Messages in this thread Patch in this message | | | Subject | Re: 2.6.17-mm2 hrtimer code wedges at boot? | From | john stultz <> | Date | Wed, 05 Jul 2006 17:51:54 -0700 |
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On Wed, 2006-07-05 at 00:29 -0400, Valdis.Kletnieks@vt.edu wrote: > On Mon, 03 Jul 2006 03:13:39 +0200, Roman Zippel said: > > Hi, > > > > On Fri, 30 Jun 2006, Valdis.Kletnieks@vt.edu wrote: > > > > > *AHA* I *found* the bugger, I think. > > > > > > In kernel/timer.c, we have: > > > > > > static void clocksource_adjust(struct clocksource *clock, s64 offset) > > > (s64 used for offset in multiple places). > > > > > > However, in other places, offset is a 'cycle_t', which is: > > > > > > include/linux/clocksource.h:typedef u64 cycle_t; > > > > > > So it looks like a signed/unsigned screwage. > > > > It's a possibility, but the signed/unsigned usage is pretty much > > intentional. The assumptation is that time only goes forward so nothing > > there should become negative, only adjustments happen in both directions. > > It's really weird why it's getting completely so out of control early > > during boot. It would be great if you could also test the patch below, it > > should trigger closer to when it goes wrong and help to analyze the > > problem (or at least rule out a number of possibilities). > > Here you go.. For what it's worth, your debugging in clocksource_adjust seems > to only pop before we start userspace, and get_realtime_clock_ts only once > userspace starts.
Once again, thanks for the testing! My observations below...
> The dmesg, with all the suggested patches so far applied. Looks like something > starts off uninitialized - we get the first 'big adj' squawk right after we > allocate the console - we don't allocate the tsc timesource for another 4 > seconds or so. > > I'll bite - what *am* I using as a timesource for those first 4 seconds? :)
The jiffies clocksource.
> [ 0.000000] Detected 1595.408 MHz processor. ... >[ 24.322196] CPU: Intel(R) Pentium(R) 4 Mobile CPU 1.60GHz stepping 04 ... > [ 29.528533] Time: tsc clocksource has been installed. > [ 29.552855] clock changed at -296333 (4294314460971008) > [ 29.577109] clock tsc: m:2628985,s:22,cl: ,ci:1595166,xn:0,xi:4193667486510,e:0
Ok, so here's our initial TSC state:
Verify the mult/shift pair: 2^s/m = 2^22/2628985 = 1.595408113777750729 cyc/ns => 1.595 GHz
Verify the cycle_interval/xtime_interval pair: xi = ci*m = 1595166 * 2628985 = 4193667486510
Convert xi to ns: xi>>s = 4193667486510>>22 = 999848.2433581352234 ns/interval
Convert ntp_tick to ns: ntp_tick>>32 = 4294314460971008>>32 = 999848 ns/tick
Ok, that all looks pretty good...
> [ 29.601869] big adj at -296332 (4294314460971008,-16,-25522656,-11031712) > [ 29.626688] clock tsc: m:2628985,s:22,cl:47288392250,ci:1595166,xn:148610636380190,xi:4193667486510,e:-76300711936
Now here's where things turn odd. Note that only one jiffy has passed (-296332 - -296333 = 1).
However, looking at the difference between cycle_last: 47288392250 - 47171945132 = 116,447,118
That's *way* larger then the 1,595,166 value expected in ci!
Same thing is seen in the later data points:
47452694348 - 47368150550 = 84,543,798 47538833312 - 47452694348 = 86,138,964 etc.
So it seems either something is causing you to take interrupts at a lower frequency then what is expected, or your cpu is ~50x faster then advertised :)
This is probably not an issue w/ the timekeeping code, however as a side-effect it appears to make the clocksource_adjust function oscillate pretty severely. I've reproduced a similar hang (not completely sure, as it occurred while X was loading) by adding the following to the top of update_wall_time:
static int droptick; if(droptick++%60) return;
Roman: While I'm not 100% confident about my assessment above, I worry this is mimicking the problems I had been seeing in my simulator w/ your clocksource_adjustment algorithm when I simulated dropping many ticks. While currently this behavior points to some other problem, with the dynticks patch, its much more likely that we will see 100s of ticks skipped.
I quickly revived my P-D adjustment patch and it does not appear to suffer from the same problem with the above droptick change (although its only been lightly tested).
I realize you may have a more trivial change to this issue, but would you consider my method again?
Vladis: Mind trying the following patch to see if it affects the behavior.
thanks -john
Implement P-D control for clocksource_adjust()
diff --git a/kernel/timer.c b/kernel/timer.c index 396a3c0..f4e7681 100644 --- a/kernel/timer.c +++ b/kernel/timer.c @@ -1007,81 +1007,108 @@ static int __init timekeeping_init_devic device_initcall(timekeeping_init_device); -/* - * If the error is already larger, we look ahead another tick, - * to compensate for late or lost adjustments. - */ -static __always_inline int clocksource_bigadjust(int sign, s64 error, s64 *interval, s64 *offset) +static int error_aproximation(u64 error, u64 unit, int max) { - int adj; - - /* - * As soon as the machine is synchronized to the external time - * source this should be the common case. - */ - error >>= 2; - if (likely(sign > 0 ? error <= *interval : error >= *interval)) - return sign; - - /* - * An extra look ahead dampens the effect of the current error, - * which can grow quite large with continously late updates, as - * it would dominate the adjustment value and can lead to - * oscillation. - */ - error += current_tick_length() >> (TICK_LENGTH_SHIFT - clock->shift + 1); - error -= clock->xtime_interval >> 1; - - adj = 0; + int adj = 0; while (1) { error >>= 1; - if (sign > 0 ? error <= *interval : error >= *interval) - break; - adj++; + if (error <= unit) + return adj; + if (!max || adj < max) + adj++; } - - /* - * Add the current adjustments to the error and take the offset - * into account, the latter can cause the error to be hardly - * reduced at the next tick. Check the error again if there's - * room for another adjustment, thus further reducing the error - * which otherwise had to be corrected at the next update. - */ - error = (error << 1) - *interval + *offset; - if (sign > 0 ? error > *interval : error < *interval) - adj++; - - *interval <<= adj; - *offset <<= adj; - return sign << adj; } +#define MAXOFFADJ 4 /* vary max oscillation vs convergance speed */ /* * Adjust the multiplier to reduce the error value, * this is optimized for the most common adjustments of -1,0,1, * for other values we can do a bit more work. */ -static void clocksource_adjust(struct clocksource *clock, s64 offset) +static void clocksource_adjust(struct clocksource *clock, s64 offset, + s64 interval_cycs, s64 interval_error) { s64 error, interval = clock->cycle_interval; - int adj; - - error = clock->error >> (TICK_LENGTH_SHIFT - clock->shift - 1); - if (error > interval) { - adj = clocksource_bigadjust(1, error, &interval, &offset); - } else if (error < -interval) { - interval = -interval; - offset = -offset; - adj = clocksource_bigadjust(-1, error, &interval, &offset); - } else - return; + + error = shift_right(clock->error, (TICK_LENGTH_SHIFT - clock->shift)); + interval_error = shift_right(interval_error, + (TICK_LENGTH_SHIFT - clock->shift)); + + if ((error > interval) + ||(error < -(interval)) ) { + + int adj, multadj = 0; + s64 offset_update = 0, snsec_update = 0; + + /* First do the frequency adjustment: + * The idea here is to look at the error + * accumulated since the last call to + * update_wall_time to determine the + * frequency adjustment needed so no new + * error will be incurred in the next + * interval. + * + * This is basically derivative control + * using the PID terminology (we're calculating + * the derivative of the slope and correcting it). + * + * The math is basically: + * multadj = interval_error/interval_cycles + * Which we fudge using binary approximation. + */ + if(interval_error >= 0) { + adj = error_aproximation(interval_error, + interval_cycs, 0); + multadj += 1 << adj; + snsec_update += interval << adj; + offset_update += offset << adj; + } else { + adj = error_aproximation(-interval_error, + interval_cycs, 0); + multadj -= 1 << adj; + snsec_update -= interval << adj; + offset_update -= offset << adj; + } + /* Now do the offset adjustment: + * Now that the frequncy is fixed, we + * want to look at the total error accumulated + * to move us back in sync using the same method. + * However, we must be careful as if we make too + * sudden an adjustment we might overshoot. So we + * limit the amount of change to spread the + * adjustment (using MAXOFFADJ) over a longer + * period of time. + * + * This is basically proportional control + * using the PID terminology. + * + * We use interval_cycs here as the divisor, which + * hopes that the next sample will be similar in + * distance from the last. + */ + if(error >= 0) { + adj = error_aproximation(error, + interval_cycs, MAXOFFADJ); + multadj += 1<<adj; + snsec_update += interval <<adj; + offset_update += offset << adj; + } else { + adj = error_aproximation(-error, + interval_cycs, MAXOFFADJ); + multadj -= 1<<adj; + snsec_update -= interval <<adj; + offset_update -= offset << adj; + } - clock->mult += adj; - clock->xtime_interval += interval; - clock->xtime_nsec -= offset; - clock->error -= (interval - offset) << (TICK_LENGTH_SHIFT - clock->shift); + clock->mult += multadj; + clock->xtime_interval += snsec_update; + clock->xtime_nsec -= offset_update; + clock->error += (offset_update) + << (TICK_LENGTH_SHIFT - clock->shift); + } } + /* * update_wall_time - Uses the current clocksource to increment the wall time * @@ -1089,7 +1116,8 @@ static void clocksource_adjust(struct cl */ static void update_wall_time(void) { - cycle_t offset; + cycle_t offset, interval_cycs = 0; + s64 interval_error = 0; clock->xtime_nsec += (s64)xtime.tv_nsec << clock->shift; @@ -1106,8 +1134,13 @@ static void update_wall_time(void) /* accumulate one interval */ clock->xtime_nsec += clock->xtime_interval; clock->cycle_last += clock->cycle_interval; + interval_cycs += clock->cycle_interval; offset -= clock->cycle_interval; + /* accumulate error between NTP and clock interval */ + interval_error += current_tick_length(); + interval_error -= clock->xtime_interval << (TICK_LENGTH_SHIFT - clock->shift); + if (clock->xtime_nsec >= (u64)NSEC_PER_SEC << clock->shift) { clock->xtime_nsec -= (u64)NSEC_PER_SEC << clock->shift; xtime.tv_sec++; @@ -1119,14 +1152,10 @@ static void update_wall_time(void) >> clock->shift); /* increment the NTP state machine */ update_ntp_one_tick(); - - /* accumulate error between NTP and clock interval */ - clock->error += current_tick_length(); - clock->error -= clock->xtime_interval << (TICK_LENGTH_SHIFT - clock->shift); } - + clock->error += interval_error; /* correct the clock when NTP error is too big */ - clocksource_adjust(clock, offset); + clocksource_adjust(clock, offset, interval_cycs, interval_error); /* store full nanoseconds into xtime */ xtime.tv_nsec = clock->xtime_nsec >> clock->shift;
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