Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754626AbZJBXTa (ORCPT ); Fri, 2 Oct 2009 19:19:30 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753857AbZJBXT3 (ORCPT ); Fri, 2 Oct 2009 19:19:29 -0400 Received: from e1.ny.us.ibm.com ([32.97.182.141]:43213 "EHLO e1.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752327AbZJBXT3 (ORCPT ); Fri, 2 Oct 2009 19:19:29 -0400 Subject: [PATCH 1/2] time: logrithmic time accumulation From: john stultz To: lkml Cc: John Kacur , Clark Williams , Ingo Molnar , Martin Schwidefsky , Thomas Gleixner , Andrew Morton Content-Type: text/plain Date: Fri, 02 Oct 2009 16:17:53 -0700 Message-Id: <1254525473.7741.88.camel@localhost.localdomain> Mime-Version: 1.0 X-Mailer: Evolution 2.24.3 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 6393 Lines: 178 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<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 */ -- 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/