This commit speeds up the low-contention case, especially for systems with large rcu_node trees, by attempting to directly acquire the ->exp_mutex. This fastpath checks the leaves and root first in order to avoid excessive memory contention on the mutex itself.
Signed-off-by: Paul E. McKenney <[email protected]> --- kernel/rcu/tree.c | 10 ++++++++++ 1 file changed, 10 insertions(+) diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index bd2658edce00..892a140ae7b6 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -3603,6 +3603,15 @@ static bool exp_funnel_lock(struct rcu_state *rsp, unsigned long s) { struct rcu_data *rdp = per_cpu_ptr(rsp->rda, raw_smp_processor_id()); struct rcu_node *rnp = rdp->mynode; + struct rcu_node *rnp_root = rcu_get_root(rsp); + + /* Low-contention fastpath. */ + if (ULONG_CMP_LT(READ_ONCE(rnp->exp_seq_rq), s) && + (rnp == rnp_root || + ULONG_CMP_LT(READ_ONCE(rnp_root->exp_seq_rq), s)) && + !mutex_is_locked(&rsp->exp_mutex) && + mutex_trylock(&rsp->exp_mutex)) + goto fastpath; /* * Each pass through the following loop works its way up @@ -3635,6 +3644,7 @@ static bool exp_funnel_lock(struct rcu_state *rsp, unsigned long s) rnp->grphi, TPS("nxtlvl")); } mutex_lock(&rsp->exp_mutex); +fastpath: if (sync_exp_work_done(rsp, &rdp->exp_workdone3, s)) { mutex_unlock(&rsp->exp_mutex); return true; -- 2.5.2
