On Fri, 30 Jun 2006 23:37:10 -0400
Shailabh Nagar [off-list ref] wrote:

quoted

Set aside the implementation details and ask "what is a good design"?

A kernel-wide constant, whether determined at build-time or by a /proc poke
isn't a nice design.

Can we permit userspace to send in a netlink message describing a cpumask? 
That's back-compatible.
 

Yes, that should be doable. And passing in a cpumask is much better 
since we no longer
have to maintain mappings.

So the strawman is:
Listener bind()s to genetlink using its real pid.
Sends a separate "registration" message with cpumask to listen to. 
Kernel stores (real) pid and cpumask.
During task exit, kernel goes through each registered listener (small 
list) and decides which
one needs to get this exit data and calls a genetlink_unicast to each 
one that does need it.

If number of listeners is small, the lookups should be swift enough. If 
it grows large, we
can consider a fancier lookup (but there I go again, delving into 
implementation too early :-)

We'll need a map.

1024 CPUs, 1024 listeners, 1000 exits/sec/CPU and we're up to a million
operations per second per CPU.  Meltdown.

But it's a pretty simple map.  A per-cpu array of pointers to the head of a
linked list.  One lock for each CPU's list.