Re: [patch 4/5] sched: RCU sched domains

[Ingo Molnar <mingo@elte.hu>]

* Nick Piggin <nickpiggin@yahoo.com.au> wrote:

> > At a minimum i think we need the fix+comment below.
> 
> Well if we say "this is actually RCU", then yes. And we should 
> probably change the preempt_{dis|en}ables in other places to 
> rcu_read_lock.
> 
> OTOH, if we say we just want all running threads to process through a 
> preemption stage, then this would just be a preempt_disable/enable 
> pair.
> 
> In practice that makes no difference yet, but it looks like you and 
> Paul are working to distinguish these two cases in the RCU code, to 
> accomodate your low latency RCU stuff?

it doesnt impact PREEMPT_RCU/PREEMPT_RT directly, because the scheduler 
itself always needs to be non-preemptible.

those few places where we currently do preempt_disable(), which should 
thus be rcu_read_lock(), are never in codepaths that can take alot of 
time.

but yes, in principle you are right, but in this particular (and 
special) case it's not a big issue. We should document the RCU read-lock 
dependencies cleanly and make all rcu-read-lock cases truly 
rcu_read_lock(), but it's not a pressing issue even considering possible 
future features like PREEMPT_RT.

the only danger in this area is to PREEMPT_RT: it is a bug on PREEMPT_RT 
if kernel code has an implicit 'spinlock means preempt-off and thus 
RCU-read-lock' assumption. Most of the time these get discovered via 
PREEMPT_DEBUG. (preempt_disable() disables preemption on PREEMPT_RT too, 
so that is not a problem either.)

	Ingo