Re: [crash, bisected] Re: [PATCH 3/4] x86_64: Fold pda into per cpu area

[ebiederm@xmission.com (Eric W. Biederman)]

Mike Travis <travis@sgi.com> writes:

> What I meant was using early_printk in place of printk, which seems to stuff the
> messages into the log buf until the serial console is setup fairly late in
> start_kernel.
> I did this by removing printk() and renaming early_printk() to be printk (and a
> couple
> other things like #define early_printk printk ...

Last I looked after the magic early_printk setup.  printk calls early_printk
and stuff messages in the log buffer.

It matters little though.  As long as you get the print messages.  Weird
cases where you don't get into C code worry me much more.

Once you get into C things are much easier to track.

>> Is stack overflow the only problem you are seeing or are there still other
> mysteries?
>
> I'm not entirely sure it's a stack overflow, the fault has a NULL dereference
> and
> then the stack overflow message.

Ok.  Interesting.

>>> Only a few of these though I would think might get called early in
>>> the boot, that might also be contributing to the stack overflow.
>> 
>> Still the call chain depth shouldn't really be changing.  So why should it
>> matter?  Ah.  The high cpu count is growing cpumask_t so when you put
>> it on the stack.  That makes sense.  So what stars out as a 4 byte
>> variable on the stack in a normal setup winds up being a 1k variable
>> with 4k cpus.
>
> Yes, it's definitely the three related:
>
> NR_CPUS Patch_Applied THREAD_ORDER Results
>   256        NO           1        works (obviously ;-)
>   256        YES          1        works
>  4096        NO           1        works
>  4096        YES          1        panics
>  4096        YES          3        works (just happened to pick 3,
> 					  2 probably will work as well.)

> I've been testing NR_CPUS=4096 for quite a while and it's been very
> reliable.  It's just weird that this config fails with this new patch
> applied.  (default configs and some fairly normal distro configs also
> work fine.)  And with the zillion config straws we now have, spotting
> the arbitrary needle is proving difficult. ;-) 

Right.  Just please split your patch up.  It would be good to see
if simply changing the per cpu segment address to 0 is related
to your problem.  Or if it the other logic changes necessary to 
put the use the pda as a per cpu variable?

I just noticed that we always allocate the pda in the per cpu section.

> One reason I've been sticking with 4.2.4.

Eric