Re: [RFT/PATCH v2 2/6] x86-64: Optimize vread_tsc's barriers

[Andi Kleen]

> Also, do we *really* have RDTSC SMP-coherency guarantees on multi-socket CPUs 
> today? It now works on multi-core, but on bigger NUMA i strongly doubt it. So 
> this hack tries to preserve something that we wont be able to offer anyway.

Some larger NUMA systems have explicit TSC consistency in hardware; on those that don't 
we disable TSC as a clocksource so this path should be never taken.

> So the much better optimization would be to give up on exact GTOD coherency and 
> just make sure the same task does not see time going backwards. If user-space 
> wants precise coherency it can use synchronization primitives itsef. By default 
> it would get the fast and possibly off by a few cycles thing instead. We'd 
> never be seriously jump in time - only small jumps would happen in practice, 
> depending on CPU parallelism effects.

That would be a big user visible break in compatibility.

Any small jump can lead to a negative time difference, and negative time differences
are known to break applications.

e.g. typical case is app using this as a event time stamp into a buffer written
from multiple CPUs, and then assuming that the time stamp always goes up.

> If we do that then the optimization would be to RDTSC and not use *any* of the 
> barriers, neither the hardware ones nor your tricky software data-dependency 
> obfuscation barrier.

The barriers were originally added because a stress test was able to observe
time going backwards without them.

-Andi

-- 
ak@xxxxxxxxxxxxxxx -- Speaking for myself only.
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