From 0f51f2852ccf0fe38a02d340d0ba625e8e32a863 Mon Sep 17 00:00:00 2001 From: Andy Lutomirski Date: Mon, 23 May 2011 09:31:27 -0400 Subject: [PATCH] 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 Cc: Andi Kleen Cc: Linus Torvalds Cc: "David S. Miller" Cc: Eric Dumazet Cc: Peter Zijlstra Cc: Borislav Petkov Link: http://lkml.kernel.org/r/%3Cd6d528d32c7a21618057cfc9005942a0fe5cb54a.1306156808.git.luto%40mit.edu%3E Signed-off-by: Thomas Gleixner --- arch/x86/vdso/vclock_gettime.c | 40 +++++++++++++++++++--------------- 1 file changed, 22 insertions(+), 18 deletions(-) diff --git a/arch/x86/vdso/vclock_gettime.c b/arch/x86/vdso/vclock_gettime.c index 0b873d455bda..28b2c00bd1be 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(>od->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(>od->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; } -- 2.20.1