Re: Real-Time Preemption and RCU

[Paul E. McKenney]

On Tue, Mar 22, 2005 at 09:55:26AM +0100, Esben Nielsen wrote:
> On Mon, 21 Mar 2005, Paul E. McKenney wrote:
[ . . . ]
> > On Mon, Mar 21, 2005 at 12:23:22AM +0100, Esben Nielsen wrote:
> > This is in some ways similar to the K42 approach to RCU (which they call
> > "generations").  Dipankar put together a similar patch for Linux, but
> > the problem was that grace periods could be deferred for an extremely
> > long time.  Which I suspect is what you were calling out as causing
> > RCU batches never to run.
> 
> That is where the preempt_by_nonrt_disable/enable() is supposed to help:
> Then it can't take longer than the normal kernel in the situation where
> there is no RT tasks running. RT tasks will prolong the grace periods if
> they go into RCU regions, but they are supposed to be relatively small -
> and deterministic!

The part that I am missing is how this helps in the case where a non-RT
task gets preempted in the middle of an RCU read-side critical section
indefinitely.  Or are you boosting the priority of any task that
enters an RCU read-side critical section?

> > > > The counter approach might work, and is also what the implementation #5
> > > > does -- check out rcu_read_lock() in Ingo's most recent patch.
> > > > 
> > > 
> > > Do you refer to your original mail with implementing it in 5 steps?
> > > In #5 in that one (-V0.7.41-00, right?) you use a lock and as you say that
> > > forces syncronization between the CPUs - bad for scaling. It does make the
> > > RCU batches somewhat deterministic, as the RCU task can boost the readers
> > > to the rcu-task's priority.
> > > The problem about this approach is that everybody calling into RCU code
> > > have a worst case behaviour of the systemwide worst case RCU reader 
> > > section - which can be pretty large (in principle infinite if somebody.)
> > > So if somebody uses a call to a function in the code containing a RCU read
> > > area the worst case behavious would be the same as teh worst case latency
> > > in the simple world where preempt_disable()/preempt_enable() was used.
> > 
> > I missed something here -- readers would see the worst-case -writer-
> > latency rather than the worst-case -reader- latency, right?  Or are you
> > concerned about the case where some reader blocks the write-side
> > acquisitions in _synchronize_kernel(), but not before the writer has
> > grabbed a lock, blocking any readers on the corresponding CPU?
> 
> I am conserned that readers block each other, too. You do need an rw-mutex
> allowing an unlimited number of readers for doing this. With the current
> rw-mutex the readers block each other. I.e. the worst case latency is the
> worst case reader latency - globally!
> On the other hand with a rw-lock being unlimited - and thus do not keep
> track of it readers - the readers can't be boosted by the writer. Then you
> are back to square 1: The grace period can take a very long time.

OK, good point.

> > Yes, but this is true of every other lock in the system as well, not?
> 
> Other locks are not globaly used but only used for a specific subsystem.
> On a real-time system you are supposed to know which subsystems you can
> call into and still have a low enough latency as each subsystem has it's
> own bound. But with a global RCU locking mechanism all RCU using code is
> to be regarded as _one_ such subsystem.

Yep.  As would the things protected by the dcache lock, task list lock,
and so on, right?

						Thanx, Paul

> > In a separate conversation a few weeks ago, Steve Hemminger suggested
> > placing checks in long RCU read-side critical sections -- this could
> > be used to keep the worst-case reader latency within the desired bounds.
> > Not pretty, but, then again, bounding lock latency will require a fair
> > number of similar changes, I suspect.
> > 
> > 						Thanx, Paul
> > 
> 
> Esben
> 
> 
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