On Tue, May 14, 2013 at 08:47:28AM -0700, Paul E. McKenney wrote: > On Tue, May 14, 2013 at 04:51:20PM +0200, Peter Zijlstra wrote: > > > In theory, yes. In practice, this requires lots of lock acquisitions > > > and releases on large systems, including some global locks. The weight > > > could be reduced, but... > > > > > > What I would like to do instead would be to specify expedited grace > > > periods during boot. > > > > But why, surely going idle without any RCU callbacks isn't completely > > unheard > > of, even outside of the boot process? > > Yep, and RCU has special-cased that for quite some time. > > > Being able to quickly drop out of the RCU state machinery would be a good > > thing IMO. > > And this is currently possible -- this is the job of rcu_idle_enter() > and friends. And it works well, at least when I get my "if" statements > set up correctly (hence the earlier patch). > > Or are you seeing a slowdown even with that earlier patch applied? If so, > please let me know what you are seeing.
I'm not running anything in particular, except maybe a broken mental model of RCU ;-) So what I'm talking about is the !rcu_cpu_has_callbacks() case, where there's absolutely nothing for RCU to do except tell the state machine its no longer participating. Your patch to rcu_needs_cpu() frobbing the lazy condition is after that and thus irrelevant for this AFAICT. Now as far as I can see, rcu_needs_cpu() will return false in this case; allowing the cpu to enter NO_HZ state. We then call rcu_idle_enter() which would call rcu_eqs_enter(). Which should put the CPU in extended quiescent state. However, you're still running into these FQSs delaying boot. Why is that? Is that because rcu_eqs_enter() doesn't really do enough? The thing is, if all other CPUs are idle, detecting the end of a grace period should be rather trivial and not involve FQSs and thus be tons faster. Clearly I'm missing something obvious and not communicating right or so. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
