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

[Nick Piggin]

Ingo Molnar wrote:
> * Nick Piggin <nickpiggin@xxxxxxxxxxxx> 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.

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