tick-sched: add specific do_timer_cpu value for nohz off mode

tick-sched: add specific do_timer_cpu value for nohz off mode

Show and modify the tick_do_timer_cpu via sysfs.  This determines the cpu
on which global time (jiffies) updates occur.  Modification can only be
done on systems with nohz mode turned off.

While not necessarily harmful, doing jiffies updates on an application cpu
does cause some extra overhead that HPC benchmarking people notice.  They
prefer to have OS activity isolated to certain cpus.  They like
reproducibility of results, and having jiffies updates bouncing around
introduces variability.

Signed-off-by: Dimitri Sivanich <sivanich@sgi.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>