From 9036c2ffd596261d2067fc2d693dc4f0d7a51214 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Tue, 10 Apr 2018 21:17:56 -0700 Subject: [PATCH] rcu: Improve non-root rcu_cbs_completed() accuracy When rcu_cbs_completed() is invoked on a non-root rcu_node structure, it unconditionally assumes that two grace periods must complete before the callbacks at hand can be invoked. This is overly conservative because if that non-root rcu_node structure believes that no grace period is in progress, and if the corresponding rcu_state structure's ->gpnum field has not yet been incremented, then these callbacks may safely be invoked after only one grace period has completed. This change is required to permit grace-period start requests to use funnel locking, which is in turn permitted to reduce root rcu_node ->lock contention, which has been observed by Nick Piggin. Furthermore, such contention will likely be increased by the merging of RCU-bh, RCU-preempt, and RCU-sched, so it makes sense to take steps to decrease it. This commit therefore improves the accuracy of rcu_cbs_completed() when invoked on a non-root rcu_node structure as described above. Reported-by: Nicholas Piggin Signed-off-by: Paul E. McKenney Tested-by: Nicholas Piggin --- kernel/rcu/tree.c | 15 +++++++++++++++ 1 file changed, 15 insertions(+) diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 2a734692a581..f5ca72f2ed43 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -1641,6 +1641,21 @@ static unsigned long rcu_cbs_completed(struct rcu_state *rsp, if (rcu_get_root(rsp) == rnp && rnp->gpnum == rnp->completed) return rnp->completed + 1; + /* + * If the current rcu_node structure believes that RCU is + * idle, and if the rcu_state structure does not yet reflect + * the start of a new grace period, then the next grace period + * will suffice. The memory barrier is needed to accurately + * sample the rsp->gpnum, and pairs with the second lock + * acquisition in rcu_gp_init(), which is augmented with + * smp_mb__after_unlock_lock() for this purpose. + */ + if (rnp->gpnum == rnp->completed) { + smp_mb(); /* See above block comment. */ + if (READ_ONCE(rsp->gpnum) == rnp->completed) + return rnp->completed + 1; + } + /* * Otherwise, wait for a possible partial grace period and * then the subsequent full grace period. -- 2.30.2