Messages in this thread | | | From | Rusty Russell <> | Subject | Re: A question on RCU vs. preempt-RCU | Date | Sun, 16 Jun 2013 16:16:15 +0930 |
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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.
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