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kern_tc.c: Scaling/large delta recalculation

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This change is a slight performance optimization for systems with a slow
64-bit division.

The th->th_scale and th->th_large_delta values only depend on the
timecounter frequency and the th->th_adjustment. The timecounter
frequency of a timehand only changes when a new timecounter is activated
for the timehand. The th->th_adjustment is only changed by the NTP
second update. The NTP second update is not done for every call of
tc_windup().

Move the code block to recalculate the scaling factor and
the large delta of a timehand to the new helper function
recalculate_scaling_factor_and_large_delta().

Call recalculate_scaling_factor_and_large_delta() when a new timecounter
is activated and a NTP second update occurred.

MFC after:	1 week
This commit is contained in:
Sebastian Huber 2021-10-28 10:22:58 +02:00 committed by Konstantin Belousov
parent a901f2af58
commit ae750fbac7
1 changed files with 50 additions and 38 deletions
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88
sys/kern/kern_tc.c
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@ -1310,6 +1310,40 @@ tc_setclock(struct timespec *ts)
}
}
/*
* Recalculate the scaling factor. We want the number of 1/2^64
* fractions of a second per period of the hardware counter, taking
* into account the th_adjustment factor which the NTP PLL/adjtime(2)
* processing provides us with.
*
* The th_adjustment is nanoseconds per second with 32 bit binary
* fraction and we want 64 bit binary fraction of second:
*
* x = a * 2^32 / 10^9 = a * 4.294967296
*
* The range of th_adjustment is +/- 5000PPM so inside a 64bit int
* we can only multiply by about 850 without overflowing, that
* leaves no suitably precise fractions for multiply before divide.
*
* Divide before multiply with a fraction of 2199/512 results in a
* systematic undercompensation of 10PPM of th_adjustment. On a
* 5000PPM adjustment this is a 0.05PPM error. This is acceptable.
*
* We happily sacrifice the lowest of the 64 bits of our result
* to the goddess of code clarity.
*/
static void
recalculate_scaling_factor_and_large_delta(struct timehands *th)
{
uint64_t scale;
scale = (uint64_t)1 << 63;
scale += (th->th_adjustment / 1024) * 2199;
scale /= th->th_counter->tc_frequency;
th->th_scale = scale * 2;
th->th_large_delta = MIN(((uint64_t)1 << 63) / scale, UINT_MAX);
}
/*
* Initialize the next struct timehands in the ring and make
* it the active timehands. Along the way we might switch to a different
@ -1320,7 +1354,6 @@ tc_windup(struct bintime *new_boottimebin)
{
struct bintime bt;
struct timehands *th, *tho;
uint64_t scale;
u_int delta, ncount, ogen;
int i;
time_t t;
@ -1382,7 +1415,7 @@ tc_windup(struct bintime *new_boottimebin)
tho->th_counter->tc_poll_pps(tho->th_counter);
/*
* Deal with NTP second processing. The for loop normally
* Deal with NTP second processing. The loop normally
* iterates at most once, but in extreme situations it might
* keep NTP sane if timeouts are not run for several seconds.
* At boot, the time step can be large when the TOD hardware
@ -1393,14 +1426,21 @@ tc_windup(struct bintime *new_boottimebin)
bt = th->th_offset;
bintime_add(&bt, &th->th_boottime);
i = bt.sec - tho->th_microtime.tv_sec;
if (i > LARGE_STEP)
i = 2;
for (; i > 0; i--) {
t = bt.sec;
ntp_update_second(&th->th_adjustment, &bt.sec);
if (bt.sec != t)
th->th_boottime.sec += bt.sec - t;
if (i > 0) {
if (i > LARGE_STEP)
i = 2;
do {
t = bt.sec;
ntp_update_second(&th->th_adjustment, &bt.sec);
if (bt.sec != t)
th->th_boottime.sec += bt.sec - t;
--i;
} while (i > 0);
recalculate_scaling_factor_and_large_delta(th);
}
/* Update the UTC timestamps used by the get*() functions. */
th->th_bintime = bt;
bintime2timeval(&bt, &th->th_microtime);
@ -1418,40 +1458,12 @@ tc_windup(struct bintime *new_boottimebin)
th->th_offset_count = ncount;
tc_min_ticktock_freq = max(1, timecounter->tc_frequency /
(((uint64_t)timecounter->tc_counter_mask + 1) / 3));
recalculate_scaling_factor_and_large_delta(th);
#ifdef FFCLOCK
ffclock_change_tc(th);
#endif
}
/*-
* Recalculate the scaling factor. We want the number of 1/2^64
* fractions of a second per period of the hardware counter, taking
* into account the th_adjustment factor which the NTP PLL/adjtime(2)
* processing provides us with.
*
* The th_adjustment is nanoseconds per second with 32 bit binary
* fraction and we want 64 bit binary fraction of second:
*
* x = a * 2^32 / 10^9 = a * 4.294967296
*
* The range of th_adjustment is +/- 5000PPM so inside a 64bit int
* we can only multiply by about 850 without overflowing, that
* leaves no suitably precise fractions for multiply before divide.
*
* Divide before multiply with a fraction of 2199/512 results in a
* systematic undercompensation of 10PPM of th_adjustment. On a
* 5000PPM adjustment this is a 0.05PPM error. This is acceptable.
*
* We happily sacrifice the lowest of the 64 bits of our result
* to the goddess of code clarity.
*
*/
scale = (uint64_t)1 << 63;
scale += (th->th_adjustment / 1024) * 2199;
scale /= th->th_counter->tc_frequency;
th->th_scale = scale * 2;
th->th_large_delta = MIN(((uint64_t)1 << 63) / scale, UINT_MAX);
/*
* Now that the struct timehands is again consistent, set the new
* generation number, making sure to not make it zero.