Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758577AbYAOWIl (ORCPT ); Tue, 15 Jan 2008 17:08:41 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751839AbYAOWIb (ORCPT ); Tue, 15 Jan 2008 17:08:31 -0500 Received: from tomts36-srv.bellnexxia.net ([209.226.175.93]:57466 "EHLO tomts36-srv.bellnexxia.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756641AbYAOWIa (ORCPT ); Tue, 15 Jan 2008 17:08:30 -0500 X-IronPort-Anti-Spam-Filtered: true X-IronPort-Anti-Spam-Result: Aq4HAFO9jEdMROHU/2dsb2JhbACBWKot Date: Tue, 15 Jan 2008 17:08:24 -0500 From: Mathieu Desnoyers To: Steven Rostedt Cc: LKML , Ingo Molnar , Linus Torvalds , Andrew Morton , Peter Zijlstra , Christoph Hellwig , Gregory Haskins , Arnaldo Carvalho de Melo , Thomas Gleixner , Tim Bird , Sam Ravnborg , "Frank Ch. Eigler" , Steven Rostedt Subject: Re: [RFC PATCH 16/22 -v2] add get_monotonic_cycles Message-ID: <20080115220824.GB22242@Krystal> References: <20080109232914.676624725@goodmis.org> <20080109233044.777564395@goodmis.org> <20080115214636.GD17439@Krystal> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Content-Disposition: inline In-Reply-To: X-Editor: vi X-Info: http://krystal.dyndns.org:8080 X-Operating-System: Linux/2.6.21.3-grsec (i686) X-Uptime: 17:07:01 up 73 days, 3:12, 6 users, load average: 0.42, 0.51, 0.63 User-Agent: Mutt/1.5.16 (2007-06-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3086 Lines: 82 * Steven Rostedt (rostedt@goodmis.org) wrote: > > > On Tue, 15 Jan 2008, Mathieu Desnoyers wrote: > > > > > > Signed-off-by: Steven Rostedt > > > --- > > > include/linux/clocksource.h | 3 ++ > > > kernel/time/timekeeping.c | 48 ++++++++++++++++++++++++++++++++++++++++++++ > > > 2 files changed, 51 insertions(+) > > > > > > Index: linux-compile-i386.git/kernel/time/timekeeping.c > > > =================================================================== > > > --- linux-compile-i386.git.orig/kernel/time/timekeeping.c 2008-01-09 14:27:26.000000000 -0500 > > > +++ linux-compile-i386.git/kernel/time/timekeeping.c 2008-01-09 14:34:40.000000000 -0500 > > > @@ -103,6 +103,54 @@ static inline void __get_realtime_clock_ > > > timespec_add_ns(ts, nsecs); > > > } > > > > > > +cycle_t notrace get_monotonic_cycles(void) > > > +{ > > > + cycle_t cycle_now, cycle_delta, cycle_raw, cycle_last; > > > + > > > + do { > > > + /* > > > + * cycle_raw and cycle_last can change on > > > + * another CPU and we need the delta calculation > > > + * of cycle_now and cycle_last happen atomic, as well > > > + * as the adding to cycle_raw. We don't need to grab > > > + * any locks, we just keep trying until get all the > > > + * calculations together in one state. > > > + * > > > + * In fact, we __cant__ grab any locks. This > > > + * function is called from the latency_tracer which can > > > + * be called anywhere. To grab any locks (including > > > + * seq_locks) we risk putting ourselves into a deadlock. > > > + */ > > > > I wonder what makes the compiler read the clock->cycle_raw and > > clock->cycle_last variables twice ? I guess some memory barriers could > > be welcome here ? > > We need both cycle_raw and cycle_last to be the same from the time we read > the clock source to the time we calculate cycle_delta. If either one > changes then delta is bogus. > Ok, but what actually insures that the clock->cycle_* reads won't be reordered across the clocksource_read() ? > Also, it just occurred to me that this is an old patch. I thought I > renamed cycle_raw to cycle_monotonic. But I must have lost that patch :-/ > > > > > > + cycle_raw = clock->cycle_raw; > > > + cycle_last = clock->cycle_last; > > > + > > > + /* read clocksource: */ > > > + cycle_now = clocksource_read(clock); > > > + > > > + /* calculate the delta since the last update_wall_time: */ > > > + cycle_delta = (cycle_now - cycle_last) & clock->mask; > > > + > > > + } while (cycle_raw != clock->cycle_raw || > > > + cycle_last != clock->cycle_last); > > > + > > > + return cycle_raw + cycle_delta; > > > +} > > -- Steve > -- Mathieu Desnoyers Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 -- 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/