Messages in this thread | | | Date | Wed, 15 May 2013 10:56:39 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH tip/core/rcu 6/7] rcu: Drive quiescent-state-forcing delay from HZ |
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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.
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