[RFC] RCU and CONFIG_PREEMPT_RT progress

[Paul E. McKenney]

Hello!

Making some progress on CONFIG_PREEMPT_RT-compatible RCU.  Am exploring
two approaches, the lock-based approach discussed earlier and a
counter-flip approach, with the latter very likely being the method
of choice.  The reason for working the former is to get myself up to
speed on details of CONFIG_PREEMPT_RT with something relatively simple.

I am basing my work off of:

	http://people.redhat.com/mingo/realtime-preempt/older/realtime-preempt-2.6.12-rc1-V0.7.41-14

since it works well on a four-CPU x86 system.  I will port forward when
I have something worth considering for inclusion.  To reiterate, the
patches referenced below are playtoys, for experimental and educational
use only.


Lock-Based Approach

1.	Trivial working patch:

	http://www.rdrop.com/users/paulmck/RCU/patches/lbdf-2a.patch

	This one uses a single rwlock and a single global callback
	list.  This of course means that only one task may be in
	an RCU read-side critical section at a time.  Even so,
	I had to split synchronize_kernel() into synchronize_rcu()
	and synchronize_sched() -- I get infrequent hangs otherwise.
	The implementation of synchronize_sched() is probably not what
	it eventually needs to be, since it simply forces each CPU to
	context switch, whether voluntary or preemption.  Will be looking
	into this later on.

2.	Slightly less trivial working patch:

	http://www.rdrop.com/users/paulmck/RCU/patches/lbdf-3a.patch

	This one uses a per-CPU rwlock, but keeps the single global
	callback list.  It is otherwise identical to #1.

Next step is to go to per-CPU callback lists.  If I was taking this
approach seriously, I would also experiment with multiple RCU read-side
locks per CPU, but I don't believe I would learn anything from that
exercise.

The reason that I am not taking this approach seriously is that it
can impose high latencies on RCU read-side critical sections, as
discussed earlier on LKML.  It also has high rcu_read_lock() and
rcu_read_unlock() overhead.


Counter-Based Approach

The current implementation in Ingo's CONFIG_PREEMPT_RT patch uses a
counter-based approach, which seems to work, but which can result in
indefinite-duration grace periods.  The following are very hazy thoughts
on how to get the benefits of this approach, but with short grace periods.

1.	The basic trick is to maintain a pair of counters per CPU.
	There would also be a global boolean variable that would select
	one or the other of each pair.  The rcu_read_lock() primitive
	would then increment the counter indicated by the boolean
	corresponding to the CPU that it is currently running on.
	It would also keep a pointer to that particular counter in
	the task structure.  The rcu_read_unlock() primitive would
	decrement this counter.  (And, yes, you would also have a
	counter in the task structure so that only the outermost of
	a set of nested rcu_read_lock()/rcu_read_unlock() pairs would
	actually increment/decrement the per-CPU counter pairs.)

	To force a grace period, one would invert the value of the
	global boolean variable.  Once all the counters indicated
	by the old value of the global boolean variable hit zero,
	the corresponding set of RCU callbacks can be safely invoked.

	The big problem with this approach is that a pair of inversions
	of the global boolean variable could be spaced arbitrarily 
	closely, especially when you consider that the read side code
	can be preempted.  This could cause RCU callbacks to be invoked
	prematurely, which could greatly reduce the life expectancy
	of your kernel.

2.	#1 above, but use a seqlock to guard the counter selection in
	rcu_read_lock().  One potential disadvantage of this approach
	is that an extremely unlucky instance of rcu_read_lock() might
	be indefinitely delayed by a series of counter flips.  I am
	concerned that this might actually happen under low-memory
	conditions.  Also requires memory barriers on the read side,
	which we might be stuck with, but still hope to be able to
	get rid of.  And the per-CPU counter manipulation must use
	atomic instructions.

3.	#1 above, but use per-CPU locks to guard the counter selection.
	I don't like this any better than #2, worse, in fact, since it
	requires expensive atomic instructions as well.

4.	The Australian National Zoo alternative:  keep the counter pairs
	in the task structure rather than keeping them per-CPU.  This
	eliminates the need for atomic operations in rcu_read_lock() and
	rcu_read_unlock(), but makes the update side do horribly expensive
	task-list trawls.  [So named because I thought of it while trying
	to jog to the Australian National Zoo.  I took a wrong turn, and
	ended up running up a valley on the other side of Black Mountain,
	so never did make it to the zoo.  On the other hand, I did encounter
	a herd of wild kangaroo and also thought up this approach, so I
	think I came out OK on the deal.]

5.	The National Train notion:  #4 above, but keep a separate list
	containing only preempted tasks that had non-zero RCU counters
	at the time of preemption.  In the (presumably) common case of
	no preemption in RCU read-side critical sections, both the
	read-side and the update-side overhead is low.  But...  There
	is a problem with detecting tasks that are in long-running
	RCU read-side critical sections that don't get preempted.
	[So named because I thought of it on the UK National Train
	somewhere between London and Winchester.]

6.	Oak Hills option:  keep per-CPU counters, which require atomic
	increment/decrement in the general case, but use a fastpath
	that (with preemption disabled) checks to see if the value of
	the counter is zero (for rcu_read_lock()) or one (for
	rcu_read_unlock()), and, if so, does the counter manipulation
	non-atomically.  Use atomics on the (presumably infrequent)
	slow path, which is taken if someone gets preempted in the middle
	of an RCU read-side critical section.

	Handle races between rcu_read_lock() and counter flips by
	having rcu_read_lock() increment the counter, then checking
	to see if it incremented the correct counter of the pair.
	If it did not (i.e., the flip just happened), increment
	the other counter of the pair as well, recording the fact that
	both were incremented in the task struct.  The rcu_read_unlock()
	primitive then decrements any/all counters that rcu_read_lock()
	incremented.

	Memory barriers are still needed in the non-atomic increment
	and decrement cases.  However, it may be possible to leverage
	naturally occuring memory barriers (see for example Joe Seigh's
	recent LKML posting on RCU+SMR: http://lkml.org/lkml/2005/5/9/129).
	If the naturally occuring memory barriers aren't happening fast
	enough (e.g., low memory situation), a round of IPIs should
	suffice, for example, smp_call_function() to a function that
	advances the callbacks on each CPU.

	If this one pans out, the common-case overhead of rcu_read_lock()
	and rcu_read_unlock() would not be much more expensive than the
	current CONFIG_PREEMPT implementations.

There are probably better approaches, but that is what I have thus far.

Thoughts?

							Thanx, Paul
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