linux-kernel - Re: [PATCH percpu/for-3.11] percpu-refcount: use RCU-sched insted of normal RCU

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Message-ID: <20130616231041.GF28664@moria.home.lan>
Date:	Sun, 16 Jun 2013 16:10:41 -0700
From:	Kent Overstreet <koverstreet@...gle.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Michal Hocko <mhocko@...e.cz>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH percpu/for-3.11] percpu-refcount: use RCU-sched insted of
 normal RCU

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@...nel.org>
> Cc: Kent Overstreet <koverstreet@...gle.com>
> Cc: Michal Hocko <mhocko@...e.cz>
> Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> Cc: Rusty Russell <rusty@...tcorp.com.au>

Acked-by: Kent Overstreet <koverstreet@...gle.com>
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