This commit makes rcu_nocb_wait_gp() check rdp->gp_seq_needed to see if the current CPU already knows about the needed grace period having already been requested. If so, it avoids acquiring the corresponding leaf rcu_node structure's ->lock, thus decreasing contention. This optimization is intended for cases where either multiple leader rcuo kthreads are running on the same CPU or these kthreads are running on a non-offloaded (e.g., housekeeping) CPU.
Signed-off-by: Paul E. McKenney <[email protected]> [ paulmck: Move lock release past "if" as suggested by Joel Fernandes. ] [ paulmck: Fix caching of furthest-future requested grace period. ] --- kernel/rcu/tree.c | 5 +++++ kernel/rcu/tree_plugin.h | 15 ++++++++++----- 2 files changed, 15 insertions(+), 5 deletions(-) diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index e5d1369e099f..0da0db1128d0 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -1618,6 +1618,11 @@ static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp, trace_rcu_grace_period(rsp->name, READ_ONCE(rsp->gpnum), TPS("newreq")); ret = true; /* Caller must wake GP kthread. */ unlock_out: + /* Push furthest requested GP to leaf node and rcu_data structure. */ + if (ULONG_CMP_LT(c, rnp_root->gp_seq_needed)) { + rnp->gp_seq_needed = rnp_root->gp_seq_needed; + rdp->gp_seq_needed = rnp_root->gp_seq_needed; + } if (rnp != rnp_root) raw_spin_unlock_rcu_node(rnp_root); return ret; diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index f4a88e3c388d..ca73931f7b30 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -2104,12 +2104,17 @@ static void rcu_nocb_wait_gp(struct rcu_data *rdp) bool needwake; struct rcu_node *rnp = rdp->mynode; - raw_spin_lock_irqsave_rcu_node(rnp, flags); + local_irq_save(flags); c = rcu_seq_snap(&rdp->rsp->gp_seq); - needwake = rcu_start_this_gp(rnp, rdp, c); - raw_spin_unlock_irqrestore_rcu_node(rnp, flags); - if (needwake) - rcu_gp_kthread_wake(rdp->rsp); + if (!rdp->gpwrap && ULONG_CMP_GE(rdp->gp_seq_needed, c)) { + local_irq_restore(flags); + } else { + raw_spin_lock_rcu_node(rnp); /* irqs already disabled. */ + needwake = rcu_start_this_gp(rnp, rdp, c); + raw_spin_unlock_irqrestore_rcu_node(rnp, flags); + if (needwake) + rcu_gp_kthread_wake(rdp->rsp); + } /* * Wait for the grace period. Do so interruptibly to avoid messing -- 2.17.1
