Re: [PATCH tip/core/rcu] Make RCU_FAST_NO_HZ respect nohz= boot parameter

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Jul 05, 2012 at 04:02:08PM -0700, Josh Triplett wrote:
> On Thu, Jul 05, 2012 at 03:37:31PM -0700, Paul E. McKenney wrote:
> > If the nohz= boot parameter disables nohz, then RCU_FAST_NO_HZ needs to
> > also disable itself.  This commit therefore checks for tick_nohz_enabled
> > being zero, disabling rcu_prepare_for_idle() if so.  This patch assumes
> > that tick_nohz_enabled can change at runtime: If this is not the case,
> > then a simpler approach suffices.
> 
> Allowing nohz to change at runtime seems like an entirely unnecessary
> bit of added complexity.  (So does having a boot parameter for it, but
> that one at least seems easier to handle.)

I will let representatives from the various distros expound to you on
their one-binary-only strategy for kernel builds.  ;-)

> What would the patch look like if you can assume nohz will never change
> at runtime?  And does anyone have a use case for changing nohz at
> runtime, rather than at boot time?

It would be a little bit simpler, but would break in very odd and
difficult-to-debug ways if anyone ever did allow it to change at runtime,
for example, to accommodate systems subject to varying workloads.

							Thanx, Paul