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

[Shailabh Nagar <nagar@watson.ibm.com>]

Andrew Morton wrote:

> Shailabh Nagar <nagar@watson.ibm.com> wrote:
>  
>
> > +/*
> > + * Per-task exit data sent from the kernel to user space
> > + * is tagged by an id based on grouping of cpus.
> > + *
> > + * If userspace specifies a non-zero P as the nl_pid field of
> > + * the sockaddr_nl structure while binding to a netlink socket,
> > + * it will receive exit data from threads that exited on cpus in the range
> > + *
> > + *    [(P-1)*Y, P*Y-1]
> > + *
> > + *  where Y = TASKSTATS_CPUS_PER_SET
> > + *  i.e. if TASKSTATS_CPUS_PER_SET is 16,
> > + *  to listen to data from cpus 0..15, specify P=1
> > + *  for cpus 16..32, specify P=2 etc.
> > + *
> > + *  To listen to data from all cpus, userspace should use P=0
> > + */
> > +
> > +#define TASKSTATS_CPUS_PER_SET	16
> >    
>
> The constant is unpleasant.
>  
I was planning to make it configurable. But that would still not be as 
flexible as below...

> 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 ?

--Shailabh
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