linux-kernel - Re: [BUG] long freezes on thinkpad t60

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Date:	Thu, 21 Jun 2007 23:06:30 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Eric Dumazet <dada1@...mosbay.com>,
	Chuck Ebbert <cebbert@...hat.com>,
	Jarek Poplawski <jarkao2@...pl>,
	Miklos Szeredi <miklos@...redi.hu>, chris@...ee.ca,
	linux-kernel@...r.kernel.org, tglx@...utronix.de,
	akpm@...ux-foundation.org
Subject: Re: [BUG] long freezes on thinkpad t60

* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Thu, 21 Jun 2007, Ingo Molnar wrote:
> > 
> > damn, i first wrote up an explanation about why that ugly __delay(1) is 
> > there (it almost hurts my eyes when i look at it!) but then deleted it 
> > as superfluous :-/
> 
> I'm fine with a delay, but the __delay(1) is simply not "correct". It 
> doesn't do anything.

it's a bit trickier than that. Yes, it's a simple 1-entry loop and thus 
makes little sense to call. But it's a loop that got boot-time 
calibrated, so we can do this in the spinlock-debug code:

        u64 loops = loops_per_jiffy * HZ;

this guarantees that we will loop for _at least_ 1 second before 
printing a message. (in practice it's much longer, especially with the 
current naive trylock approach)

Why? Because as part of the activities that the spin-loop does, we also 
do everything that an mdelay(1000) would do. We do it 'piecemail-wise', 
and we do it very inefficiently, but the lower time-bound should be 
guaranteed. This is done because most of the problems were caused by too 
short looping and bogus debug printouts. So this is basically an 
open-coded udelay implementation.

Furthermore, with the spin_is_locked() fix i just sent, the __loop(1) 
solution should actually be quite close to a real udelay() thing, 
without the delay effect. It would probably more accurate to increase it 
to loops_per_jiffy*10*HZ/16 and to call a __loop(16) thing, to get 
closer timings. (but then some people would argue 'why dont you take the 
lock as soon as it's released'.)

	Ingo
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