Commit-ID: 057e6a8c660e95c3f4e7162e00e2fee1fc90c50d Gitweb: http://git.kernel.org/tip/057e6a8c660e95c3f4e7162e00e2fee1fc90c50d Author: Andy Lutomirski <luto@xxxxxxx> AuthorDate: Mon, 23 May 2011 09:31:25 -0400 Committer: Thomas Gleixner <tglx@xxxxxxxxxxxxx> CommitDate: Tue, 24 May 2011 14:51:28 +0200 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@xxxxxxx> Cc: Andi Kleen <andi@xxxxxxxxxxxxxx> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Cc: "David S. Miller" <davem@xxxxxxxxxxxxx> Cc: Eric Dumazet <eric.dumazet@xxxxxxxxx> Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> Cc: Borislav Petkov <bp@xxxxxxxxx> Link: http://lkml.kernel.org/r/%3C1c158b9d74338aa5361f96dd473d0e6a58235302.1306156808.git.luto%40mit.edu%3E Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> --- 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 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; -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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