linux-kernel - Re: 2.6.23-rc1: BUG_ON in kmap_atomic

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Message-ID: <20070726180721.GN3572@stusta.de>
Date:	Thu, 26 Jul 2007 20:07:21 +0200
From:	Adrian Bunk <bunk@...sta.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Alexey Dobriyan <adobriyan@...il.com>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: 2.6.23-rc1: BUG_ON in kmap_atomic_prot()

On Tue, Jul 24, 2007 at 12:48:13PM -0700, Linus Torvalds wrote:
> 
> 
> On Tue, 24 Jul 2007, Adrian Bunk wrote:
> > 
> > > But do we 
> > > care so much that it's worth inlining something like buffered_rmqueue()? 
> > >...
> > 
> > Where is the problem with having buffered_rmqueue() inlined?
> 
> In this case, it was a pain to just even try to find the call chain, or 
> read the asm.

Optimization versus debugging is a common issue...

As I said, it might make sense to disable this optimization depending on 
some debugging option.

> I would encourage lots of kernel hackers to read the assembler code gcc 
> generates. I suspect people being aware of code generation issues (and 
> writing their code with that in mind) is a *much* bigger performance 
> impact than gcc inlining random functions.
> 
> So maybe I'm old-fashioned and crazy, but "readability of the asm result" 
> actually is a worthwhile goal. Not because we care directly, but because 
> I'd like to encourage people to do it, due to the *indirect* benefits.

This would lead to people trying to optimize code for one gcc version - 
and the code might stay this way for 10 years.

People should write readable C code. This also has the best chances of 
resulting in good performance with the next gcc version on the next 
generation hardware.

> 		Linus

cu
Adrian

-- 

       "Is there not promise of rain?" Ling Tan asked suddenly out
        of the darkness. There had been need of rain for many days.
       "Only a promise," Lao Er said.
                                       Pearl S. Buck - Dragon Seed

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