Messages in this thread | | | Date | Tue, 14 May 2013 08:47:28 -0700 | From | "Paul E. McKenney" <> | 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 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.
> > The challenge here appears to be somehow telling > > RCU when boot is done. The APIs are there from an RCU viewpoint: boot > > with rcupdate.rcu_expedited=1, then, once boot is complete (whatever > > that means on your platform) "echo 0 > /sys/kernel/rcu_expedited". > > Ha, and here you assume userspace is sane and co-operative. Fail in my book ;-)
If they are insane or uncooperative, they pay the price. ;-)
Thanx, Paul
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