kthread: Document ways of reducing OS jitter due to per-CPU kthreads

kthread: Document ways of reducing OS jitter due to per-CPU kthreads

The Linux kernel uses a number of per-CPU kthreads, any of which might
contribute to OS jitter at any time.  The usual approach to normal
kthreads, namely to bind them to a "housekeeping" CPU, does not work
with these kthreads because they cannot operate correctly if moved to
some other CPU.  This commit therefore lists ways of controlling OS
jitter from the Linux kernel's per-CPU kthreads.  It also lists some
ways of diagnosing excessive jitter.

Signed-off-by: Paul E. McKenney <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Arjan van de Ven <[email protected]>
Cc: Kevin Hilman <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Olivier Baetz <[email protected]>
Cc: Pradeep Satyanarayana <[email protected]>
Reviewed-by: Randy Dunlap <[email protected]>
Reviewed-by: Borislav Petkov <[email protected]>