Re: 2.6.24-rc6-git12: Reported regressions from 2.6.23

[Arjan van de Ven <arjan@infradead.org>]

On Sun, 6 Jan 2008 10:55:01 +0100
Ingo Molnar <mingo@elte.hu> wrote:

> 
> * Mark Lord <lkml@rtr.ca> wrote:
> 
> > Rafael J. Wysocki wrote:
> >> This message contains a list of some regressions from 2.6.23
> >> reported since 2.6.24-rc1 was released, for which there are no
> >> fixes in the mainline I know of.  If any of them have been fixed
> >> already, please let me know.
> > ..
> >> Subject		: 20000+ wake-ups/second in 2.6.24
> >> Submitter	: Mark Lord <lkml@rtr.ca>
> >> Date		: 2007-12-02 04:23
> >> References	: http://lkml.org/lkml/2007/12/1/141
> >> 		  http://bugzilla.kernel.org/show_bug.cgi?id=9489
> >> Handled-By	: Arjan van de Ven <arjan@infradead.org>
> >>
> >
> > I wonder if it's just a babbling IRQ on resume, before the driver
> > has run it's resume code or something ?
> 
> i've read the discussions, and i cannot see it analyzed anywhere
> _what_ causes the wakeups. And how are these wakeups counted? Is this
> based on powertop output:
> 
>  Wakeups-from-idle per second : 20.4     interval: 1.8s
> 
> ? Somewhere i saw it mentioned that "the CPU throws out of C mode".
> What does that mean - does it mean we try to idle again and again,
> but we immediately return from C mode - while this all looks like
> "idle" time to the scheduler (so 'top' will show lots of idle time),
> but the ACPI wakeup counters are going up like mad? What
> is /proc/interrupts doing when this happens - is any of the irq
> sources going upwards?
> 

what seems to happen (and this is based on seeing this on my own devel laptop, as well
as several other reports; Mark is by far not the only one) is something hardware
related, it's been seen on lots of different kernel versions.

It seems to mostly (but not 100%) happen with TI cardbus bridges, where for some reason,
once the yenta driver is loaded (unloading it later makes no difference), once in a while
we get into a mode where the CPU always immediately goes out of the C-state again.
On a hardware level, there are only a few things that cause a CPU to exit a C-state,
and one of them is a pending interrupt of some kind, which is the most likely thing going on here;
some device or apic being stuck with interrupt high, but somehow the CPU isn't actually
seeing the interrupt itself (or has it blocked!)

To call this a 2.6.24 regression is a mistake (as I've said before), it's not new to .24
by any means.

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