Re: [PATCH 4/3] x86/vdso: Optimize setup_additional_pages()

[Ingo Molnar <mingo@kernel.org>]

* Davidlohr Bueso <davidlohr@hp.com> wrote:

> > 2)
> > 
> > I don't see the justification: this code gets executed in exec() where 
> > a new mm has just been allocated. There's only a single user of the mm 
> > and thus the critical section width of mmap_sem is more or less 
> > irrelevant.
> > 
> > mmap_sem critical section size only matters for codepaths that 
> > threaded programs can hit.
> 
> Yes, I was surprised by the performance boost I noticed when running 
> this patch. This weekend I re-ran the tests (including your 4/3 patch) 
> and noticed that while we're still getting some benefits (more like in 
> the +5% throughput range), it's not as good as I originally reported. I 
> believe the reason is because I had ran the tests on the vanilla kernel 
> without the max clock frequency, so the comparison was obviously not 
> fair. That said, I still think it's worth adding this patch, as it does 
> help at a micro-optimization level, and it's one less mmap_sem user we 
> have to worry about.

But it's a mmap_sem user that is essentially _guaranteed_ to have only a 
single user at that point, in the exec() path!

So I don't see how this can show _any_ measurable speedup, let alone a 5% 
speedup in a macro test. If our understanding is correct then the patch 
does nothing but shuffle around a flag setting operation. (the mmap_sem is 
equivalent to setting a single flag, in the single-user case.)

Now, if our understanding is incorrect then we need to improve our 
understanding.

Thanks,

	Ingo

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