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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> |
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