**diff options**

author | Sebastian Huber <sebastian.huber@embedded-brains.de> | 2021-10-28 10:22:58 +0200 |
---|---|---|

committer | Sebastian Huber <sebastian.huber@embedded-brains.de> | 2021-11-15 08:56:54 +0100 |

commit | 346755874b5e0b4446bd21036a56574081b43e3f (patch) | |

tree | b1714b322539f3a03c91689776fb072fd4c8b596 | |

parent | ea389aaaddefd1606632843e38923ddba098574b (diff) | |

download | rtems-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.c | 88 |

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. |