rcu: Extend expedited funnel locking to rcu_data structure

The strictly rcu_node based funnel-locking scheme works well in many
cases, but systems with CONFIG_RCU_FANOUT_LEAF=64 won't necessarily get
all that much concurrency.  This commit therefore extends the funnel
locking into the per-CPU rcu_data structure, providing concurrency equal
to the number of CPUs.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

3 files changed, 21 insertions, 5 deletions

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index a905d3ba8673..e45097fc39fa 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3312,11 +3312,14 @@ static bool rcu_exp_gp_seq_done(struct rcu_state *rsp, unsigned long s)
 
 /* Common code for synchronize_{rcu,sched}_expedited() work-done checking. */
 static bool sync_exp_work_done(struct rcu_state *rsp, struct rcu_node *rnp,
+                               struct rcu_data *rdp,
                                atomic_long_t *stat, unsigned long s)
 {
         if (rcu_exp_gp_seq_done(rsp, s)) {
                 if (rnp)
                         mutex_unlock(&rnp->exp_funnel_mutex);
+                else if (rdp)
+                        mutex_unlock(&rdp->exp_funnel_mutex);
                 /* Ensure test happens before caller kfree(). */
                 smp_mb__before_atomic(); /* ^^^ */
                 atomic_long_inc(stat);
@@ -3332,6 +3335,7 @@ static bool sync_exp_work_done(struct rcu_state *rsp, struct rcu_node *rnp,
  */
 static struct rcu_node *exp_funnel_lock(struct rcu_state *rsp, unsigned long s)
 {
+        struct rcu_data *rdp;
         struct rcu_node *rnp0;
         struct rcu_node *rnp1 = NULL;
 
@@ -3343,16 +3347,24 @@ static struct rcu_node *exp_funnel_lock(struct rcu_state *rsp, unsigned long s)
          * can be inexact, as it is just promoting locality and is not
          * strictly needed for correctness.
          */
-        rnp0 = per_cpu_ptr(rsp->rda, raw_smp_processor_id())->mynode;
+        rdp = per_cpu_ptr(rsp->rda, raw_smp_processor_id());
+        if (sync_exp_work_done(rsp, NULL, NULL, &rsp->expedited_workdone1, s))
+                return NULL;
+        mutex_lock(&rdp->exp_funnel_mutex);
+        rnp0 = rdp->mynode;
         for (; rnp0 != NULL; rnp0 = rnp0->parent) {
-                if (sync_exp_work_done(rsp, rnp1, &rsp->expedited_workdone1, s))
+                if (sync_exp_work_done(rsp, rnp1, rdp,
+                                       &rsp->expedited_workdone2, s))
                         return NULL;
                 mutex_lock(&rnp0->exp_funnel_mutex);
                 if (rnp1)
                         mutex_unlock(&rnp1->exp_funnel_mutex);
+                else
+                        mutex_unlock(&rdp->exp_funnel_mutex);
                 rnp1 = rnp0;
         }
-        if (sync_exp_work_done(rsp, rnp1, &rsp->expedited_workdone2, s))
+        if (sync_exp_work_done(rsp, rnp1, rdp,
+                               &rsp->expedited_workdone3, s))
                 return NULL;
         return rnp1;
 }
@@ -3733,6 +3745,7 @@ rcu_boot_init_percpu_data(int cpu, struct rcu_state *rsp)
         WARN_ON_ONCE(atomic_read(&rdp->dynticks->dynticks) != 1);
         rdp->cpu = cpu;
         rdp->rsp = rsp;
+        mutex_init(&rdp->exp_funnel_mutex);
         rcu_boot_init_nocb_percpu_data(rdp);
         raw_spin_unlock_irqrestore(&rnp->lock, flags);
 }
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 5c1042d9c310..efee84ce1e08 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -364,11 +364,12 @@ struct rcu_data {
         unsigned long n_rp_nocb_defer_wakeup;
         unsigned long n_rp_need_nothing;
 
-        /* 6) _rcu_barrier() and OOM callbacks. */
+        /* 6) _rcu_barrier(), OOM callbacks, and expediting. */
         struct rcu_head barrier_head;
 #ifdef CONFIG_RCU_FAST_NO_HZ
         struct rcu_head oom_head;
 #endif /* #ifdef CONFIG_RCU_FAST_NO_HZ */
+        struct mutex exp_funnel_mutex;
 
         /* 7) Callback offloading. */
 #ifdef CONFIG_RCU_NOCB_CPU
@@ -494,6 +495,7 @@ struct rcu_state {
         atomic_long_t expedited_tryfail;        /* # acquisition failures. */
         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. */
         atomic_long_t expedited_normal;         /* # fallbacks to normal. */
         atomic_t expedited_need_qs;             /* # CPUs left to check in. */
         wait_queue_head_t expedited_wq;         /* Wait for check-ins. */
diff --git a/kernel/rcu/tree_trace.c b/kernel/rcu/tree_trace.c
index d9982a2ce305..ec62369f1b02 100644
--- a/kernel/rcu/tree_trace.c
+++ b/kernel/rcu/tree_trace.c
@@ -185,11 +185,12 @@ 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 n=%lu enq=%d sc=%lu\n",
+        seq_printf(m, "t=%lu tf=%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_workdone1),
                    atomic_long_read(&rsp->expedited_workdone2),
+                   atomic_long_read(&rsp->expedited_workdone3),
                    atomic_long_read(&rsp->expedited_normal),
                    atomic_read(&rsp->expedited_need_qs),
                    rsp->expedited_sequence / 2);