From: Nick Piggin <nickpiggin@yahoo.com.au>
Date: Wed, 27 Aug 2008 17:47:14 +1000
There is a lot of indirect costs that are hard to see as well.
Two things a lot of these cross-call dispatch paths do is:
1) Clear self-cpu
2) AND with cpus_online
#1 can normally be a simple bit clear, but some places can also
implement this with something like "cpus_andn(X, cpumask_of_cpu(cpu))"
It's simply easier to move those two things down to the bottom of
the APIC programming code, they just loop over the cpumask doing
an expensive APIC I/O operation anyways, might as well overlap it
with these "skip self-cpu" and "skip not-online cpus" checks.
And oh yeah we get the stack wastage fixed too, isn't what what we
were talking about? :-)
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