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>

diff --git a/util-linux/hwclock.c b/util-linux/hwclock.c
index b830057..416271b 100644 (file)
--- 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);