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Commit a4244454 authored by Tejun Heo's avatar Tejun Heo
Browse files

percpu-refcount: use RCU-sched insted of normal RCU 



percpu-refcount was incorrectly using preempt_disable/enable() for RCU
critical sections against call_rcu().  6a24474d ("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: default avatarTejun Heo <tj@kernel.org>
Acked-by: default avatarKent Overstreet <koverstreet@google.com>
Acked-by: default avatar"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Rusty Russell <rusty@rustcorp.com.au>
parent dbece3a0

include/linux/percpu-refcount.h

+6 −6
Original line number Diff line number Diff line
@@ -105,7 +105,7 @@ static inline void percpu_ref_get(struct percpu_ref *ref) 
{

	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 percpu_ref *ref) 
	else

		atomic_inc(&ref->count);



	rcu_read_unlock();

	rcu_read_unlock_sched();

}



/**
@@ -134,7 +134,7 @@ static inline bool percpu_ref_tryget(struct percpu_ref *ref) 
	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(struct percpu_ref *ref) 
		ret = true;

	}



	rcu_read_unlock();

	rcu_read_unlock_sched();



	return ret;

}
@@ -159,7 +159,7 @@ static inline void percpu_ref_put(struct percpu_ref *ref) 
{

	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 percpu_ref *ref) 
	else if (unlikely(atomic_dec_and_test(&ref->count)))

		ref->release(ref);



	rcu_read_unlock();

	rcu_read_unlock_sched();

}



#endif

lib/percpu-refcount.c

+1 −1
Original line number Diff line number Diff line
@@ -154,5 +154,5 @@ void percpu_ref_kill_and_confirm(struct percpu_ref *ref, 
		(((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);

}