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x86-64: Vclock_gettime(CLOCK_MONOTONIC) can't ever see nsec < 0

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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>
This commit is contained in:
Andy Lutomirski 2011-05-23 09:31:27 -04:00 committed by Thomas Gleixner
parent 3729db5ca2
commit 0f51f2852c
1 changed files with 22 additions and 18 deletions
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40
arch/x86/vdso/vclock_gettime.c
View file

@ -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;
}