Re: [PATCH V2 7/7] x86,rcu: use percpu rcu_preempt_depth

[Sebastian Andrzej Siewior]

On 2019-11-02 12:45:59 [+0000], Lai Jiangshan wrote:
> Convert x86 to use a per-cpu rcu_preempt_depth. The reason for doing so
> is that accessing per-cpu variables is a lot cheaper than accessing
> task_struct or thread_info variables.

Is there a benchmark saying how much we gain from this?

> We need to save/restore the actual rcu_preempt_depth when switch.
> We also place the per-cpu rcu_preempt_depth close to __preempt_count
> and current_task variable.
> 
> Using the idea of per-cpu __preempt_count.
> 
> No function call when using rcu_read_[un]lock().
> Single instruction for rcu_read_lock().
> 2 instructions for fast path of rcu_read_unlock().

I think these were not inlined due to the header requirements.

Boris pointed one thing, there is also DEFINE_PERCPU_RCU_PREEMP_DEPTH.

Sebastian