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>
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@ -55,22 +55,6 @@ notrace static noinline int do_realtime(struct timespec *ts)
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return 0;
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}
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/* Copy of the version in kernel/time.c which we cannot directly access */
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notrace static void
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vset_normalized_timespec(struct timespec *ts, long sec, long nsec)
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{
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while (nsec >= NSEC_PER_SEC) {
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nsec -= NSEC_PER_SEC;
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++sec;
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}
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while (nsec < 0) {
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nsec += NSEC_PER_SEC;
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--sec;
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}
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ts->tv_sec = sec;
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ts->tv_nsec = nsec;
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}
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notrace static noinline int do_monotonic(struct timespec *ts)
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{
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unsigned long seq, ns, secs;
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@ -81,7 +65,17 @@ notrace static noinline int do_monotonic(struct timespec *ts)
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secs += gtod->wall_to_monotonic.tv_sec;
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ns += gtod->wall_to_monotonic.tv_nsec;
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} while (unlikely(read_seqretry(>od->lock, seq)));
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vset_normalized_timespec(ts, secs, ns);
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/* wall_time_nsec, vgetns(), and wall_to_monotonic.tv_nsec
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* are all guaranteed to be nonnegative.
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*/
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while (ns >= NSEC_PER_SEC) {
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ns -= NSEC_PER_SEC;
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++secs;
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}
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ts->tv_sec = secs;
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ts->tv_nsec = ns;
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return 0;
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}
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@ -106,7 +100,17 @@ notrace static noinline int do_monotonic_coarse(struct timespec *ts)
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secs += gtod->wall_to_monotonic.tv_sec;
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ns += gtod->wall_to_monotonic.tv_nsec;
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} while (unlikely(read_seqretry(>od->lock, seq)));
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vset_normalized_timespec(ts, secs, ns);
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/* wall_time_nsec and wall_to_monotonic.tv_nsec are
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* guaranteed to be between 0 and NSEC_PER_SEC.
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*/
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if (ns >= NSEC_PER_SEC) {
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ns -= NSEC_PER_SEC;
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++secs;
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}
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ts->tv_sec = secs;
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ts->tv_nsec = ns;
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return 0;
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}
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