| From | Luis Henriques <> | Subject | [PATCH 3.16.y-ckt 250/254] rcu: Use rcu_gp_kthread_wake() to wake up grace period kthreads | Date | Tue, 25 Nov 2014 10:40:00 +0000 |
| |
3.16.7-ckt2 -stable review patch. If anyone has any objections, please let me know.
------------------
From: Pranith Kumar <bobby.prani@gmail.com>
commit 2aa792e6faf1a00f5accf1f69e87e11a390ba2cd 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> Signed-off-by: Luis Henriques <luis.henriques@canonical.com> --- kernel/rcu/tree.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 625d0b0cd75a..627180d3bed4 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -1931,7 +1931,7 @@ static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags) { WARN_ON_ONCE(!rcu_gp_in_progress(rsp)); raw_spin_unlock_irqrestore(&rcu_get_root(rsp)->lock, flags); - wake_up(&rsp->gp_wq); /* Memory barrier implied by wake_up() path. */ + rcu_gp_kthread_wake(rsp); } /* @@ -2510,7 +2510,7 @@ static void force_quiescent_state(struct rcu_state *rsp) } ACCESS_ONCE(rsp->gp_flags) |= RCU_GP_FLAG_FQS; raw_spin_unlock_irqrestore(&rnp_old->lock, flags); - wake_up(&rsp->gp_wq); /* Memory barrier implied by wake_up() path. */ + rcu_gp_kthread_wake(rsp); } /* -- 2.1.0
|