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

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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@kernel.org>
Acked-by: Kent Overstreet <koverstreet@google.com>
Acked-by: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Rusty Russell <rusty@rustcorp.com.au>
This commit is contained in:
Tejun Heo 2013-06-16 16:12:26 -07:00
parent dbece3a0f1
commit a4244454df
2 changed files with 7 additions and 7 deletions
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12
include/linux/percpu-refcount.h
View File

@ -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

2
lib/percpu-refcount.c
View File

@ -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);
}