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Subject[PATCH v5 4/8] x86-64: vclock_gettime(CLOCK_MONOTONIC) can't ever see nsec < 0
Date
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>
---
arch/x86/vdso/vclock_gettime.c | 40 ++++++++++++++++++++++------------------
1 files changed, 22 insertions(+), 18 deletions(-)

diff --git a/arch/x86/vdso/vclock_gettime.c b/arch/x86/vdso/vclock_gettime.c
index 0b873d4..28b2c00 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;
}

--
1.7.5.1


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