On Mon, 2011-09-26 at 09:54 +0300, Pekka Enberg wrote:
> 
> AFAICT, flush_all() isn't all that performance sensitive. Why do we
> want to reduce IPIs here? 

Because it can wake up otherwise idle CPUs, wasting power. Or for the
case I care more about, unnecessarily perturb a CPU that didn't actually
have anything to flush but was running something, introducing jitter.

on_each_cpu() things are bad when you have a ton of CPUs (which is
pretty normal these days).