Re: Efficient x86 and x86_64 NOP microbenchmarks

[Mathieu Desnoyers]

* Andi Kleen (andi@xxxxxxxxxxxxxx) 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
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