From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andrew Morton <akpm@osdl.org>
Subject: Re: [Patch][RFC] Disabling per-tgid stats on task exit in taskstats
Date: Fri, 30 Jun 2006 19:43:53 -0700
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To: Shailabh Nagar <nagar@watson.ibm.com>
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On Fri, 30 Jun 2006 22:20:23 -0400
Shailabh Nagar <nagar@watson.ibm.com> wrote:

> >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.