Messages in this thread | | | From | Andrew Lutomirski <> | Date | Tue, 29 Mar 2011 07:54:32 -0400 | Subject | Re: [PATCH 4/6] x86-64: vclock_gettime(CLOCK_MONOTONIC) can't ever see nsec < 0 |
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On Tue, Mar 29, 2011 at 2:21 AM, Ingo Molnar <mingo@elte.hu> wrote: > > * Andy Lutomirski <luto@MIT.EDU> wrote: > >> 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. 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 ee55754..67d54bb 100644 >> --- a/arch/x86/vdso/vclock_gettime.c >> +++ b/arch/x86/vdso/vclock_gettime.c >> @@ -56,22 +56,6 @@ 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; >> @@ -82,7 +66,17 @@ 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; >> } >> >> @@ -107,7 +101,17 @@ 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; > > Beyond the change you describe in the changelog, you also uninlined the helper > function. > > You can use __always_inline instead and still keep the code maintainable. >
I think I should fix the changelog instead. The two copies are different (one uses while because nsec > 2e9 is possible and one uses if because nsec < 2e9 always).
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