Messages in this thread | | | Date | Wed, 13 Aug 2008 15:30:11 -0400 | From | Mathieu Desnoyers <> | Subject | Re: Efficient x86 and x86_64 NOP microbenchmarks |
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* Andi Kleen (andi@firstfloor.org) wrote: > > So microbenchmarking this way will probably make some things look > > unrealistically good. > > Must be careful to miss the big picture here. > > We have two assumptions here in this thread: > > - Normal alternative() nops are relatively infrequent, typically > in points with enough pipeline bubbles anyways, and it likely doesn't > matter how they are encode. And also they don't have an issue > with mult part instructions anyways because they're not patched > at runtime, so always the best known can be used. > > - The one case where nops are very frequent and matter and multipart > is a problem is with ftrace noping out the call to mcount at runtime > because that happens on every function entry. > Even there the overhead is not that big, but at least measurable > in kernel builds. > > Now the numbers have shown that just by not using frame pointer ( > -pg right now implies frame pointer) you can get more benefit > than what you lose from using non optimal nops. > > So for me the best strategy would be to get rid of the frame pointer > and ignore the nops. This unfortunately would require going away > from -pg and instead post process gcc output to insert "call mcount" > manually. But the nice advantage of that is that you could actually > set up a custom table of callers built in a ELF section and with > that you don't actually need the runtime patching (which is only > done currently because there's no global table of mcount calls), > but could do everything in stop_machine(). Without > runtime patching you also don't need single part nops. >
I agree that if frame pointer brings a too big overhead, it should not be used.
Sorry to ask, I feel I must be missing something, but I'm trying to figure out where you propose to add the "call mcount" ? In the caller or in the callee ?
In the caller, I guess it would replace the normal function call, call a trampoline which would jump to the normal code.
In the callee, as what is currently done with -pg, the callee would have a call mcount at the beginning of the function.
Or is it a different scheme I don't see ? I am trying to figure out how you happen to do all that without dynamic code modification and manage not to hurt performance.
Mathieu
> I think that would be the best option. I especially like it because > it would prevent forcing frame pointer which seems to be costlier > than any kinds of nosp. > > -Andi >
-- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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