time: Optimize ns_to_timespec64()

ns_to_timespec64() calls div_s64_rem(), which is a rather slow function on
32-bit architectures, as it cannot take advantage of the do_div()
optimizations for constant arguments.

Open-code the div_s64_rem() function in ns_to_timespec64(), so a constant
divider can be passed into the optimized div_u64_rem() function.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20191108203435.112759-3-arnd@arndb.de

diff --git a/kernel/time/time.c b/kernel/time/time.c
index 5c54ca6..45a3589 100644 (file)
--- a/kernel/time/time.c
+++ b/kernel/time/time.c
@@ -550,18 +550,21 @@ EXPORT_SYMBOL(set_normalized_timespec64);
  */
 struct timespec64 ns_to_timespec64(const s64 nsec)
 {
-       struct timespec64 ts;
+       struct timespec64 ts = { 0, 0 };
        s32 rem;
 
-       if (!nsec)
-               return (struct timespec64) {0, 0};
-
-       ts.tv_sec = div_s64_rem(nsec, NSEC_PER_SEC, &rem);
-       if (unlikely(rem < 0)) {
-               ts.tv_sec--;
-               rem += NSEC_PER_SEC;
+       if (likely(nsec > 0)) {
+               ts.tv_sec = div_u64_rem(nsec, NSEC_PER_SEC, &rem);
+               ts.tv_nsec = rem;
+       } else if (nsec < 0) {
+               /*
+                * With negative times, tv_sec points to the earlier
+                * second, and tv_nsec counts the nanoseconds since
+                * then, so tv_nsec is always a positive number.
+                */
+               ts.tv_sec = -div_u64_rem(-nsec - 1, NSEC_PER_SEC, &rem) - 1;
+               ts.tv_nsec = NSEC_PER_SEC - rem - 1;
        }
-       ts.tv_nsec = rem;
 
        return ts;
 }