Messages in this thread | | | Date | Tue, 29 Mar 2011 08:21:12 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH 4/6] x86-64: vclock_gettime(CLOCK_MONOTONIC) can't ever see nsec < 0 |
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* 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.
Thanks,
Ingo
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