Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751749AbaJSWUQ (ORCPT ); Sun, 19 Oct 2014 18:20:16 -0400 Received: from bombadil.infradead.org ([198.137.202.9]:52837 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751476AbaJSWUN (ORCPT ); Sun, 19 Oct 2014 18:20:13 -0400 Date: Mon, 20 Oct 2014 00:20:04 +0200 From: Peter Zijlstra To: Andy Lutomirski Cc: Erik Bosman , Ingo Molnar , "linux-kernel@vger.kernel.org" , Paul Mackerras , Kees Cook , Arnaldo Carvalho de Melo , Andrea Arcangeli , Valdis Kletnieks Subject: Re: [RFC 5/5] x86,perf: Only allow rdpmc if a perf_event is mapped Message-ID: <20141019222004.GI23531@worktop.programming.kicks-ass.net> References: <20141019213341.GF23531@worktop.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.22.1 (2013-10-16) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Oct 19, 2014 at 03:05:42PM -0700, Andy Lutomirski wrote: > On Oct 19, 2014 2:33 PM, "Peter Zijlstra" wrote: > > > Also, I don't understand the purpose of cap_user_time. Wouldn't it be > > > easier to just record the last CLOCK_MONOTONIC time and let the user > > > call __vdso_clock_gettime if they need an updated time? > > > > Because perf doesn't use CLOCK_MONOTONIC. Due to performance > > considerations we used the sched_clock stuff, which tries its best to > > make the best of the TSC without reverting to HPET and the like. > > > > Not to mention that CLOCK_MONOTONIC was not available from NMI context > > until very recently. > > I'm only talking about the userspace access to when an event was > enabled and how long it's been running. I think that's what the > cap_user_time stuff is for. I don't think those parameters are > touched from NMI, right? > > Point taken about sched_clock, though. Well, mixing two time bases, one TSC based and one CLOCK_MONOTONIC is just asking for trouble IMO ;-) > > Also, things like c73deb6aecda ("perf/x86: Add ability to calculate TSC > > from perf sample timestamps") seem to suggest people actually use TSC > > for things as well. > > > > Now we might change to using the new NMI safe CLOCK_MONOTONIC (with a > > fallback to use the sched_clock stuff on time challenged hardware) in > > order to ease the correlation between other trace thingies, but even > > then it makes sense to have this, having it here and reading the TSC > > within the seqcount loop ensures you've got consistent data and touch > > less cachelines for reading. > > True. > > OTOH, people (i.e. I) have optimized the crap out of > __vdso_clock_gettime, and __vdso_perf_event_whatever could be > similarly optimized. Maybe, but at that point we commit to yet another ABI... I'd rather just put a 'sane' implementation in a library or so. -- 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/