Re: A question on RCU vs. preempt-RCU

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Sat, Jun 15, 2013 at 07:36:11PM -0700, Tejun Heo wrote:
> Hello, guys.
> 
> Kent recently implemented a generic percpu reference counter.  It's
> scheduled to be merged in the coming merge window and some part of
> cgroup refcnting is already converted to it.
> 
>  https://git.kernel.org/cgit/linux/kernel/git/tj/percpu.git/tree/include/linux/percpu-refcount.h?h=for-3.11
> 
>  https://git.kernel.org/cgit/linux/kernel/git/tj/percpu.git/tree/lib/percpu-refcount.c?h=for-3.11
> 
> It's essentially a generalized form of module refcnting but uses
> regular RCU instead of toggling preemption for local atomicity.
> 
> I've been running some performance tests with different preemption
> levels and, with CONFIG_PREEMPT, the percpu ref could be slower by
> around 10% or at the worst contrived case maybe even close to 20% when
> compared to simple atomic_t on a single CPU (when hit by multiple CPUs
> concurrently, it of course destroys atomic_t).  Most of the slow down
> seems to come from the preempt tree RCU calls and there no longer
> seems to be a way to opt out of that RCU implementation when
> CONFIG_PREEMPT.

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().

> For most use cases, the trade-off should be fine.  With any kind of
> cross-cpu traffic, which there usually will be, it should be an easy
> win for the percpu-refcount even when CONFIG_PREEMPT; however, I've
> been looking to replace the module ref with the generic one and the
> performance degradation there has low but existing possibility of
> being noticeable in some edge use cases.
> 
> We can convert the percpu-refcount to use preempt_disable/enable()
> paired with call_rcu_sched() but IIUC that would have latency
> implications from the callback processing side, right?  Given that
> module ref killing would be very low-frequency, it shouldn't
> contribute significant amount of callbacks but I'd like to avoid
> providing two separate implementations if at all possible.

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.

> So, what would be the right thing to do here?  How bad would
> converting percpu-refcount to sched-RCU by default be?  Would the
> extra overhead on module ref be acceptable when CONFIG_PREEMPT?
> What do you guys think?

The big question is "how long are the RCU read-side critical sections?"
My guess is that module references can have arbitrarily long lifetimes,
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.

							Thanx, Paul