03:01:43PM +0100, Ingo Molnar wrote: > > So the problem I see with this one is that because you're allowed to > > call sched_setparam() or whatever it will be called next on another > > task; a task can very easily fail its sched_getparam() call. > > > > Suppose the application is 'old' and only supports a subset of the > > fields; but its wants to get, modify and set its params. This will > > work as long nothing will set anything it doesn't know about. > > > > As soon as some external entity -- say a sysad using schedtool -- > > sets a param field it doesn't support the get, modify, set routing > > completely fails. > > There are two approaches to this that I can see: > > 1) > > allow partial information to be returned to user-space, for existing > input parameters. The new fields won't be displayed, but the tool > doesn't know about them anyway so it's OK. The tool can still display > all the other existing parameters. But suppose a task simply wants to lower/raise its static (FIFO) priority and does: sched_getparam(¶ms); params.prio += 1; sched_setparam(¶ms); If anything outside of the known param fields was set, we just silently lost it, for the setparam() call will fill out 0s for the unprovided fields. > 2) > > Return -ENOSYS if the 'extra' fields are nonzero. In this case the > usual case of old tooling + new kernel will still work just fine, > because old tooling won't set the new fields to any non-default > (nonzero) values. In the 'mixed' case old tooling will not be able to > change/display those fields. > > I tend to lean towards #1. What do you think? As per the above that can result in silent unexpected behavioural changes. I'd much rather be explicit and break hard; so 2). So mixing new tools (schedtool, chrt etc) and old apps will give pain, but at least not silent surprises.