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authorSebastian Huber <sebastian.huber@embedded-brains.de>2021-10-28 10:22:58 +0200
committerSebastian Huber <sebastian.huber@embedded-brains.de>2021-11-15 08:56:54 +0100
commit346755874b5e0b4446bd21036a56574081b43e3f (patch)
treeb1714b322539f3a03c91689776fb072fd4c8b596
parentea389aaaddefd1606632843e38923ddba098574b (diff)
downloadrtems-346755874b5e0b4446bd21036a56574081b43e3f.tar.bz2
kern_tc.c: Scaling/large delta recalculation
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
-rw-r--r--cpukit/score/src/kern_tc.c88
1 files changed, 50 insertions, 38 deletions
diff --git a/cpukit/score/src/kern_tc.c b/cpukit/score/src/kern_tc.c
index d8f92e11dd..a2e79b0b25 100644
--- a/cpukit/score/src/kern_tc.c
+++ b/cpukit/score/src/kern_tc.c
@@ -1522,6 +1522,40 @@ _Timecounter_Set_clock(const struct bintime *_bt,
}
/*
+ * 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
* timecounter and/or do seconds processing in NTP. Slightly magic.
@@ -1543,7 +1577,6 @@ _Timecounter_Windup(struct bintime *new_boottimebin,
{
struct bintime bt;
struct timehands *th, *tho;
- uint64_t scale;
uint32_t delta, ncount, ogen;
int i;
time_t t;
@@ -1613,7 +1646,7 @@ _Timecounter_Windup(struct bintime *new_boottimebin,
#endif /* __rtems__ */
/*
- * 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
@@ -1624,14 +1657,21 @@ _Timecounter_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);
@@ -1653,40 +1693,12 @@ _Timecounter_Windup(struct bintime *new_boottimebin,
tc_min_ticktock_freq = max(1, timecounter->tc_frequency /
(((uint64_t)timecounter->tc_counter_mask + 1) / 3));
#endif /* __rtems__ */
+ 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.