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 <[email protected]>
> Cc: Kent Overstreet <[email protected]>
> Cc: Michal Hocko <[email protected]>
> Cc: "Paul E. McKenney" <[email protected]>
Acked-by: Paul E. McKenney <[email protected]>
> Cc: Rusty Russell <[email protected]>
> ---
> include/linux/percpu-refcount.h | 12 ++++++------
> lib/percpu-refcount.c | 2 +-
> 2 files changed, 7 insertions(+), 7 deletions(-)
>
> --- a/include/linux/percpu-refcount.h
> +++ b/include/linux/percpu-refcount.h
> @@ -105,7 +105,7 @@ static inline void percpu_ref_get(struct
> {
> unsigned __percpu *pcpu_count;
>
> - rcu_read_lock();
> + rcu_read_lock_sched();
>
> pcpu_count = ACCESS_ONCE(ref->pcpu_count);
>
> @@ -114,7 +114,7 @@ static inline void percpu_ref_get(struct
> else
> atomic_inc(&ref->count);
>
> - rcu_read_unlock();
> + rcu_read_unlock_sched();
> }
>
> /**
> @@ -134,7 +134,7 @@ static inline bool percpu_ref_tryget(str
> unsigned __percpu *pcpu_count;
> int ret = false;
>
> - rcu_read_lock();
> + rcu_read_lock_sched();
>
> pcpu_count = ACCESS_ONCE(ref->pcpu_count);
>
> @@ -143,7 +143,7 @@ static inline bool percpu_ref_tryget(str
> ret = true;
> }
>
> - rcu_read_unlock();
> + rcu_read_unlock_sched();
>
> return ret;
> }
> @@ -159,7 +159,7 @@ static inline void percpu_ref_put(struct
> {
> unsigned __percpu *pcpu_count;
>
> - rcu_read_lock();
> + rcu_read_lock_sched();
>
> pcpu_count = ACCESS_ONCE(ref->pcpu_count);
>
> @@ -168,7 +168,7 @@ static inline void percpu_ref_put(struct
> else if (unlikely(atomic_dec_and_test(&ref->count)))
> ref->release(ref);
>
> - rcu_read_unlock();
> + rcu_read_unlock_sched();
> }
>
> #endif
> --- a/lib/percpu-refcount.c
> +++ b/lib/percpu-refcount.c
> @@ -154,5 +154,5 @@ void percpu_ref_kill_and_confirm(struct
> (((unsigned long) ref->pcpu_count)|PCPU_REF_DEAD);
> ref->confirm_kill = confirm_kill;
>
> - call_rcu(&ref->rcu, percpu_ref_kill_rcu);
> + call_rcu_sched(&ref->rcu, percpu_ref_kill_rcu);
> }
>
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