Re: [PATCH RFC tip/core/rcu] accelerate grace period if last non-dynticked CPU

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

On Wed, Jan 27, 2010 at 10:50:50AM +0100, Peter Zijlstra wrote:
> On Wed, 2010-01-27 at 10:43 +0100, Andi Kleen wrote:
> > 
> > Can't you simply check that at runtime then?
> > 
> > if (num_possible_cpus() > 20) 
> >         ...
> > 
> > BTW the new small is large. This years high end desktop PC will come with 
> > upto 12 CPU threads. It would likely be challenging to find a good
> > number for 20 that holds up with the future. 
> 
> If only scalability were that easy :/
> 
> These massive core/thread count things are causing more problems as
> well, the cpus/node ratios are constantly growing, giving grief in the
> page allocator as well as other places that used to scale per node.
> 
> As to the current problem, the call_rcu() interface doesn't make a hard
> promise that the callback will be done on the same cpu, right? So why
> not simply move the callback list over to a more active cpu?

I could indeed do that.  However, there is nothing stopping the
more-active CPU from going into dynticks-idle mode between the time
that I decide to push the callback to it and the time I actually do
the pushing.  :-(

I considered pushing the callbacks to the orphanage, but that is a
global lock that I would rather not acquire on each dyntick-idle
transition.

This conversation is having the effect of making me much more comfortable
adding a kernel configuration parameter.  Might not have been the intent,
but there you have it!  ;-)

							Thanx, Paul