Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754517AbYAPXeL (ORCPT ); Wed, 16 Jan 2008 18:34:11 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751761AbYAPXd5 (ORCPT ); Wed, 16 Jan 2008 18:33:57 -0500 Received: from ms-smtp-05.nyroc.rr.com ([24.24.2.59]:51809 "EHLO ms-smtp-05.nyroc.rr.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751262AbYAPXd4 (ORCPT ); Wed, 16 Jan 2008 18:33:56 -0500 Date: Wed, 16 Jan 2008 18:33:50 -0500 (EST) From: Steven Rostedt X-X-Sender: rostedt@gandalf.stny.rr.com To: john stultz cc: Mathieu Desnoyers , 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 , Paul Mackerras , Daniel Walker Subject: Re: [RFC PATCH 16/22 -v2] add get_monotonic_cycles In-Reply-To: <1200523867.6127.5.camel@localhost.localdomain> Message-ID: References: <20080109232914.676624725@goodmis.org> <20080109233044.777564395@goodmis.org> <20080115214636.GD17439@Krystal> <20080115220824.GB22242@Krystal> <20080116031730.GA2164@Krystal> <20080116145604.GB31329@Krystal> <1f1b08da0801161436k4a7ac1e3kd83590951e7bebb9@mail.gmail.com> <1200523867.6127.5.camel@localhost.localdomain> 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: 5907 Lines: 179 Thanks John for doing this! (comments imbedded) On Wed, 16 Jan 2008, john stultz wrote: > > On Wed, 2008-01-16 at 14:36 -0800, john stultz wrote: > > Completely un-tested, but it builds, so I figured I'd send it out for > review. heh, ok, I'll take it and run it. > > I'm not super sure the update or the read doesn't need something > additional to force a memory access, but as I didn't see anything > special in Mathieu's implementation, I'm going to guess this is ok. > > Mathieu, Let me know if this isn't what you're suggesting. > > Signed-off-by: John Stultz > > Index: monotonic-cleanup/include/linux/clocksource.h > =================================================================== > --- monotonic-cleanup.orig/include/linux/clocksource.h 2008-01-16 12:22:04.000000000 -0800 > +++ monotonic-cleanup/include/linux/clocksource.h 2008-01-16 14:41:31.000000000 -0800 > @@ -87,9 +87,17 @@ > * more than one cache line. > */ > struct { > - cycle_t cycle_last, cycle_accumulated, cycle_raw; > - } ____cacheline_aligned_in_smp; > + cycle_t cycle_last, cycle_accumulated; > > + /* base structure provides lock-free read > + * access to a virtualized 64bit counter > + * Uses RCU-like update. > + */ > + struct { > + cycle_t cycle_base_last, cycle_base; > + } base[2]; > + int base_num; > + } ____cacheline_aligned_in_smp; > u64 xtime_nsec; > s64 error; > > @@ -175,19 +183,21 @@ > } > > /** > - * clocksource_get_cycles: - Access the clocksource's accumulated cycle value > + * clocksource_get_basecycles: - get the clocksource's accumulated cycle value > * @cs: pointer to clocksource being read > * @now: current cycle value > * > * Uses the clocksource to return the current cycle_t value. > * NOTE!!!: This is different from clocksource_read, because it > - * returns the accumulated cycle value! Must hold xtime lock! > + * returns a 64bit wide accumulated value. > */ > static inline cycle_t > -clocksource_get_cycles(struct clocksource *cs, cycle_t now) > +clocksource_get_basecycles(struct clocksource *cs, cycle_t now) > { > - cycle_t offset = (now - cs->cycle_last) & cs->mask; > - offset += cs->cycle_accumulated; > + int num = cs->base_num; > + cycle_t offset = (now - cs->base[num].cycle_base_last); > + offset &= cs->mask; > + offset += cs->base[num].cycle_base; > return offset; > } > > @@ -197,14 +207,25 @@ > * @now: current cycle value > * > * Used to avoids clocksource hardware overflow by periodically > - * accumulating the current cycle delta. Must hold xtime write lock! > + * accumulating the current cycle delta. Uses RCU-like update, but > + * ***still requires the xtime_lock is held for writing!*** > */ > static inline void clocksource_accumulate(struct clocksource *cs, cycle_t now) > { > - cycle_t offset = (now - cs->cycle_last) & cs->mask; > + /* First update the monotonic base portion. > + * The dual array update method allows for lock-free reading. > + */ > + int num = !cs->base_num; > + cycle_t offset = (now - cs->base[!num].cycle_base_last); > + offset &= cs->mask; > + cs->base[num].cycle_base = cs->base[!num].cycle_base + offset; > + cs->base[num].cycle_base_last = now; I would think that we would need some sort of barrier here. Otherwise, base_num could be updated before all the cycle_base. I'd expect a smp_wmb is needed. > + cs->base_num = num; > + > + /* Now update the cycle_accumulated portion */ > + offset = (now - cs->cycle_last) & cs->mask; > cs->cycle_last = now; > cs->cycle_accumulated += offset; > - cs->cycle_raw += offset; > } > > /** > Index: monotonic-cleanup/kernel/time/timekeeping.c > =================================================================== > --- monotonic-cleanup.orig/kernel/time/timekeeping.c 2008-01-16 12:21:46.000000000 -0800 > +++ monotonic-cleanup/kernel/time/timekeeping.c 2008-01-16 14:15:31.000000000 -0800 > @@ -71,10 +71,12 @@ > */ > static inline s64 __get_nsec_offset(void) > { > - cycle_t cycle_delta; > + cycle_t now, cycle_delta; > s64 ns_offset; > > - cycle_delta = clocksource_get_cycles(clock, clocksource_read(clock)); > + now = clocksource_read(clock); > + cycle_delta = (now - clock->cycle_last) & clock->mask; > + cycle_delta += clock->cycle_accumulated; Is the above just to decouple the two methods? > ns_offset = cyc2ns(clock, cycle_delta); > > return ns_offset; > @@ -105,35 +107,7 @@ > > 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. > - */ > - 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; > + return clocksource_get_basecycles(clock, clocksource_read(clock)); Nice ;-) > } > > unsigned long notrace cycles_to_usecs(cycle_t cycles) -- 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/