linux-kernel - Re: [PATCH 4/3] x86/vdso: Optimize setup_additional

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Date:	Mon, 21 Oct 2013 07:27:31 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Davidlohr Bueso <davidlohr@...com>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Michel Lespinasse <walken@...gle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Rik van Riel <riel@...hat.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>, aswin@...com,
	linux-mm <linux-mm@...ck.org>, linux-kernel@...r.kernel.org,
	Russell King <linux@....linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Richard Kuo <rkuo@...eaurora.org>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Paul Mundt <lethal@...ux-sh.org>,
	Chris Metcalf <cmetcalf@...era.com>,
	Jeff Dike <jdike@...toit.com>,
	Richard Weinberger <richard@....at>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 4/3] x86/vdso: Optimize setup_additional_pages()


* Davidlohr Bueso <davidlohr@...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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