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

[Peter Zijlstra]

On Thu, Apr 13, 2017 at 11:19:26AM -0700, Paul E. McKenney wrote:

> First get me some system-level data showing that the current layout is
> causing a real problem.  RCU's fastpath code doesn't come anywhere near
> the rcu_node tree, so in the absence of such data, I of course remain
> quite doubtful that there is a real need.  And painfully aware of the
> required increase in complexity.
> 
> But if there is a real need demonstrated by real system-level data,
> I will of course make the needed changes, as I have done many times in
> the past in response to other requests.

I read what you wrote here:

> > > Increasing it reduces the number of rcu_node structures, and thus the
> > > number of cache misses during grace-period initialization and cleanup.
> > > This has proven necessary in the past on large machines having long
> > > memory latencies.  And there are starting to be some pretty big machines
> > > running in production, and even for typical commerical workloads.

to mean you had exactly that pain. Or am I now totally not understanding
you?