On Mon, Jul 21, 2014 at 02:56:49PM -0700, Andy Lutomirski wrote:
> I expect that users of __vdso_clock_gettime (e.g. glibc) will get the
> correct time :) They use vread_tsc, and they can't use
> preempt_disable, because they're in userspace. They also can't
> directly access per-cpu variables.
>
> Turning off vdso tsc support on these machines would be an option.
Right, this is what I was going to propose to tglx on IRC. Or we can try
to come up with something working for the vdso too, say RDTSCP :-)
But that still won't work as it needs the per-cpu variables. So I guess
vdso loses...
> I actually own one of these systems. It's a Sandy Bridge Core-i7
> Extreme or something like that.
Ha, cool, so I've got my tester! :-)
> I wonder if that's a bug in get_cycles.
>
> The basic issue is that rdtsc is not ordered with respect to nearby
> loads, so it's fairly easy to see it behaving non-monotonically across
> CPUs. rdtscp is ordered, but it's a little slower.
Yah, that I know. But I don't see get_cycles() having the barriers. So
it might be a bug. We can certainly try to "fix" it and see what happens
:-)
diff --git a/arch/x86/include/asm/tsc.h b/arch/x86/include/asm/tsc.h
index 94605c0e9cee..ad7d5e449c0b 100644
--- a/arch/x86/include/asm/tsc.h
+++ b/arch/x86/include/asm/tsc.h
@@ -27,7 +27,9 @@ static inline cycles_t get_cycles(void)
if (!cpu_has_tsc)
return 0;
#endif
+ rdtsc_barrier();
rdtscll(ret);
+ rdtsc_barrier();
return ret;
}
--
Regards/Gruss,
Boris.
Sent from a fat crate under my desk. Formatting is fine.
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