ok - i wish such threads were on lkml so that everyone not just the
netdev kabal can read it. It's quite ugly, but if we want to check stack
free space i'd suggest for you to put a stack_can_recurse() call into
arch/x86/kernel/process.c and offer a default __weak implementation in
kernel/fork.c that always returns 0.
the rule on x86 should be something like this: on 4K stacks and 64-bit
[which have irqstacks] free stack space can go as low as 25%. On 8K
stacks [which doesnt have irqstacks but nests irqs] it should not go
below 50% before falling back to the explicitly queued packet branch.
this way other pieces of kernel code code can choose between on-stack
fast recursion and explicit iterators. Although i'm not sure i like the
whole concept to begin with ...