Re: A question on RCU vs. preempt-RCU

[Rusty Russell <rusty@rustcorp.com.au>]

Tejun Heo <tj@kernel.org> writes:
> 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.
>
> 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.

I'm confused: is it actually 10% slower than the existing module
refcount code, or 10% slower than atomic inc?

> 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.
>
> 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?

CONFIG_PREEMPT, now with more preempt!  Sure, that has a cost, but
you're arguably fixing a bug.

If we want to improve CONFIG_PREEMPT performance, we can probably use a
trick I wanted to try long ago:

1) Use a per-cpu counter rather than a per-task counter for preempt.
2) Lay out preempt_counter so it covers NR_CPU pages, one per page.
3) When you want to preempt a CPU and counter isn't zero, make the page RO.
4) Handle preemption enable in the fault handler.

Then there's no branch in preempt_enable().

At a glance, the same trick could apply to t->rcu_read_unlock_special,
but I'd have to offload that to my RCU coprocessor.  Paul? :)

Cheers,
Rusty.