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

[Nick Piggin <nickpiggin@yahoo.com.au>]

Ingo Molnar wrote:
> * 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.)
> 

OK thanks for the good explanation. So I'll keep it as is for now,
and whatever needs cleaning up later can be worked out as it comes
up.

-- 
SUSE Labs, Novell Inc.