Re: A question on RCU vs. preempt-RCU

[Tejun Heo]

Hello, Paul.

On Sun, Jun 16, 2013 at 07:13:35AM -0700, Paul E. McKenney wrote:
> CONFIG_TREE_PREEMPT_RCU does have an increment, decrement (sort of),
> and check in its rcu_read_lock() and rcu_read_unlock(), which will
> add overhead that might well be noticeable compared to CONFIG_TREE_RCU's
> zero-code implementation of rcu_read_lock() and rcu_read_unlock().

Yeah, I should have added one more data point.  I was testing atomic_t
vs. percpu-ref and saw the overhead and worrying that it would show
regression against preempt_disable/enable() implementation.

Just ran some tests and preempt_disable/enable() based implementation
is about 18% faster than rcu_read_lock/unlock() based one.

Compared to atomic_t, in a horribly contrived test case, normal RCU
would be slower by around 20% while the preemption one would be slower
by 7.5%.

> The main source of longer latency from preempt_disable/enable()
> (or rcu_read_{,un}lock_sched()) will be on the read side.
> The callback-processing is very nearly identical.

Ah, right.  I was completely confused there.  The goal of
CONFIG_TREE_PREEMPT_RCU is to allow preemption inside RCU read
critical sections.  I knew that at one point and completely forgot
about it, so using preemption based one is fine as long as the length
of critical section is short.

> The big question is "how long are the RCU read-side critical sections?"

Extremely short.  It's gonna be like five instructions.

> My guess is that module references can have arbitrarily long lifetimes,

Preemption is disabled only while the refcnt operations are actually
going on.

> which would argue strongly against use of RCU-sched.  But if the lifetimes
> are always short (say, sub-microsecond), then RCU-sched should be fine.

So, RCU-sched, it is.

Thanks a lot for the help!

-- 
tejun
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