Why does $$parms show value as "?"

[Josh Stone]

On 08/25/2011 10:15 AM, Grant Edwards wrote:
>[...]
>   skb=?
>   skb=?
>   ^Csystemtap end
>   # 
> 
> Why isn't systemtap printing the value of the parameter "skb"?

This means that the debuginfo has defined this parameter, but we're
unable to access it for some reason, most likely because gcc didn't
output location information in the debuginfo.  The variable may not even
exist at that point anymore due to optimization.  That function is so
short that it quite likely has been inlined into its callers.

> The module in question is built "out of tree" and then installed in 
> /lib/modules/VERSION/misc, and according to "file" it's not stripped.

Since this is out of tree, you could try building the module without
optimization, and that should keep all variables around for your
perusal.  Or it may be enough just to mark that function noinline.


Josh