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>

1 files changed, 22 insertions, 18 deletions

diff --git a/arch/x86/vdso/vclock_gettime.c b/arch/x86/vdso/vclock_gettime.c
index 0b873d455bd..28b2c00bd1b 100644
--- 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;
 }