• Tejun Heo's avatar
    percpu-refcount: use RCU-sched insted of normal RCU · a4244454
    Tejun Heo authored
    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: 's avatarTejun Heo <tj@kernel.org>
    Acked-by: 's avatarKent Overstreet <koverstreet@google.com>
    Acked-by: 's avatar"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
    Cc: Michal Hocko <mhocko@suse.cz>
    Cc: Rusty Russell <rusty@rustcorp.com.au>
    a4244454
percpu-refcount.c 5.53 KB