• Pranith Kumar's avatar
    rcu: Use rcu_gp_kthread_wake() to wake up grace period kthreads · 2aa792e6
    Pranith Kumar authored
    The rcu_gp_kthread_wake() function checks for three conditions before
    waking up grace period kthreads:
    
    *  Is the thread we are trying to wake up the current thread?
    *  Are the gp_flags zero? (all threads wait on non-zero gp_flags condition)
    *  Is there no thread created for this flavour, hence nothing to wake up?
    
    If any one of these condition is true, we do not call wake_up().
    It was found that there are quite a few avoidable wake ups both during
    idle time and under stress induced by rcutorture.
    
    Idle:
    
    Total:66000, unnecessary:66000, case1:61827, case2:66000, case3:0
    Total:68000, unnecessary:68000, case1:63696, case2:68000, case3:0
    
    rcutorture:
    
    Total:254000, unnecessary:254000, case1:199913, case2:254000, case3:0
    Total:256000, unnecessary:256000, case1:201784, case2:256000, case3:0
    
    Here case{1-3} are the cases listed above. We can avoid these wake
    ups by using rcu_gp_kthread_wake() to conditionally wake up the grace
    period kthreads.
    
    There is a comment about an implied barrier supplied by the wake_up()
    logic.  This barrier is necessary for the awakened thread to see the
    updated ->gp_flags.  This flag is always being updated with the root node
    lock held. Also, the awakened thread tries to acquire the root node lock
    before reading ->gp_flags because of which there is proper ordering.
    
    Hence this commit tries to avoid calling wake_up() whenever we can by
    using rcu_gp_kthread_wake() function.
    Signed-off-by: default avatarPranith Kumar <bobby.prani@gmail.com>
    CC: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
    Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
    2aa792e6
tree.c 116 KB