percpu-refcount: use RCU-sched insted of normal RCU (a4244454) · Commits · 戴 / test

percpu-refcount was incorrectly using preempt_disable/enable() for RCU critical sections against call_rcu(). ("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>

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);
		}

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