[RFC 0/5] rcu: Introduce leaf_node_for_each_mask_possible_cpu() and its friend

[Boqun Feng]

Hi Paul,

While reading the discussion at:

https://marc.info/?l=linux-kernel&m=148044253400769

I figured we might use this fact to save some extra checks in RCU core code,
currently we iterate over all the possible CPUs on a leaf node, check whether
they were masked in a certain mask and do something. However, given the fact
that the masks on a leaf node should always be sparse than the corresponding
part of cpu_possible_mask, we'd better iterate over all bits in a mask and
check whether the corresponding CPU is possible or not.

So I made this RFC, I did a simple build/boot/rcutorture test on my box with
SMP=4, nothing bad happens. Currently I'm waiting for the 0day and trying to
test this one a bigger system, in the meanwhile, looking forwards to any
comment and suggestion.

So thoughts?

Regards,
Boqun