rcu: Abstract rcu_start_future_gp() from rcu_nocb_wait_gp()

CPUs going idle will need to record the need for a future grace
period, but won't actually need to block waiting on it.  This commit
therefore splits rcu_start_future_gp(), which does the recording, from
rcu_nocb_wait_gp(), which now invokes rcu_start_future_gp() to do the
recording, after which rcu_nocb_wait_gp() does the waiting.

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

3 files changed, 130 insertions, 99 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 0d532950baa3..f4b23f16677a 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -224,6 +224,7 @@ static ulong jiffies_till_next_fqs = RCU_JIFFIES_TILL_FORCE_QS;
 module_param(jiffies_till_first_fqs, ulong, 0644);
 module_param(jiffies_till_next_fqs, ulong, 0644);
 
+static void rcu_start_gp(struct rcu_state *rsp);
 static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *));
 static void force_quiescent_state(struct rcu_state *rsp);
 static int rcu_pending(int cpu);
@@ -1075,6 +1076,120 @@ static unsigned long rcu_cbs_completed(struct rcu_state *rsp,
 }
 
 /*
+ * Trace-event helper function for rcu_start_future_gp() and
+ * rcu_nocb_wait_gp().
+ */
+static void trace_rcu_future_gp(struct rcu_node *rnp, struct rcu_data *rdp,
+                                unsigned long c, char *s)
+{
+        trace_rcu_future_grace_period(rdp->rsp->name, rnp->gpnum,
+                                      rnp->completed, c, rnp->level,
+                                      rnp->grplo, rnp->grphi, s);
+}
+
+/*
+ * Start some future 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.
+ *
+ * The caller must hold the specified rcu_node structure's ->lock.
+ */
+static unsigned long __maybe_unused
+rcu_start_future_gp(struct rcu_node *rnp, struct rcu_data *rdp)
+{
+        unsigned long c;
+        int i;
+        struct rcu_node *rnp_root = rcu_get_root(rdp->rsp);
+
+        /*
+         * Pick up grace-period number for new callbacks.  If this
+         * grace period is already marked as needed, return to the caller.
+         */
+        c = rcu_cbs_completed(rdp->rsp, rnp);
+        trace_rcu_future_gp(rnp, rdp, c, "Startleaf");
+        if (rnp->need_future_gp[c & 0x1]) {
+                trace_rcu_future_gp(rnp, rdp, c, "Prestartleaf");
+                return c;
+        }
+
+        /*
+         * If either this rcu_node structure or the root rcu_node structure
+         * believe that a grace period is in progress, then we must wait
+         * for the one following, which is in "c".  Because our request
+         * will be noticed at the end of the current grace period, we don't
+         * need to explicitly start one.
+         */
+        if (rnp->gpnum != rnp->completed ||
+            ACCESS_ONCE(rnp->gpnum) != ACCESS_ONCE(rnp->completed)) {
+                rnp->need_future_gp[c & 0x1]++;
+                trace_rcu_future_gp(rnp, rdp, c, "Startedleaf");
+                return c;
+        }
+
+        /*
+         * There might be no grace period in progress.  If we don't already
+         * hold it, acquire the root rcu_node structure's lock in order to
+         * start one (if needed).
+         */
+        if (rnp != rnp_root)
+                raw_spin_lock(&rnp_root->lock);
+
+        /*
+         * Get a new grace-period number.  If there really is no grace
+         * period in progress, it will be smaller than the one we obtained
+         * earlier.  Adjust callbacks as needed.  Note that even no-CBs
+         * CPUs have a ->nxtcompleted[] array, so no no-CBs checks needed.
+         */
+        c = rcu_cbs_completed(rdp->rsp, rnp_root);
+        for (i = RCU_DONE_TAIL; i < RCU_NEXT_TAIL; i++)
+                if (ULONG_CMP_LT(c, rdp->nxtcompleted[i]))
+                        rdp->nxtcompleted[i] = c;
+
+        /*
+         * If the needed for the required grace period is already
+         * recorded, trace and leave.
+         */
+        if (rnp_root->need_future_gp[c & 0x1]) {
+                trace_rcu_future_gp(rnp, rdp, c, "Prestartedroot");
+                goto unlock_out;
+        }
+
+        /* Record the need for the future grace period. */
+        rnp_root->need_future_gp[c & 0x1]++;
+
+        /* If a grace period is not already in progress, start one. */
+        if (rnp_root->gpnum != rnp_root->completed) {
+                trace_rcu_future_gp(rnp, rdp, c, "Startedleafroot");
+        } else {
+                trace_rcu_future_gp(rnp, rdp, c, "Startedroot");
+                rcu_start_gp(rdp->rsp);
+        }
+unlock_out:
+        if (rnp != rnp_root)
+                raw_spin_unlock(&rnp_root->lock);
+        return c;
+}
+
+/*
+ * Clean up any old requests for the just-ended grace period.  Also return
+ * whether any additional grace periods have been requested.  Also invoke
+ * rcu_nocb_gp_cleanup() in order to wake up any no-callbacks kthreads
+ * waiting for this grace period to complete.
+ */
+static int rcu_future_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp)
+{
+        int c = rnp->completed;
+        int needmore;
+        struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
+
+        rcu_nocb_gp_cleanup(rsp, rnp);
+        rnp->need_future_gp[c & 0x1] = 0;
+        needmore = rnp->need_future_gp[(c + 1) & 0x1];
+        trace_rcu_future_gp(rnp, rdp, c, needmore ? "CleanupMore" : "Cleanup");
+        return needmore;
+}
+
+/*
  * If there is room, assign a ->completed number to any callbacks on
  * this CPU that have not already been assigned.  Also accelerate any
  * callbacks that were previously assigned a ->completed number that has
@@ -1312,9 +1427,9 @@ static int rcu_gp_init(struct rcu_state *rsp)
                 rdp = this_cpu_ptr(rsp->rda);
                 rcu_preempt_check_blocked_tasks(rnp);
                 rnp->qsmask = rnp->qsmaskinit;
-                rnp->gpnum = rsp->gpnum;
+                ACCESS_ONCE(rnp->gpnum) = rsp->gpnum;
                 WARN_ON_ONCE(rnp->completed != rsp->completed);
-                rnp->completed = rsp->completed;
+                ACCESS_ONCE(rnp->completed) = rsp->completed;
                 if (rnp == rdp->mynode)
                         rcu_start_gp_per_cpu(rsp, rnp, rdp);
                 rcu_preempt_boost_start_gp(rnp);
@@ -1395,11 +1510,11 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
          */
         rcu_for_each_node_breadth_first(rsp, rnp) {
                 raw_spin_lock_irq(&rnp->lock);
-                rnp->completed = rsp->gpnum;
+                ACCESS_ONCE(rnp->completed) = rsp->gpnum;
                 rdp = this_cpu_ptr(rsp->rda);
                 if (rnp == rdp->mynode)
                         __rcu_process_gp_end(rsp, rnp, rdp);
-                nocb += rcu_nocb_gp_cleanup(rsp, rnp);
+                nocb += rcu_future_gp_cleanup(rsp, rnp);
                 raw_spin_unlock_irq(&rnp->lock);
                 cond_resched();
         }
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 034b524594bd..c11b46c33159 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -526,7 +526,7 @@ static void zero_cpu_stall_ticks(struct rcu_data *rdp);
 static void increment_cpu_stall_ticks(void);
 static int rcu_nocb_needs_gp(struct rcu_state *rsp);
 static void rcu_nocb_gp_set(struct rcu_node *rnp, int nrq);
-static int rcu_nocb_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp);
+static void rcu_nocb_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp);
 static void rcu_init_one_nocb(struct rcu_node *rnp);
 static bool is_nocb_cpu(int cpu);
 static bool __call_rcu_nocb(struct rcu_data *rdp, struct rcu_head *rhp,
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index f3f0020b5b58..723af5f707f0 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -2022,22 +2022,12 @@ static int rcu_nocb_needs_gp(struct rcu_state *rsp)
 }
 
 /*
- * Clean up this rcu_node structure's no-CBs state at the end of
- * a grace period, and also return whether any no-CBs CPU associated
- * with this rcu_node structure needs another grace period.
+ * Wake up any no-CBs CPUs' kthreads that were waiting on the just-ended
+ * grace period.
  */
-static int rcu_nocb_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp)
+static void rcu_nocb_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp)
 {
-        int c = rnp->completed;
-        int needmore;
-
-        wake_up_all(&rnp->nocb_gp_wq[c & 0x1]);
-        rnp->need_future_gp[c & 0x1] = 0;
-        needmore = rnp->need_future_gp[(c + 1) & 0x1];
-        trace_rcu_future_grace_period(rsp->name, rnp->gpnum, rnp->completed,
-                                      c, rnp->level, rnp->grplo, rnp->grphi,
-                                      needmore ? "CleanupMore" : "Cleanup");
-        return needmore;
+        wake_up_all(&rnp->nocb_gp_wq[rnp->completed & 0x1]);
 }
 
 /*
@@ -2175,84 +2165,16 @@ static void rcu_nocb_wait_gp(struct rcu_data *rdp)
         bool d;
         unsigned long flags;
         struct rcu_node *rnp = rdp->mynode;
-        struct rcu_node *rnp_root = rcu_get_root(rdp->rsp);
 
         raw_spin_lock_irqsave(&rnp->lock, flags);
-        c = rnp->completed + 2;
-
-        /* Count our request for a grace period. */
-        rnp->need_future_gp[c & 0x1]++;
-        trace_rcu_future_grace_period(rdp->rsp->name, rnp->gpnum,
-                                      rnp->completed, c, rnp->level,
-                                      rnp->grplo, rnp->grphi, "Startleaf");
-
-        if (rnp->gpnum != rnp->completed) {
-
-                /*
-                 * This rcu_node structure believes that a grace period
-                 * is in progress, so we are done.  When this grace
-                 * period ends, our request will be acted upon.
-                 */
-                trace_rcu_future_grace_period(rdp->rsp->name, rnp->gpnum,
-                                              rnp->completed, c, rnp->level,
-                                              rnp->grplo, rnp->grphi,
-                                              "Startedleaf");
-                raw_spin_unlock_irqrestore(&rnp->lock, flags);
-
-        } else {
-
-                /*
-                 * Might not be a grace period, check root rcu_node
-                 * structure to see if we must start one.
-                 */
-                if (rnp != rnp_root)
-                        raw_spin_lock(&rnp_root->lock); /* irqs disabled. */
-                if (rnp_root->gpnum != rnp_root->completed) {
-                        trace_rcu_future_grace_period(rdp->rsp->name,
-                                                      rnp->gpnum,
-                                                      rnp->completed,
-                                                      c, rnp->level,
-                                                      rnp->grplo, rnp->grphi,
-                                                      "Startedleafroot");
-                        raw_spin_unlock(&rnp_root->lock); /* irqs disabled. */
-                } else {
-
-                        /*
-                         * No grace period, so we need to start one.
-                         * The good news is that we can wait for exactly
-                         * one grace period instead of part of the current
-                         * grace period and all of the next grace period.
-                         * Adjust counters accordingly and start the
-                         * needed grace period.
-                         */
-                        rnp->need_future_gp[c & 0x1]--;
-                        c = rnp_root->completed + 1;
-                        rnp->need_future_gp[c & 0x1]++;
-                        rnp_root->need_future_gp[c & 0x1]++;
-                        trace_rcu_future_grace_period(rdp->rsp->name,
-                                                      rnp->gpnum,
-                                                      rnp->completed,
-                                                      c, rnp->level,
-                                                      rnp->grplo, rnp->grphi,
-                                                      "Startedroot");
-                        rcu_start_gp(rdp->rsp);
-                        raw_spin_unlock(&rnp->lock);
-                }
-
-                /* Clean up locking and irq state. */
-                if (rnp != rnp_root)
-                        raw_spin_unlock_irqrestore(&rnp->lock, flags);
-                else
-                        local_irq_restore(flags);
-        }
+        c = rcu_start_future_gp(rnp, rdp);
+        raw_spin_unlock_irqrestore(&rnp->lock, flags);
 
         /*
          * Wait for the grace period.  Do so interruptibly to avoid messing
          * up the load average.
          */
-        trace_rcu_future_grace_period(rdp->rsp->name, rnp->gpnum,
-                                      rnp->completed, c, rnp->level,
-                                      rnp->grplo, rnp->grphi, "StartWait");
+        trace_rcu_future_gp(rnp, rdp, c, "StartWait");
         for (;;) {
                 wait_event_interruptible(
                         rnp->nocb_gp_wq[c & 0x1],
@@ -2260,14 +2182,9 @@ static void rcu_nocb_wait_gp(struct rcu_data *rdp)
                 if (likely(d))
                         break;
                 flush_signals(current);
-                trace_rcu_future_grace_period(rdp->rsp->name,
-                                              rnp->gpnum, rnp->completed, c,
-                                              rnp->level, rnp->grplo,
-                                              rnp->grphi, "ResumeWait");
+                trace_rcu_future_gp(rnp, rdp, c, "ResumeWait");
         }
-        trace_rcu_future_grace_period(rdp->rsp->name, rnp->gpnum,
-                                      rnp->completed, c, rnp->level,
-                                      rnp->grplo, rnp->grphi, "EndWait");
+        trace_rcu_future_gp(rnp, rdp, c, "EndWait");
         smp_mb(); /* Ensure that CB invocation happens after GP end. */
 }
 
@@ -2375,9 +2292,8 @@ static int rcu_nocb_needs_gp(struct rcu_state *rsp)
         return 0;
 }
 
-static int rcu_nocb_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp)
+static void rcu_nocb_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp)
 {
-        return 0;
 }
 
 static void rcu_nocb_gp_set(struct rcu_node *rnp, int nrq)