Date: Thu, 28 Oct 2021 10:22:58 +0200 From: Sebastian Huber <sebastian.huber@embedded-brains.de> To: freebsd-hackers@freebsd.org Subject: [PATCH] kern_tc.c: Scaling/large delta recalculation Message-ID: <20211028082258.82847-1-sebastian.huber@embedded-brains.de>
next in thread | raw e-mail | index | archive | help
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 timecou=
nter
frequency and the th->th_adjustment. The timecounter frequency of a time=
hand
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 seco=
nd
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.
---
sys/kern/kern_tc.c | 88 ++++++++++++++++++++++++++--------------------
1 file changed, 50 insertions(+), 38 deletions(-)
diff --git a/sys/kern/kern_tc.c b/sys/kern/kern_tc.c
index 467e5cf8ee3a..c7309f487dc3 100644
--- a/sys/kern/kern_tc.c
+++ b/sys/kern/kern_tc.c
@@ -1274,6 +1274,40 @@ tc_setclock(struct timespec *ts)
}
}
=20
+/*
+ * 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 =3D a * 2^32 / 10^9 =3D 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 =3D (uint64_t)1 << 63;
+ scale +=3D (th->th_adjustment / 1024) * 2199;
+ scale /=3D th->th_counter->tc_frequency;
+ th->th_scale =3D scale * 2;
+ th->th_large_delta =3D 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 differen=
t
@@ -1284,7 +1318,6 @@ 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;
@@ -1346,7 +1379,7 @@ tc_windup(struct bintime *new_boottimebin)
tho->th_counter->tc_poll_pps(tho->th_counter);
=20
/*
- * 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
@@ -1357,14 +1390,21 @@ tc_windup(struct bintime *new_boottimebin)
bt =3D th->th_offset;
bintime_add(&bt, &th->th_boottime);
i =3D bt.sec - tho->th_microtime.tv_sec;
- if (i > LARGE_STEP)
- i =3D 2;
- for (; i > 0; i--) {
- t =3D bt.sec;
- ntp_update_second(&th->th_adjustment, &bt.sec);
- if (bt.sec !=3D t)
- th->th_boottime.sec +=3D bt.sec - t;
+ if (i > 0) {
+ if (i > LARGE_STEP)
+ i =3D 2;
+
+ do {
+ t =3D bt.sec;
+ ntp_update_second(&th->th_adjustment, &bt.sec);
+ if (bt.sec !=3D t)
+ th->th_boottime.sec +=3D 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 =3D bt;
bintime2timeval(&bt, &th->th_microtime);
@@ -1382,40 +1422,12 @@ tc_windup(struct bintime *new_boottimebin)
th->th_offset_count =3D ncount;
tc_min_ticktock_freq =3D 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
}
=20
- /*-
- * 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 =3D a * 2^32 / 10^9 =3D 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 =3D (uint64_t)1 << 63;
- scale +=3D (th->th_adjustment / 1024) * 2199;
- scale /=3D th->th_counter->tc_frequency;
- th->th_scale =3D scale * 2;
- th->th_large_delta =3D 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.
--=20
2.26.2
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20211028082258.82847-1-sebastian.huber>