Messages in this thread | | | Subject | Re: [PATCH 05/32] nohz: Move rcu dynticks idle mode handling to idle enter/exit APIs | From | Peter Zijlstra <> | Date | Mon, 29 Aug 2011 20:06:00 +0200 |
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On Mon, 2011-08-29 at 19:59 +0200, Frederic Weisbecker wrote: > On Mon, Aug 29, 2011 at 07:49:15PM +0200, Peter Zijlstra wrote: > > On Mon, 2011-08-29 at 19:11 +0200, Frederic Weisbecker wrote: > > > On Mon, Aug 29, 2011 at 04:25:22PM +0200, Peter Zijlstra wrote: > > > > On Mon, 2011-08-15 at 17:52 +0200, Frederic Weisbecker wrote: > > > > > To prepare for nohz / idle logic split, pull out the rcu dynticks > > > > > idle mode switching to strict idle entry/exit areas. > > > > > > > > > > So we make the dyntick mode possible without always involving rcu > > > > > extended quiescent state. > > > > > > > > Why is this a good thing? I would be thinking that if we're a userspace > > > > bound task and we disable the tick rcu would be finished on this cpu and > > > > thus the extended quiescent state is just what we want? > > > > > > But we can stop the tick from the kernel, not just userspace. > > > > Humm!? I'm confused, I thought the idea was to only stop the tick when > > we're 'stuck' in a user bound task. Now I get that we have to stop the > > tick from kernel space (as in the interrupt will clearly run in kernel > > space), but assuming the normal return from interrupt path doesn't use > > rcu, and using rcu (as per a later patch) re-enables the tick again, it > > doesn't matter, right? > > Yeah. Either the interrupt returns to userspace and then we call > rcu_enter_nohz() or we return to kernel space and then a further > use of rcu will restart the tick. > > Now this is not any use of rcu. Uses of rcu read side critical section > don't need the tick.
But but but, then how is it going to stop a grace period from happening? The grace period state is per-cpu and the whole state machine is tick driven.
Now some of the new RCU things go kick cpus with IPIs to push grace periods along, but I would expect you don't want that to happen either, the whole purpose here is to leave a cpu alone, unperturbed.
That means it has to be in an extended grace period when we stop the tick.
> But we need it as long as there is an RCU callback > enqueued on some CPU.
Well, no, only if there's one enqueued on this cpu because then we can't enter the extended grace period.
> > Also, RCU needs the tick to drive the state machine, so how can you stop > > the tick and not also stop the RCU state machine? > > This is why we have rcu_needs_cpu() and rcu_pending() checks before > stopping the tick. > > rcu_needs_cpu() checks we have no local callback enqueued, in which > case the local CPU is responsible of the RCU state machine. > > rcu_pending() is there to know if another CPU started a grace period > so we need the tick to complete it.
Hence the extended grace period, so we don't need to complete grace periods.
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