Re: [PATCH tip/core/rcu 6/7] rcu: Drive quiescent-state-forcing delay from HZ

[Peter Zijlstra <peterz@infradead.org>]

On Thu, May 16, 2013 at 06:22:10AM -0700, Paul E. McKenney wrote:
> > But somehow I imagined making a CPU part of the GP would be easier than taking
> > it out. After all, taking it out is dangerous and careful work, one is not to
> > accidentally execute a callback or otherwise end a GP before time.
> > 
> > When entering the GP cycle there is no such concern, the CPU state is clean
> > after all.
> 
> But that would increase the overhead of GP initialization.  Right now,
> GP initialization touches only the leaf rcu_node structures, of which
> there are by default one per 16 CPUs (and can be configured up to one per
> 64 CPUs, which it is on really big systems).  So on busy mixed-workload
> systems, this approach increases GP initialization overhead for no
> good reason -- and on systems running these sorts of workloads, there
> usually aren't "sacrificial lamb" timekeeping CPUs whose utilization
> doesn't matter.

Right, so I read through some of the fqs code to get a better feel for
things and I suppose I see what you're talking about :-)

The only thing I could come up with is making fqslock a global/local
style lock, so that individual CPUs can adjust their own state without
bouncing the lock around.

It would make the fqs itself a 'bit' more expensive but ideally those
don't happen that often, ha!.

But yeah, every time you let the fqs propagate 'idle' state up the tree
your join becomes more expensive too.