linux-kernel - Re: gcc inlining heuristics was Re: [PATCH -v7][RFC]: mutex: implement adaptive spinning

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Date:	Tue, 20 Jan 2009 13:38:24 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Bernd Schmidt <bernds_cb1@...nline.de>,
	Andi Kleen <andi@...stfloor.org>,
	David Woodhouse <dwmw2@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Harvey Harrison <harvey.harrison@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Chris Mason <chris.mason@...cle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	paulmck@...ux.vnet.ibm.com, Gregory Haskins <ghaskins@...ell.com>,
	Matthew Wilcox <matthew@....cx>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-btrfs <linux-btrfs@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Morreale <pmorreale@...ell.com>,
	Sven Dietrich <SDietrich@...ell.com>, jh@...e.cz
Subject: Re: gcc inlining heuristics was Re: [PATCH -v7][RFC]: mutex:
	implement adaptive spinning

* Nick Piggin <npiggin@...e.de> wrote:

> > > it seems like a nice opt-in thing that can be used where the aliases 
> > > are verified and the code is particularly performance critical...
> > 
> > Yes. I think we could use it in the kernel, although I'm not sure how 
> > many cases we would ever find where we really care.
> 
> Yeah, we don't tend to do a lot of intensive data processing, so it is 
> normally the cache misses that hurt most as you noted earlier.
> 
> Some places it might be appropriate, though. It might be nice if it can 
> bring code size down too...

I checked, its size effects were miniscule [0.17%] on the x86 defconfig 
kernel and it seems to be a clear loss in total cost as there would be an 
ongoing maintenance cost of this weird new variant of C that language 
lawyers legislated out of thin air and which departs so significantly from 
time-tested C coding concepts and practices.

We'd have to work around aliasing warnings of the compiler again and again 
with no upside and in fact i'd argue that the resulting code is _less_ 
clean.

The lack of data processing complexity in the kernel is not a surprise: 
the kernel is really just a conduit/abstractor between hw and apps, and 
rarely generates genuinely new information. (In fact it can be generally 
considered a broken system call concept if such data processing _has_ to 
be conducted somewhere in the kernel.)

( Notable exceptions would be the crypto code and the RAID5 [XOR checksum]
  and RAID6 [polinomial checksums] code - but those tend to be seriously
  hand-optimized already, with the most critical bits written in assembly. )

	Ingo
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