Re: Strange hang on ia64 with CONFIG_PRINTK_TIME=y

[David Miller <davem@davemloft.net>]

From: Roland Dreier <rdreier@cisco.com>
Date: Tue, 12 Feb 2008 22:24:05 -0800

> I'm seeing a strange hang with current git (head 96b5a46e) on an ia64
> box -- an Intel SDV with 2 dual core hyperthreaded Itanium 2 CPUs (so
> 8 logical CPUs to the kernel).  It hangs without printing anything
> ("Uncompressing Linux... done" from ELILO is the last thing I see) if
> I have CONFIG_PRINTK_TIME=y; it works fine with CONFIG_PRINTK_TIME=n.
> 
> The really strange thing is that I have bisected this down to 326e96b9
> ("printk: revert ktime_get() timestamps"), and verified that if revert
> this one patch on top of my current git tree, then the kernel boots
> fine with CONFIG_PRINTK_TIME=y.  The strange thing is that I have also
> checked that the real v2.6.24 kernel boots fine on this system, and as
> far as I can tell, 2.6.24 didn't have the commit that 326e96b9 reverts
> (19ef9309), so there is some interaction with another patch that made
> 19ef9309 necessary on my system.
> 
> Any good idea how to debug this, given that the broken kernels don't
> give any output at all?

The kernel now derefernces per-cpu variables very early, essentially
in the very first printk() (via printk()'s call to cpu_clock()).

This bit me on sparc64 because of how I do the per-cpu address
formation.  If I booted on a non-zero cpuid things would explode.

You might be hitting something similar.