Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1762758AbZFLN4U (ORCPT ); Fri, 12 Jun 2009 09:56:20 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755027AbZFLN4K (ORCPT ); Fri, 12 Jun 2009 09:56:10 -0400 Received: from mx2.mail.elte.hu ([157.181.151.9]:59358 "EHLO mx2.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753696AbZFLN4J (ORCPT ); Fri, 12 Jun 2009 09:56:09 -0400 Date: Fri, 12 Jun 2009 15:56:03 +0200 From: Ingo Molnar To: Mike Frysinger Cc: Peter Zijlstra , Paul Mackerras , linux-kernel@vger.kernel.org, Thomas Gleixner Subject: Re: [PATCH] scripts/checksyscalls.sh: only whine perf_counter_open when supported Message-ID: <20090612135603.GF32105@elte.hu> References: <20090612120507.GH16044@elte.hu> <8bd0f97a0906120513u4e823460t8192f00fd0460ab@mail.gmail.com> <20090612121757.GD31845@elte.hu> <8bd0f97a0906120522v51ae0151i48d5f6846ddcff10@mail.gmail.com> <20090612123152.GE31845@elte.hu> <8bd0f97a0906120541x739b7902pe40ff7aa91337bac@mail.gmail.com> <20090612125941.GI31845@elte.hu> <8bd0f97a0906120604o1e90ac93l469e86173224364e@mail.gmail.com> <20090612130949.GJ31845@elte.hu> <8bd0f97a0906120621o609d7489v83cd3efc0102412d@mail.gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <8bd0f97a0906120621o609d7489v83cd3efc0102412d@mail.gmail.com> User-Agent: Mutt/1.5.18 (2008-05-17) X-ELTE-SpamScore: -1.5 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-1.5 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.2.5 -1.5 BAYES_00 BODY: Bayesian spam probability is 0 to 1% [score: 0.0000] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5006 Lines: 104 * Mike Frysinger wrote: > On Fri, Jun 12, 2009 at 09:09, Ingo Molnar wrote: > > * Mike Frysinger wrote: > >> On Fri, Jun 12, 2009 at 08:59, Ingo Molnar wrote: > >> > * Mike Frysinger wrote: > >> >> On Fri, Jun 12, 2009 at 08:31, Ingo Molnar wrote: > >> >> > * Mike Frysinger wrote: > >> >> >> On Fri, Jun 12, 2009 at 08:17, Ingo Molnar wrote: > >> >> >> > * Mike Frysinger wrote: > >> >> >> >> On Fri, Jun 12, 2009 at 08:05, Ingo Molnar wrote: > >> >> >> >> > * Mike Frysinger wrote: > >> >> >> >> >> If the port does not support HAVE_PERF_COUNTERS, then they can't > >> >> >> >> >> support the perf_counter_open syscall either. ?Rather than forcing > >> >> >> >> >> everyone to add an ignore (or suffer the warning until they get > >> >> >> >> >> around to implementing support), only whine about the syscall when > >> >> >> >> >> applicable. > >> >> >> >> > > >> >> >> >> > No, this patch is wrong - it's really easy to add support: just hook > >> >> >> >> > up the syscall. This should happen for every architecture really, so > >> >> >> >> > the warning is correct and it should not be patched out. > >> >> >> >> > > >> >> >> >> > PMU support is not required to get perfcounters support: if an > >> >> >> >> > architecture hooks up the syscall it will get generic software > >> >> >> >> > counters and the tools will work as well. > >> >> >> >> > > >> >> >> >> > Profiling falls back to a hrtimer-based sampling method - this is a > >> >> >> >> > much better fallback than oprofile's fall-back to the timer tick. > >> >> >> >> > This hrtimer based sampling is dynticks/nohz-correct and can go > >> >> >> >> > beyond HZ if the architecture supports hrtimers. > >> >> >> >> > >> >> >> >> if there is generic support available, why must every arch select > >> >> >> >> HAVE_PERF_COUNTERS in their Kconfig ? > >> >> >> > > >> >> >> > Because we only want to enable it on architectures that have tested > >> >> >> > it. It should only need a syscall addition, but nothing beats having > >> >> >> > tested things, hence we have that additional Kconfig symbol. > >> >> >> > >> >> >> that is a pretty weak reason. [...] > >> >> > > >> >> > It isnt - this is proper isolation - dont offer something to the > >> >> > user to enable that 1) cannot be used due to the lack of a syscall > >> >> > 2) has not been tested by anyone on that architecture, ever. > >> >> > > >> >> > That way say build breakages or runtime failures due to perfcounters > >> >> > only become possible on an architecture if the architecture > >> >> > maintainer has hooked up the syscall and has provided > >> >> > HAVE_PERF_COUNTERS explicitly. > >> >> > >> >> except that the syscall presence is trivial to detect in the code by > >> >> something like: > >> >> #ifndef __NR_perf_counter_open > >> >> # error sorry, your arch has not hooked up perf_counter_open syscall yet > >> >> #endif > >> >> > >> >> as for "no arch testing yet", there are plenty of drivers which lack > >> >> arch depends in the Kconfig specifically so that it can be *easily* > >> >> tested on random systems out there without requiring manual twiddling. > >> > > >> > This is a new kernel subsystem, not just yet another driver. > >> > >> so what ? ?if it has generic pieces, it is exactly the same as yet > >> another generic driver. ?people should be able to randomly test > >> build it when possible to discover latent issues that your testing > >> limited to one arch did not find. > >> > >> > Which bit of: "we dont want perfcounters to be enabled in the > >> > Kconfig on architectures that have no syscalls and no testing for > >> > it" is hard to understand? It is a valid technical concern. > >> > >> your (1) is valid but i already pointed out a simple fix for that. > >> your (2) is not. > > > > Uhm, your 'fix': > > > > ?#ifndef __NR_perf_counter_open > > ?# error sorry, your arch has not hooked up perf_counter_open syscall yet > > ?#endif > > > > is completely unacceptable. We dont propagate build failures via > > user-enable config options, we never did. There's a lot of people > > doing randconfig builds - if it randomly failed due to your 'fix' > > that would upset a lot of testing for no good reason. > > accept that is a valid bug: the arch is missing the syscall and it > should hook it up Uhm, that's ridiculous, observe lkml for a few weeks and see what happens when any subsystem fails to build in a user-configurable variation. Even if it's "just" because something like a syscall definition is missing. Anyway, i have no time to teach you about kernel mainteinance basics really so i probably wont follow up on future emails. Thanks, Ingo -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/