hwclock: improve, and then disable clever sync code: it's bloat

...and hardware is too stupid to benefit from it anyway

function                                             old     new   delta
hwclock_main                                         439     319    -120

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 82 insertions, 30 deletions

diff --git a/util-linux/hwclock.c b/util-linux/hwclock.c
index b8300570e..416271b31 100644
--- a/util-linux/hwclock.c
+++ b/util-linux/hwclock.c
@@ -109,10 +109,53 @@ static void to_sys_clock(const char **pp_rtcname, int utc)
 
 static void from_sys_clock(const char **pp_rtcname, int utc)
 {
-#define TWEAK_USEC 200
-        struct tm tm_time;
+#if 1
         struct timeval tv;
+        struct tm tm_time;
+        int rtc;
+
+        rtc = rtc_xopen(pp_rtcname, O_WRONLY);
+        gettimeofday(&tv, NULL);
+        /* Prepare tm_time */
+        if (sizeof(time_t) == sizeof(tv.tv_sec)) {
+                if (utc)
+                        gmtime_r((time_t*)&tv.tv_sec, &tm_time);
+                else
+                        localtime_r((time_t*)&tv.tv_sec, &tm_time);
+        } else {
+                time_t t = tv.tv_sec;
+                if (utc)
+                        gmtime_r(&t, &tm_time);
+                else
+                        localtime_r(&t, &tm_time);
+        }
+#else
+/* Bloated code which tries to set hw clock with better precision.
+ * On x86, even though code does set hw clock within <1ms of exact
+ * whole seconds, apparently hw clock (at least on some machines)
+ * doesn't reset internal fractional seconds to 0,
+ * making all this a pointless excercise.
+ */
+        /* If we see that we are N usec away from whole second,
+         * we'll sleep for N-ADJ usecs. ADJ corrects for the fact
+         * that CPU is not infinitely fast.
+         * On infinitely fast CPU, next wakeup would be
+         * on (exactly_next_whole_second - ADJ). On real CPUs,
+         * this difference between current time and whole second
+         * is less than ADJ (assuming system isn't heavily loaded).
+         */
+        /* Small value of 256us gives very precise sync for 2+ GHz CPUs.
+         * Slower CPUs will fail to sync and will go to bigger
+         * ADJ values. qemu-emulated armv4tl with ~100 MHz
+         * performance ends up using ADJ ~= 4*1024 and it takes
+         * 2+ secs (2 tries with successively larger ADJ)
+         * to sync. Even straced one on the same qemu (very slow)
+         * takes only 4 tries.
+         */
+#define TWEAK_USEC 256
         unsigned adj = TWEAK_USEC;
+        struct tm tm_time;
+        struct timeval tv;
         int rtc = rtc_xopen(pp_rtcname, O_WRONLY);
 
         /* Try to catch the moment when whole second is close */
@@ -124,55 +167,64 @@ static void from_sys_clock(const char **pp_rtcname, int utc)
 
                 t = tv.tv_sec;
                 rem_usec = 1000000 - tv.tv_usec;
-                if (rem_usec < 1024) {
-                        /* Less than 1ms to next second. Good enough */
+                if (rem_usec < adj) {
+                        /* Close enough */
  small_rem:
                         t++;
                 }
 
-                /* Prepare tm */
+                /* Prepare tm_time from t */
                 if (utc)
                         gmtime_r(&t, &tm_time); /* may read /etc/xxx (it takes time) */
                 else
                         localtime_r(&t, &tm_time); /* same */
-                tm_time.tm_isdst = 0;
+
+                if (adj >= 32*1024) {
+                        break; /* 32 ms diff and still no luck?? give up trying to sync */
+                }
 
                 /* gmtime/localtime took some time, re-get cur time */
                 gettimeofday(&tv, NULL);
 
-                if (tv.tv_sec < t /* may happen if rem_usec was < 1024 */
-                 || (tv.tv_sec == t && tv.tv_usec < 1024)
+                if (tv.tv_sec < t /* we are still in old second */
+                 || (tv.tv_sec == t && tv.tv_usec < adj) /* not too far into next second */
                 ) {
-                        /* We are not too far into next second. Good. */
-                        break;
-                }
-                adj += 32; /* 2^(10-5) = 2^5 = 32 iterations max */
-                if (adj >= 1024) {
-                        /* Give up trying to sync */
-                        break;
+                        break; /* good, we are in sync! */
                 }
 
-                /* Try to sync up by sleeping */
                 rem_usec = 1000000 - tv.tv_usec;
-                if (rem_usec < 1024) {
-                        goto small_rem; /* already close, don't sleep */
+                if (rem_usec < adj) {
+                        t = tv.tv_sec;
+                        goto small_rem; /* already close to next sec, don't sleep */
                 }
-                /* Need to sleep.
-                 * Note that small adj on slow processors can make us
-                 * to always overshoot tv.tv_usec < 1024 check on next
-                 * iteration. That's why adj is increased on each iteration.
-                 * This also allows it to be reused as a loop limiter.
-                 */
-                usleep(rem_usec - adj);
-        }
 
-        xioctl(rtc, RTC_SET_TIME, &tm_time);
+                /* Try to sync up by sleeping */
+                usleep(rem_usec - adj);
 
-        /* Debug aid to find "good" TWEAK_USEC.
+                /* Jump to 1ms diff, then increase fast (x2): EVERY loop
+                 * takes ~1 sec, people won't like slowly converging code here!
+                 */
+        //bb_error_msg("adj:%d tv.tv_usec:%d", adj, (int)tv.tv_usec);
+                if (adj < 512)
+                        adj = 512;
+                /* ... and if last "overshoot" does not look insanely big,
+                 * just use it as adj increment. This makes convergence faster.
+                 */
+                if (tv.tv_usec < adj * 8) {
+                        adj += tv.tv_usec;
+                        continue;
+                }
+                adj *= 2;
+        }
+        /* Debug aid to find "optimal" TWEAK_USEC with nearly exact sync.
          * Look for a value which makes tv_usec close to 999999 or 0.
-         * for 2.20GHz Intel Core 2: TWEAK_USEC ~= 200
+         * For 2.20GHz Intel Core 2: optimal TWEAK_USEC ~= 200
          */
-        //bb_error_msg("tv.tv_usec:%d adj:%d", (int)tv.tv_usec, adj);
+        //bb_error_msg("tv.tv_usec:%d", (int)tv.tv_usec);
+#endif
+
+        tm_time.tm_isdst = 0;
+        xioctl(rtc, RTC_SET_TIME, &tm_time);
 
         if (ENABLE_FEATURE_CLEAN_UP)
                 close(rtc);