Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757885AbYAOWBo (ORCPT ); Tue, 15 Jan 2008 17:01:44 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753399AbYAOWBf (ORCPT ); Tue, 15 Jan 2008 17:01:35 -0500 Received: from ms-smtp-04.nyroc.rr.com ([24.24.2.58]:41315 "EHLO ms-smtp-04.nyroc.rr.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752256AbYAOWBe (ORCPT ); Tue, 15 Jan 2008 17:01:34 -0500 Date: Tue, 15 Jan 2008 17:01:30 -0500 (EST) From: Steven Rostedt X-X-Sender: rostedt@gandalf.stny.rr.com To: Mathieu Desnoyers 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 In-Reply-To: <20080115214636.GD17439@Krystal> Message-ID: References: <20080109232914.676624725@goodmis.org> <20080109233044.777564395@goodmis.org> <20080115214636.GD17439@Krystal> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2627 Lines: 71 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. 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 -- 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/