Re: Final: Add 32 bit VDSO time function support

[Andy Lutomirski]

On Thu, Feb 27, 2014 at 11:22 PM, Stefani Seibold <stefani@xxxxxxxxxxx> wrote:
> Am Mittwoch, den 26.02.2014, 16:55 -0800 schrieb Andy Lutomirski:
>>
>> Once I patch it to work, your 32-bit code is considerably faster than
>> the 64-bit case.  It's enough faster that I suspect a bug.  Dumping
>> the in-memory shows some rather suspicious nops before the rdtsc
>> instruction.  I suspect that you've forgotten to run the 32-bit vdso
>> through the alternatives code.  The is a nasty bug: it will appear to
>> work, but you'll see non-monotonic times on some SMP systems.
>>
>
> I didn't know this. My basic test case is a KVM which defaults to 1 cpu.
> Thanks for discovering the issue.

This leads to a potentially interesting question: is rdtsc_barrier()
actually necessary on UP?  IIRC the point is that, if an
rdtsc_barrier(); rdtsc in one thread is "before" (in the sense of
being synchronized by some memory operation) an rdtsc_barrier(); rdtsc
in another thread, then the first rdtsc needs to return an earlier or
equal time to the second one.

I assume that no UP CPU is silly enough to execute two rdtsc
instructions out of order relative to each other in the absence of
barriers.  So this is a nonissue on UP.

On the other hand, suppose that some code does:

volatile long x = *(something that's not in cache)
clock_gettime

I can imagine a modern CPU speculating far enough ahead that the rdtsc
happens *before* the cache miss.  This won't cause visible
non-monotonicity as far as I can see, but it might annoy people who
try to benchmark their code.

Note: actually making this change might be a bit tricky.  I don't know
if the alternatives code is smart enough.

--Andy
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