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

[Andrew Morton <akpm@osdl.org>]

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.

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..