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Message-Id: <d6d528d32c7a21618057cfc9005942a0fe5cb54a.1306156808.git.luto@mit.edu>
Date: Mon, 23 May 2011 09:31:27 -0400
From: Andy Lutomirski <luto@....EDU>
To: x86@...nel.org
Cc: linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
Andi Kleen <andi@...stfloor.org>,
Linus Torvalds <torvalds@...ux-foundation.org>,
"David S. Miller" <davem@...emloft.net>,
Eric Dumazet <eric.dumazet@...il.com>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Thomas Gleixner <tglx@...utronix.de>,
Borislav Petkov <bp@...64.org>, Andy Lutomirski <luto@....edu>
Subject: [PATCH v5 4/8] 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@....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 0b873d4..28b2c00 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;
}
--
1.7.5.1
--
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