From: Andy Lutomirski <luto@MIT.EDU>
To: x86@kernel.org
Cc: Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@elte.hu>,
Andi Kleen <andi@firstfloor.org>,
linux-kernel@vger.kernel.org, Andy Lutomirski <luto@MIT.EDU>
Subject: [RFT/PATCH v2 4/6] x86-64: vclock_gettime(CLOCK_MONOTONIC) can't ever see nsec < 0
Date: Wed, 6 Apr 2011 22:04:01 -0400 [thread overview]
Message-ID: <e210b8eea527894eaf3aec8c30c8d2747e87a524.1302137785.git.luto@mit.edu> (raw)
In-Reply-To: <cover.1302137785.git.luto@mit.edu>
In-Reply-To: <cover.1302137785.git.luto@mit.edu>
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>
---
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;
}
--
1.7.4
next prev parent reply other threads:[~2011-04-07 2:04 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-04-07 2:03 [RFT/PATCH v2 0/6] Micro-optimize vclock_gettime Andy Lutomirski
2011-04-07 2:03 ` [RFT/PATCH v2 1/6] x86-64: Clean up vdso/kernel shared variables Andy Lutomirski
2011-04-07 8:08 ` Ingo Molnar
2011-04-07 2:03 ` [RFT/PATCH v2 2/6] x86-64: Optimize vread_tsc's barriers Andy Lutomirski
2011-04-07 8:25 ` Ingo Molnar
2011-04-07 11:44 ` Andrew Lutomirski
2011-04-07 15:23 ` Andi Kleen
2011-04-07 17:28 ` Ingo Molnar
2011-04-07 16:18 ` Linus Torvalds
2011-04-07 16:42 ` Andi Kleen
2011-04-07 17:20 ` Linus Torvalds
2011-04-07 18:15 ` Andi Kleen
2011-04-07 18:30 ` Linus Torvalds
2011-04-07 21:26 ` Andrew Lutomirski
2011-04-08 17:59 ` Andrew Lutomirski
2011-04-09 11:51 ` Ingo Molnar
2011-04-07 21:43 ` Raghavendra D Prabhu
2011-04-07 22:52 ` Andi Kleen
2011-04-07 2:04 ` [RFT/PATCH v2 3/6] x86-64: Don't generate cmov in vread_tsc Andy Lutomirski
2011-04-07 7:54 ` Ingo Molnar
2011-04-07 11:25 ` Andrew Lutomirski
2011-04-07 2:04 ` Andy Lutomirski [this message]
2011-04-07 7:57 ` [RFT/PATCH v2 4/6] x86-64: vclock_gettime(CLOCK_MONOTONIC) can't ever see nsec < 0 Ingo Molnar
2011-04-07 11:27 ` Andrew Lutomirski
2011-04-07 2:04 ` [RFT/PATCH v2 5/6] x86-64: Move vread_tsc into a new file with sensible options Andy Lutomirski
2011-04-07 2:04 ` [RFT/PATCH v2 6/6] x86-64: Turn off -pg and turn on -foptimize-sibling-calls for vDSO Andy Lutomirski
2011-04-07 8:03 ` Ingo Molnar
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