commit 2aa792e6fa upstream.
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: Pranith Kumar <bobby.prani@gmail.com>
CC: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
42d49f4525