linux-kernel - Re: [PATCH 4/4] x86_64 ioapic: Improve the heuristics for when check

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Date:	Mon, 8 Jan 2007 23:33:55 +0100
From:	Adrian Bunk <bunk@...sta.de>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Linus Torvalds <torvalds@...l.org>,
	Tobias Diedrich <ranma+kernel@...edrich.de>,
	Yinghai Lu <yinghai.lu@....com>, Andrew Morton <akpm@...l.org>,
	Andi Kleen <ak@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	mingo@...hat.com, discuss@...-64.org
Subject: Re: [PATCH 4/4] x86_64 ioapic: Improve the heuristics for when check_timer fails.

On Mon, Jan 08, 2007 at 02:45:00PM -0700, Eric W. Biederman wrote:
> Adrian Bunk <bunk@...sta.de> writes:
> 
> > On Mon, Jan 08, 2007 at 09:11:24AM -0700, Eric W. Biederman wrote:
> >> 
> >> To a large extent this reverts b026872601976f666bae77b609dc490d1834bf77
> >> while still keeping to the spirits of it's goal, the ability to
> >> make smart guesses about how the timer irq is routed when the BIOS
> >> gets it wrong.
> >>...
> >
> > That's code where every changed line has a great potential of causing a 
> > different kind of breakage on someone else's computer.
> 
> Why does this piece of code give every one the screaming hebie jebies?
> I read it I understand it, it is code.
> 
> This code is not a terribly sensitive delicate heuristic, and Andi has
> already broken it as much as it can possibly be broken.  It's not like
> the code is on a SMP fastpath full of carefully orchestrated races
> that are safe because within certain limits even stale values are ok.
> 
> This is code is straight forward logic, you tell the computer what to
> do and it does it.  Of those things we can do only very few of
> them are correct, and we are seeking to enhance our ability to find
> correct solutions by adding intelligent guesses.  As long as the first
> guess is trust the BIOS the rest of this code is largely a don't
> care.  As Andi proved by breaking all the rest of this.  Or why
> don't I have more testers just crawling out of the wood work,
> screaming for this code to be fixed?
> 
> Plus this code can only cause one type of breakage.   A failure to
> work around a broken BIOS and make the IRQs work.

We just got a completely different bug reported that was confirmed to be 
caused by Andi's patch:
   AMD64/ATI : timer is running twice as fast as it should [1]

> > Your comment therefore translates to "rexvert commit 
> > b026872601976f666bae77b609dc490d1834bf77 for 2.6.20 and try to find a 
> > better solution for 2.6.21".
> 
> If that is the practical translation I am fine with it.
>...
> I really don't care how we do it, or in what timeframe.  But what I have
> posted is the only way I can see of making it better, than what we had
> in 2.6.19.
>...

My whole point is that for 2.6.20, we can live with simply reverting 
Andi's commit.

What to do for 2.6.21 is a completely different story.

> Eric

cu
Adrian

[1] http://bugzilla.kernel.org/show_bug.cgi?id=7789

-- 

       "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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