Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

[Paul E. McKenney]

On Wed, Apr 19, 2017 at 03:22:26PM +0200, Peter Zijlstra wrote:
> On Thu, Apr 13, 2017 at 11:42:32AM -0700, Paul E. McKenney wrote:
> 
> > I believe that you are missing the fact that RCU grace-period
> > initialization and cleanup walks through the rcu_node tree breadth
> > first, using rcu_for_each_node_breadth_first().
> 
> Indeed. That is the part I completely missed.
> 
> >                                                 This macro (shown below)
> > implements this breadth-first walk using a simple sequential traversal of
> > the ->node[] array that provides the structures making up the rcu_node
> > tree.  As you can see, this scan is completely independent of how CPU
> > numbers might be mapped to rcu_data slots in the leaf rcu_node structures.
> 
> So this code is clearly not a hotpath, but still its performance
> matters?
> 
> Seems like you cannot win here :/

Welcome to my world!!!  ;-)

But yes, running on 4096-CPU systems can put some serious stress on
some surprising areas.  Especially when those systems have cache-miss
latencies well in excess of a microsecond, and the users are nevertheless
expecting scheduling latencies well below 100 microseconds.

It was a fun challenge, I grant you that!

							Thanx, Paul