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authorAndy Lutomirski <luto@MIT.EDU>2011-05-23 09:31:25 -0400
committerThomas Gleixner <tglx@linutronix.de>2011-05-24 14:51:28 +0200
commit057e6a8c660e95c3f4e7162e00e2fee1fc90c50d (patch)
tree5c0a81327964affd44137754b64fbaf93ceba6fa
parent8c49d9a74bac5ea3f18480307057241b808fcc0c (diff)
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x86-64: Remove unnecessary barrier in vread_tsc
RDTSC is completely unordered on modern Intel and AMD CPUs. The Intel manual says that lfence;rdtsc causes all previous instructions to complete before the tsc is read, and the AMD manual says to use mfence;rdtsc to do the same thing. From a decent amount of testing [1] this is enough to make rdtsc be ordered with respect to subsequent loads across a wide variety of CPUs. On Sandy Bridge (i7-2600), this improves a loop of clock_gettime(CLOCK_MONOTONIC) by more than 5 ns/iter. [1] https://lkml.org/lkml/2011/4/18/350 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/%3C1c158b9d74338aa5361f96dd473d0e6a58235302.1306156808.git.luto%40mit.edu%3E Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
-rw-r--r--arch/x86/kernel/tsc.c9
1 files changed, 5 insertions, 4 deletions
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index db697b8..1e62442 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -769,13 +769,14 @@ static cycle_t __vsyscall_fn vread_tsc(void)
cycle_t ret;
/*
- * Surround the RDTSC by barriers, to make sure it's not
- * speculated to outside the seqlock critical section and
- * does not cause time warps:
+ * Empirically, a fence (of type that depends on the CPU)
+ * before rdtsc is enough to ensure that rdtsc is ordered
+ * with respect to loads. The various CPU manuals are unclear
+ * as to whether rdtsc can be reordered with later loads,
+ * but no one has ever seen it happen.
*/
rdtsc_barrier();
ret = (cycle_t)vget_cycles();
- rdtsc_barrier();
return ret >= VVAR(vsyscall_gtod_data).clock.cycle_last ?
ret : VVAR(vsyscall_gtod_data).clock.cycle_last;