Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932602AbZJEKpc (ORCPT ); Mon, 5 Oct 2009 06:45:32 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S932583AbZJEKpc (ORCPT ); Mon, 5 Oct 2009 06:45:32 -0400 Received: from mx1.redhat.com ([209.132.183.28]:55941 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932490AbZJEKpb (ORCPT ); Mon, 5 Oct 2009 06:45:31 -0400 Date: Mon, 5 Oct 2009 12:43:37 +0200 (CEST) From: John Kacur X-X-Sender: jkacur@localhost.localdomain To: john stultz cc: lkml , Clark Williams , Ingo Molnar , Martin Schwidefsky , Thomas Gleixner , Andrew Morton Subject: Re: [PATCH 1/2] time: logrithmic time accumulation In-Reply-To: <1254525473.7741.88.camel@localhost.localdomain> Message-ID: References: <1254525473.7741.88.camel@localhost.localdomain> User-Agent: Alpine 2.00 (LFD 1167 2008-08-23) 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: 6927 Lines: 187 On Fri, 2 Oct 2009, john stultz wrote: > Accumulating one tick at a time works well unless we're using NOHZ. Then > it can be an issue, since we may have to run through the loop a few > thousand times, which can increase timer interrupt caused latency. > > The current solution was to accumulate in half-second intervals with > NOHZ. This kept the number of loops down, however it did slightly change > how we make NTP adjustments. While not an issue with NTPd users, as NTPd > makes adjustments over a longer period of time, other adjtimex() users > have noticed the half-second granularity with which we can apply > frequency changes to the clock. > > For instance, if a application tries to apply a 100ppm frequency > correction for 20ms to correct a 2us offset, with NOHZ they either get > no correction, or a 50us correction. > > Now, there will always be some granularity error for applying frequency > corrections. However with users sensitive to this error have seen a > 50-500x increase with NOHZ compared to running without NOHZ. > > So I figured I'd try another approach then just simply increasing the > interval. My approach is to consume the time interval logarithmically. > This reduces the number of times through the loop needed keeping > latency down, while still preserving the original granularity error for > adjtimex() changes. > > Further, this change allows us to remove the xtime_cache code (patch to > follow), as xtime is always within one tick of the current time, instead > of the half-second updates it saw before. > > An earlier version of this patch has been shipping to x86 users in the > RedHat MRG releases for awhile without issue, but I've reworked this > version to be even more careful about avoiding possible overflows if the > shift value gets too large. > > Since this is not the most trivial code, and its slightly different then > whats been tested for awhile, it would be good to get this into some > trees for testing. Be it -tip or -mm, either would work. If there's no > problems it could be a 2.6.33 or 2.6.34 item. > > Any comments or feedback would be appreciated! > > Signed-off-by: John Stultz > > diff --git a/include/linux/timex.h b/include/linux/timex.h > index e6967d1..0c0ef7d 100644 > --- a/include/linux/timex.h > +++ b/include/linux/timex.h > @@ -261,11 +261,7 @@ static inline int ntp_synced(void) > > #define NTP_SCALE_SHIFT 32 > > -#ifdef CONFIG_NO_HZ > -#define NTP_INTERVAL_FREQ (2) > -#else > #define NTP_INTERVAL_FREQ (HZ) > -#endif > #define NTP_INTERVAL_LENGTH (NSEC_PER_SEC/NTP_INTERVAL_FREQ) > > /* Returns how long ticks are at present, in ns / 2^NTP_SCALE_SHIFT. */ > diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c > index fb0f46f..4cc5656 100644 > --- a/kernel/time/timekeeping.c > +++ b/kernel/time/timekeeping.c > @@ -721,6 +721,51 @@ static void timekeeping_adjust(s64 offset) > timekeeper.ntp_error_shift; > } > > + > +/** > + * logarithmic_accumulation - shifted accumulation of cycles > + * > + * This functions accumulates a shifted interval of cycles into > + * into a shifted interval nanoseconds. Allows for O(log) accumulation > + * loop. > + * > + * Returns the unconsumed cycles. > + */ > +static cycle_t logarithmic_accumulation(cycle_t offset, int shift) > +{ > + u64 nsecps = (u64)NSEC_PER_SEC << timekeeper.shift; > + > + /* If the offset is smaller then a shifted interval, do nothing */ > + if (offset < timekeeper.cycle_interval< + return offset; > + > + /* accumulate one shifted interval */ > + offset -= timekeeper.cycle_interval << shift; > + timekeeper.clock->cycle_last += timekeeper.cycle_interval << shift; > + > + timekeeper.xtime_nsec += timekeeper.xtime_interval << shift; > + while (timekeeper.xtime_nsec >= nsecps) { > + timekeeper.xtime_nsec -= nsecps; > + xtime.tv_sec++; > + second_overflow(); > + } > + > + /* accumulate into raw time */ > + raw_time.tv_nsec += timekeeper.raw_interval << shift;; > + while (raw_time.tv_nsec >= NSEC_PER_SEC) { > + raw_time.tv_nsec -= NSEC_PER_SEC; > + raw_time.tv_sec++; > + } > + > + /* accumulate error between NTP and clock interval */ > + timekeeper.ntp_error += tick_length << shift; > + timekeeper.ntp_error -= timekeeper.xtime_interval << > + (timekeeper.ntp_error_shift + shift); > + > + return offset; > +} > + > + > /** > * update_wall_time - Uses the current clocksource to increment the wall time > * > @@ -731,6 +776,7 @@ void update_wall_time(void) > struct clocksource *clock; > cycle_t offset; > u64 nsecs; > + int shift = 0, maxshift; > > /* Make sure we're fully resumed: */ > if (unlikely(timekeeping_suspended)) > @@ -744,33 +790,22 @@ void update_wall_time(void) > #endif > timekeeper.xtime_nsec = (s64)xtime.tv_nsec << timekeeper.shift; > > - /* normally this loop will run just once, however in the > - * case of lost or late ticks, it will accumulate correctly. > + /* > + * With NO_HZ we may have to accumulate many cycle_intervals > + * (think "ticks") worth of time at once. To do this efficiently, > + * we calculate the largest doubling multiple of cycle_intervals > + * that is smaller then the offset. We then accumulate that > + * chunk in one go, and then try to consume the next smaller > + * doubled multiple. > */ > + shift = ilog2(offset) - ilog2(timekeeper.cycle_interval); > + shift = max(0, shift); > + /* Bound shift to one less then what overflows tick_length */ > + maxshift = (8*sizeof(tick_length) - (ilog2(tick_length)+1)) - 1; > + shift = min(shift, maxshift); > while (offset >= timekeeper.cycle_interval) { > - u64 nsecps = (u64)NSEC_PER_SEC << timekeeper.shift; > - > - /* accumulate one interval */ > - offset -= timekeeper.cycle_interval; > - clock->cycle_last += timekeeper.cycle_interval; > - > - timekeeper.xtime_nsec += timekeeper.xtime_interval; > - if (timekeeper.xtime_nsec >= nsecps) { > - timekeeper.xtime_nsec -= nsecps; > - xtime.tv_sec++; > - second_overflow(); > - } > - > - raw_time.tv_nsec += timekeeper.raw_interval; > - if (raw_time.tv_nsec >= NSEC_PER_SEC) { > - raw_time.tv_nsec -= NSEC_PER_SEC; > - raw_time.tv_sec++; > - } > - > - /* accumulate error between NTP and clock interval */ > - timekeeper.ntp_error += tick_length; > - timekeeper.ntp_error -= timekeeper.xtime_interval << > - timekeeper.ntp_error_shift; > + offset = logarithmic_accumulation(offset, shift); > + shift--; > } > > /* correct the clock when NTP error is too big */ > > > There are several (6) trailing whitespace errors that checkpatch exposes, but other than that: Signed-off-by: John Kacur -- 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/