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

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

On Tue, May 21, 2013 at 11:45:31AM +0200, Peter Zijlstra wrote:
> 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.

Maybe...  The current design uses bitmasks at each level, and avoiding the
upper-level locks would mean making RCU work with out-of-date bitmasks
at the upper levels.  Might be possible, but it is not clear to me that
this would be a win.

I could also maintain yet another bitmask at the bottom level to record
the idle CPUs, but it is not clear that this is a win, either, especially
on systems with frequent idle/busy transitions.

> 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.

Yep!  :-/

							Thanx, Paul