Re: [PATCH percpu/for-3.11] percpu-refcount: use RCU-sched insted ofnormal RCU

From: Kent Overstreet
Date: Sun Jun 16 2013 - 19:10:45 EST

On Sun, Jun 16, 2013 at 02:55:46PM -0700, Tejun Heo wrote:
> percpu-refcount was incorrectly using preempt_disable/enable() for RCU
> critical sections against call_rcu(). 6a24474da8 ("percpu-refcount:
> consistently use plain (non-sched) RCU") fixed it by converting the
> preepmtion operations with rcu_read_[un]lock() citing that there isn't
> any advantage in using sched-RCU over using the usual one; however,
> rcu_read_[un]lock() for the preemptible RCU implementation -
> CONFIG_TREE_PREEMPT_RCU, chosen when CONFIG_PREEMPT - are slightly
> more expensive than preempt_disable/enable().
> In a contrived microbench which repeats the followings,
> - percpu_ref_get()
> - copy 32 bytes of data into percpu buffer
> - percpu_put_get()
> - copy 32 bytes of data into percpu buffer
> rcu_read_[un]lock() used in percpu_ref_get/put() makes it go slower by
> about 15% when compared to using sched-RCU.
> As the RCU critical sections are extremely short, using sched-RCU
> shouldn't have any latency implications. Convert to RCU-sched.
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
> Cc: Kent Overstreet <koverstreet@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxx>
> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
> Cc: Rusty Russell <rusty@xxxxxxxxxxxxxxx>

Acked-by: Kent Overstreet <koverstreet@xxxxxxxxxx>
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