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| author | Andy Lutomirski <luto@MIT.EDU> | 2011-05-23 09:31:25 -0400 |
|---|---|---|
| committer | Thomas Gleixner <tglx@linutronix.de> | 2011-05-24 14:51:28 +0200 |
| commit | 057e6a8c660e95c3f4e7162e00e2fee1fc90c50d (patch) | |
| tree | 5c0a81327964affd44137754b64fbaf93ceba6fa | |
| parent | 8c49d9a74bac5ea3f18480307057241b808fcc0c (diff) | |
| download | linux-057e6a8c660e95c3f4e7162e00e2fee1fc90c50d.tar.bz2 | |
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.c | 9 |
1 files changed, 5 insertions, 4 deletions
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c index db697b81b8b1..1e6244202612 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; |