Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chain supportto use NMI-safe methods

[Linus Torvalds]

On Mon, 15 Jun 2009, Ingo Molnar wrote:
> 
> [ If on the other hand it's a speedup of a few cycles then we have 
>   the problem of me suddenly liking this patch a whole lot more ;-) ]

I missed the patch.

It's quite possible that replacing "iret" with a regular "ret" (for the 
kernel->kernel transition) is a real speedup. That said, there's a few 
things to think about:

 - CPU return stack caches/predictors. I suspect that "iret" and 
   exceptions don't generally touch them (but who knows - maybe they do), 
   while a regular "ret" definitely does. I dunno about "retf".

   This can cause very subtle performance slowdowns, where the slowdown 
   happens somewhere else. And it could be _very_ uarch-dependent (ie only 
   happen on some architectures, while having no performance downside on 
   others)

 - kernel->kernel exceptions _should_ be rare, with the exception of 
   actual real external interrupts. So the path to optimize should always 
   be the user-space exception path. That one will need 'iret', but I'd 
   also not want to see more testing in that hot-path. I suspect we 
   already always test for user-mode anyway (due to signal handling etc 
   work), but if it adds new tests to that path, any kernel->kernel 
   speedup is likely totally pointless.

That said, it would be nice to avoid 'iret' if only because of its subtle 
interactions with the while NMI flag.

		Linus
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