Re: [Patch][RFC] Disabling per-tgid stats on task exit in taskstats

On Fri, 30 Jun 2006 22:20:23 -0400
Shailabh Nagar [off-list ref] wrote:

quoted

If we're going to abuse nl_pid then how about we design things so that
nl_pid is treated as two 16-bit words - one word is the start CPU and the
other word is the end cpu?

Or, if a 65536-CPU limit is too scary, make the bottom 8 bits of nl_pid be
the number of CPUS (ie: TASKSTATS_CPUS_PER_SET) and the top 24 bits is the
starting CPU.  

<avoids mentioning nl_pad>

It'd be better to use a cpumask, of course..
 

All these options mean each listener gets to pick a "custom" range of 
cpus to listen on, 
rather than choose one of pre-defined ranges (even if the pre-defined 
ranges can change
by a configurable TASKSTATS_CPUS_PER_SET). Which means the kernel side 
has to
figure out which of the listeners cpu range includes the currently 
exiting task's cpu. To do
this, we'll need a callback from the binding of the netlink socket (so 
taskstats can maintain
the cpu -> nl_pid mappings at any exit).
The current genetlink interface doesn't have that kind of flexibility 
(though it can be added
I'm sure).

Seems a bit involved if the primary aim is to restrict the number of 
cpus that one listener
 wants to listen, rather than be able to pick which ones.

A configurable range won't suffice ?

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.