From cdacbe1f91264687af956e810278030f2ab5a3d0 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Sat, 11 Jul 2015 16:24:45 -0700 Subject: rcu: Add fastpath bypassing funnel locking In the common case, there will be only one expedited grace period in the system at a given time, in which case it is not helpful to use funnel locking. This commit therefore adds a fastpath that bypasses funnel locking when the root ->exp_funnel_mutex is not held. Signed-off-by: Paul E. McKenney --- kernel/rcu/tree.c | 16 ++++++++++++++++ kernel/rcu/tree.h | 2 +- kernel/rcu/tree_trace.c | 4 ++-- 3 files changed, 19 insertions(+), 3 deletions(-) (limited to 'kernel/rcu') diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index f66f6e7730bc..3af0dee2d045 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -3355,6 +3355,22 @@ static struct rcu_node *exp_funnel_lock(struct rcu_state *rsp, unsigned long s) struct rcu_node *rnp0; struct rcu_node *rnp1 = NULL; + /* + * First try directly acquiring the root lock in order to reduce + * latency in the common case where expedited grace periods are + * rare. We check mutex_is_locked() to avoid pathological levels of + * memory contention on ->exp_funnel_mutex in the heavy-load case. + */ + rnp0 = rcu_get_root(rsp); + if (!mutex_is_locked(&rnp0->exp_funnel_mutex)) { + if (mutex_trylock(&rnp0->exp_funnel_mutex)) { + if (sync_exp_work_done(rsp, rnp0, NULL, + &rsp->expedited_workdone0, s)) + return NULL; + return rnp0; + } + } + /* * Each pass through the following loop works its way * up the rcu_node tree, returning if others have done the diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h index 543ba726396c..80d974df0ea0 100644 --- a/kernel/rcu/tree.h +++ b/kernel/rcu/tree.h @@ -493,7 +493,7 @@ struct rcu_state { /* End of fields guarded by barrier_mutex. */ unsigned long expedited_sequence; /* Take a ticket. */ - atomic_long_t expedited_tryfail; /* # acquisition failures. */ + atomic_long_t expedited_workdone0; /* # done by others #0. */ atomic_long_t expedited_workdone1; /* # done by others #1. */ atomic_long_t expedited_workdone2; /* # done by others #2. */ atomic_long_t expedited_workdone3; /* # done by others #3. */ diff --git a/kernel/rcu/tree_trace.c b/kernel/rcu/tree_trace.c index ec62369f1b02..6fc4c5ff3bb5 100644 --- a/kernel/rcu/tree_trace.c +++ b/kernel/rcu/tree_trace.c @@ -185,9 +185,9 @@ static int show_rcuexp(struct seq_file *m, void *v) { struct rcu_state *rsp = (struct rcu_state *)m->private; - seq_printf(m, "t=%lu tf=%lu wd1=%lu wd2=%lu wd3=%lu n=%lu enq=%d sc=%lu\n", + seq_printf(m, "s=%lu wd0=%lu wd1=%lu wd2=%lu wd3=%lu n=%lu enq=%d sc=%lu\n", rsp->expedited_sequence, - atomic_long_read(&rsp->expedited_tryfail), + atomic_long_read(&rsp->expedited_workdone0), atomic_long_read(&rsp->expedited_workdone1), atomic_long_read(&rsp->expedited_workdone2), atomic_long_read(&rsp->expedited_workdone3), -- cgit v1.2.3-59-g8ed1b