quoted

Maybe its low cost, (apparently, it is, since I can reach ~900.000
ipis on my 16 cores machine) but multiply this by 16 or 32 or 64
cpus, and clockevents_notify() cost appears to be a killer, all cpus
compete on a single lock.

Maybe this notifier could use RCU ?

could this be an artifact of the local apic stopping in deeper C states?
(which is finally fixed in the Westmere generation)

Yes it is I think.

But I suspect Eric wants a solution for Nehalem.

-Andi
-- 
ak@linux.intel.com -- Speaking for myself only.