rcu: Move from ->need_future_gp[] to ->gp_seq_needed

One problem with the ->need_future_gp[] array is that the grace-period
assignment of each element changes as the grace periods complete.
This means that it is necessary to hold a lock when checking this
array to learn if a given grace period has already been requested.
This increase lock contention, which is the opposite of helpful.
This commit therefore replaces the ->need_future_gp[] with a single
->gp_seq_needed value and keeps it updated in the rcu_data structure.

This will enable reliable lockless checking of whether or not a given
grace period has already been requested.

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

2 files changed, 20 insertions, 33 deletions

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 4a528a062cd4..1ede51690e4a 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1560,7 +1560,7 @@ static void trace_rcu_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
 /*
  * Start the specified grace period, as needed to handle newly arrived
  * callbacks.  The required future grace periods are recorded in each
- * rcu_node structure's ->need_future_gp[] field.  Returns true if there
+ * rcu_node structure's ->gp_seq_needed field.  Returns true if there
  * is reason to awaken the grace-period kthread.
  *
  * The caller must hold the specified rcu_node structure's ->lock, which
@@ -1589,14 +1589,14 @@ static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
         for (rnp_root = rnp; 1; rnp_root = rnp_root->parent) {
                 if (rnp_root != rnp)
                         raw_spin_lock_rcu_node(rnp_root);
-                if (need_future_gp_element(rnp_root, c) ||
+                if (ULONG_CMP_GE(rnp_root->gp_seq_needed, c) ||
                     rcu_seq_done(&rnp_root->gp_seq, c) ||
                     (rnp != rnp_root &&
                      rcu_seq_state(rcu_seq_current(&rnp_root->gp_seq)))) {
                         trace_rcu_this_gp(rnp_root, rdp, c, TPS("Prestarted"));
                         goto unlock_out;
                 }
-                need_future_gp_element(rnp_root, c) = true;
+                rnp_root->gp_seq_needed = c;
                 if (rnp_root != rnp && rnp_root->parent != NULL)
                         raw_spin_unlock_rcu_node(rnp_root);
                 if (!rnp_root->parent)
@@ -1633,8 +1633,9 @@ static bool rcu_future_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp)
         bool needmore;
         struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
 
-        need_future_gp_element(rnp, c) = false;
-        needmore = need_any_future_gp(rnp);
+        needmore = ULONG_CMP_LT(rnp->gp_seq, rnp->gp_seq_needed);
+        if (!needmore)
+                rnp->gp_seq_needed = rnp->gp_seq; /* Avoid counter wrap. */
         trace_rcu_this_gp(rnp, rdp, c,
                           needmore ? TPS("CleanupMore") : TPS("Cleanup"));
         return needmore;
@@ -2046,7 +2047,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
         rsp->gp_state = RCU_GP_IDLE;
         /* Check for GP requests since above loop. */
         rdp = this_cpu_ptr(rsp->rda);
-        if (need_any_future_gp(rnp)) {
+        if (ULONG_CMP_LT(rnp->gp_seq, rnp->gp_seq_needed)) {
                 trace_rcu_this_gp(rnp, rdp, rsp->completed - 1,
                                   TPS("CleanupMore"));
                 needgp = true;
@@ -2700,8 +2701,8 @@ rcu_check_gp_start_stall(struct rcu_state *rsp, struct rcu_node *rnp,
         struct rcu_node *rnp_root = rcu_get_root(rsp);
         static atomic_t warned = ATOMIC_INIT(0);
 
-        if (!IS_ENABLED(CONFIG_PROVE_RCU) ||
-            rcu_gp_in_progress(rsp) || !need_any_future_gp(rcu_get_root(rsp)))
+        if (!IS_ENABLED(CONFIG_PROVE_RCU) || rcu_gp_in_progress(rsp) ||
+            ULONG_CMP_GE(rnp_root->gp_seq, rnp_root->gp_seq_needed))
                 return;
         j = jiffies; /* Expensive access, and in common case don't get here. */
         if (time_before(j, READ_ONCE(rsp->gp_req_activity) + HZ) ||
@@ -2711,7 +2712,8 @@ rcu_check_gp_start_stall(struct rcu_state *rsp, struct rcu_node *rnp,
 
         raw_spin_lock_irqsave_rcu_node(rnp, flags);
         j = jiffies;
-        if (rcu_gp_in_progress(rsp) || !need_any_future_gp(rcu_get_root(rsp)) ||
+        if (rcu_gp_in_progress(rsp) ||
+            ULONG_CMP_GE(rnp_root->gp_seq, rnp_root->gp_seq_needed) ||
             time_before(j, READ_ONCE(rsp->gp_req_activity) + HZ) ||
             time_before(j, READ_ONCE(rsp->gp_activity) + HZ) ||
             atomic_read(&warned)) {
@@ -2723,7 +2725,8 @@ rcu_check_gp_start_stall(struct rcu_state *rsp, struct rcu_node *rnp,
         if (rnp_root != rnp)
                 raw_spin_lock_rcu_node(rnp_root); /* irqs already disabled. */
         j = jiffies;
-        if (rcu_gp_in_progress(rsp) || !need_any_future_gp(rcu_get_root(rsp)) ||
+        if (rcu_gp_in_progress(rsp) ||
+            ULONG_CMP_GE(rnp_root->gp_seq, rnp_root->gp_seq_needed) ||
             time_before(j, rsp->gp_req_activity + HZ) ||
             time_before(j, rsp->gp_activity + HZ) ||
             atomic_xchg(&warned, 1)) {
@@ -2731,12 +2734,9 @@ rcu_check_gp_start_stall(struct rcu_state *rsp, struct rcu_node *rnp,
                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
                 return;
         }
-        pr_alert("%s: g%lu %d%d%d%d gar:%lu ga:%lu f%#x %s->state:%#lx\n",
-                 __func__, READ_ONCE(rsp->gpnum),
-                 need_future_gp_element(rcu_get_root(rsp), 0),
-                 need_future_gp_element(rcu_get_root(rsp), 1),
-                 need_future_gp_element(rcu_get_root(rsp), 2),
-                 need_future_gp_element(rcu_get_root(rsp), 3),
+        pr_alert("%s: g%ld->%ld gar:%lu ga:%lu f%#x %s->state:%#lx\n",
+                 __func__, (long)READ_ONCE(rsp->gp_seq),
+                 (long)READ_ONCE(rnp_root->gp_seq_needed),
                  j - rsp->gp_req_activity, j - rsp->gp_activity,
                  rsp->gp_flags, rsp->name,
                  rsp->gp_kthread ? rsp->gp_kthread->state : 0x1ffffL);
@@ -3527,6 +3527,7 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp)
         rdp->gpnum = rnp->completed; /* Make CPU later note any new GP. */
         rdp->completed = rnp->completed;
         rdp->gp_seq = rnp->gp_seq;
+        rdp->gp_seq_needed = rnp->gp_seq;
         rdp->cpu_no_qs.b.norm = true;
         rdp->rcu_qs_ctr_snap = per_cpu(rcu_dynticks.rcu_qs_ctr, cpu);
         rdp->core_needs_qs = false;
@@ -3907,6 +3908,7 @@ static void __init rcu_init_one(struct rcu_state *rsp)
                         rnp->gpnum = rsp->gpnum;
                         rnp->completed = rsp->completed;
                         rnp->gp_seq = rsp->gp_seq;
+                        rnp->gp_seq_needed = rsp->gp_seq;
                         rnp->completedqs = rsp->gp_seq;
                         rnp->qsmask = 0;
                         rnp->qsmaskinit = 0;
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index a21d403a6010..9329c1ff695f 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -88,6 +88,7 @@ struct rcu_node {
                                 /*  This will either be equal to or one */
                                 /*  behind the root rcu_node's gpnum. */
         unsigned long gp_seq;   /* Track rsp->rcu_gp_seq. */
+        unsigned long gp_seq_needed; /* Track rsp->rcu_gp_seq_needed. */
         unsigned long completedqs; /* All QSes done for this node. */
         unsigned long qsmask;   /* CPUs or groups that need to switch in */
                                 /*  order for current grace period to proceed.*/
@@ -160,7 +161,6 @@ struct rcu_node {
         struct swait_queue_head nocb_gp_wq[2];
                                 /* Place for rcu_nocb_kthread() to wait GP. */
 #endif /* #ifdef CONFIG_RCU_NOCB_CPU */
-        u8 need_future_gp[4];   /* Counts of upcoming GP requests. */
         raw_spinlock_t fqslock ____cacheline_internodealigned_in_smp;
 
         spinlock_t exp_lock ____cacheline_internodealigned_in_smp;
@@ -170,22 +170,6 @@ struct rcu_node {
         bool exp_need_flush;    /* Need to flush workitem? */
 } ____cacheline_internodealigned_in_smp;
 
-/* Accessors for ->need_future_gp[] array. */
-#define need_future_gp_mask() \
-        (ARRAY_SIZE(((struct rcu_node *)NULL)->need_future_gp) - 1)
-#define need_future_gp_element(rnp, c) \
-        ((rnp)->need_future_gp[(c >> RCU_SEQ_CTR_SHIFT) & need_future_gp_mask()])
-#define need_any_future_gp(rnp)                                         \
-({                                                                      \
-        int __i;                                                        \
-        bool __nonzero = false;                                         \
-                                                                        \
-        for (__i = 0; __i < ARRAY_SIZE((rnp)->need_future_gp); __i++)   \
-                __nonzero = __nonzero ||                                \
-                            READ_ONCE((rnp)->need_future_gp[__i]);      \
-        __nonzero;                                                      \
-})
-
 /*
  * Bitmasks in an rcu_node cover the interval [grplo, grphi] of CPU IDs, and
  * are indexed relative to this interval rather than the global CPU ID space.
@@ -213,6 +197,7 @@ struct rcu_data {
         unsigned long   gpnum;          /* Highest gp number that this CPU */
                                         /*  is aware of having started. */
         unsigned long   gp_seq;         /* Track rsp->rcu_gp_seq counter. */
+        unsigned long   gp_seq_needed;  /* Track rsp->rcu_gp_seq_needed ctr. */
         unsigned long   rcu_qs_ctr_snap;/* Snapshot of rcu_qs_ctr to check */
                                         /*  for rcu_all_qs() invocations. */
         union rcu_noqs  cpu_no_qs;      /* No QSes yet for this CPU. */