linux-kernel - Re: HPET regression in 2.6.26 versus 2.6.25 -- RCU problem

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Date:	Mon, 11 Aug 2008 09:15:52 -0700
From:	"Yinghai Lu" <yhlu.kernel@...il.com>
To:	"Ingo Molnar" <mingo@...e.hu>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"David Witbrodt" <dawitbro@...global.net>,
	"Peter Zijlstra" <peterz@...radead.org>,
	linux-kernel@...r.kernel.org,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, netdev <netdev@...r.kernel.org>
Subject: Re: HPET regression in 2.6.26 versus 2.6.25 -- RCU problem

On Mon, Aug 11, 2008 at 4:25 AM, Ingo Molnar <mingo@...e.hu> wrote:
>
> * Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
>
>> > I'm _way_ over my head in this discussion, but here's some more food
>> > for thought.  Last weekend, when I first tried 2.6.26 and discovered
>> > the freeze, I thought an error of my own in .config was causing it.
>> > Before I ever sought help, I made about a dozen experiments with
>> > different .config files.
>> >
>> > One series of those experiments involved turning off most of the
>> > kernel... including CONFIG_INET.  The kernel still froze, but when
>> > entering pci_init().  (This info can be read in my original post to
>> > the Debian BTS, which I have provided links for a couple of times in
>> > this LKML thread.  I even went further and removed enough that the
>> > freeze was avoided, but so much of the kernel was missing that my
>> > init scripts couldn't mount a hard disk any more.  Trying to restore
>> > enough to allow HD mounting just brought back the freeze.)
> [...]
>>
>> RCU doesn't use HPET directly.  Most of its time-dependent behavior
>> comes from its being invoked from the scheduling-clock interrupt.
>
> such freezes frequently occur due to the plain lack of timer interrupts.
>
> As networking's rcu_synchronize() is one of the first calls in the
> kernel that relies on a timer IRQ hitting the CPU, it would be the first
> one that "freezes". It's not a real freeze though: it's the lack of
> timer events breaking RCU completion. (RCU has an implicit and somewhat
> subtle dependency on timer irqs periodically hitting the CPU)
>
> You can probably verify this by adding something like this to
> kernel/timer.c's do_timer() function:
>
>   if (printk_ratelimit())
>        printk("timer irq hit, jiffies: %ld\n", jiffies);
>
> Yinghai, do you have any ideas about this particular problem? One theory
> would be that your e820 changes might have caused a shuffling of
> resources that made the hpet's timer IRQ generation inoperable.

the hpet request_resource() calling fail?

YH
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