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

[Andi Kleen]

>   Instruction scheduling isn't some kind of theoretical game. It's a
> very practical issue, and CPU schedulers are constrained to do a good
> job quickly and _effectively_. In other words, instructions don't just
> schedule randomly. In the presense of the barrier, is there any reason
> to believe that the rdtsc would really schedule oddly? There is never
> any reason to _delay_ an rdtsc (it can take no cache misses or wait on
> any other resources), so when it is not able to move up, where would
> it move?

CPUs are complex beasts and I'm sure there are scheduling
constraints neither you nor me ever heard of :-)

There are always odd corner cases, e.g. if you have a correctable
error somewhere internally it may add a stall on some unit but not
on others, which may delay an arbitary uop.

Also there can be reordering against reading xtime and friends.

>  - the reason "back-to-back" (with the extreme example being in a
> tight loop) matters is that if something isn't in a tight loop, any
> jitter we see in the time counting wouldn't be visible anyway. One
> random timestamp is meaningless on its own. It's only when you have
> multiple ones that you can compare them. No?

There's also the multiple CPUs logging to a shared buffer case.

I thought Vojtech's original test case was something like that in fact.

> So _before_ we try some really clever data dependency trick with new
> inline asm and magic "double shifts to create a zero" things, I really
> would suggest just trying to remove the second lfence entirely and see
> how that works. Maybe it doesn't work, but ...

I would prefer to be safe than sorry. Also there are still other things
to optimize anyways (I suggested a few in my earlier mail) which
are 100% safe unlike this. Maybe those would be enough to offset
the cost of the "paranoid lfence"


-Andi

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