x86-64: Vclock_gettime(CLOCK_MONOTONIC) can't ever see nsec < 0

vclock_gettime's do_monotonic helper can't ever generate a negative
nsec value, so it doesn't need to check whether it's negative.  In
the CLOCK_MONOTONIC_COARSE case, ns can't ever exceed 2e9-1, so we
can avoid the loop entirely.  This saves a single easily-predicted
branch.

Signed-off-by: Andy Lutomirski <luto@mit.edu>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Borislav Petkov <bp@amd64.org>
Link: http://lkml.kernel.org/r/%3Cd6d528d32c7a21618057cfc9005942a0fe5cb54a.1306156808.git.luto%40mit.edu%3E
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

diff --git a/arch/x86/vdso/vclock_gettime.c b/arch/x86/vdso/vclock_gettime.c
index 0b873d4..28b2c00 100644 (file)
--- a/arch/x86/vdso/vclock_gettime.c
+++ b/arch/x86/vdso/vclock_gettime.c
@@ -55,22 +55,6 @@ notrace static noinline int do_realtime(struct timespec *ts)
        return 0;
 }
 
-/* Copy of the version in kernel/time.c which we cannot directly access */
-notrace static void
-vset_normalized_timespec(struct timespec *ts, long sec, long nsec)
-{
-       while (nsec >= NSEC_PER_SEC) {
-               nsec -= NSEC_PER_SEC;
-               ++sec;
-       }
-       while (nsec < 0) {
-               nsec += NSEC_PER_SEC;
-               --sec;
-       }
-       ts->tv_sec = sec;
-       ts->tv_nsec = nsec;
-}
-
 notrace static noinline int do_monotonic(struct timespec *ts)
 {
        unsigned long seq, ns, secs;
@@ -81,7 +65,17 @@ notrace static noinline int do_monotonic(struct timespec *ts)
                secs += gtod->wall_to_monotonic.tv_sec;
                ns += gtod->wall_to_monotonic.tv_nsec;
        } while (unlikely(read_seqretry(&gtod->lock, seq)));
-       vset_normalized_timespec(ts, secs, ns);
+
+       /* wall_time_nsec, vgetns(), and wall_to_monotonic.tv_nsec
+        * are all guaranteed to be nonnegative.
+        */
+       while (ns >= NSEC_PER_SEC) {
+               ns -= NSEC_PER_SEC;
+               ++secs;
+       }
+       ts->tv_sec = secs;
+       ts->tv_nsec = ns;
+
        return 0;
 }
 
@@ -106,7 +100,17 @@ notrace static noinline int do_monotonic_coarse(struct timespec *ts)
                secs += gtod->wall_to_monotonic.tv_sec;
                ns += gtod->wall_to_monotonic.tv_nsec;
        } while (unlikely(read_seqretry(&gtod->lock, seq)));
-       vset_normalized_timespec(ts, secs, ns);
+
+       /* wall_time_nsec and wall_to_monotonic.tv_nsec are
+        * guaranteed to be between 0 and NSEC_PER_SEC.
+        */
+       if (ns >= NSEC_PER_SEC) {
+               ns -= NSEC_PER_SEC;
+               ++secs;
+       }
+       ts->tv_sec = secs;
+       ts->tv_nsec = ns;
+
        return 0;
 }