rcu/nocb: Leave ->cblist enabled for no-CBs CPUs

As a first step towards making no-CBs CPUs use the ->cblist, this commit
leaves the ->cblist enabled for these CPUs.  The main reason to make
no-CBs CPUs use ->cblist is to take advantage of callback numbering,
which will reduce the effects of missed grace periods which in turn will
reduce forward-progress problems for no-CBs CPUs.

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

kernel/rcu/rcu_segcblist.c

@@ -79,9 +79,6 @@ void rcu_segcblist_disable(struct rcu_segcblist *rsclp)
  */
 void rcu_segcblist_offload(struct rcu_segcblist *rsclp)
 {
-	WARN_ON_ONCE(!rcu_segcblist_empty(rsclp));
-	WARN_ON_ONCE(rcu_segcblist_n_cbs(rsclp));
-	WARN_ON_ONCE(rcu_segcblist_n_lazy_cbs(rsclp));
 	rsclp->offloaded = 1;
 }
 

kernel/rcu/rcu_segcblist.h

@@ -59,7 +59,7 @@ static inline long rcu_segcblist_n_nonlazy_cbs(struct rcu_segcblist *rsclp)
 
 /*
  * Is the specified rcu_segcblist enabled, for example, not corresponding
- * to an offline or callback-offloaded CPU?
+ * to an offline CPU?
  */
 static inline bool rcu_segcblist_is_enabled(struct rcu_segcblist *rsclp)
 {

kernel/rcu/tree.c

@@ -2964,7 +2964,8 @@ rcu_boot_init_percpu_data(int cpu)
  * Initializes a CPU's per-CPU RCU data.  Note that only one online or
  * offline event can be happening at a given time.  Note also that we can
  * accept some slop in the rsp->gp_seq access due to the fact that this
- * CPU cannot possibly have any RCU callbacks in flight yet.
+ * CPU cannot possibly have any non-offloaded RCU callbacks in flight yet.
+ * And any offloaded callbacks are being numbered elsewhere.
  */
 int rcutree_prepare_cpu(unsigned int cpu)
 {
@@ -2978,7 +2979,7 @@ int rcutree_prepare_cpu(unsigned int cpu)
 	rdp->n_force_qs_snap = rcu_state.n_force_qs;
 	rdp->blimit = blimit;
 	if (rcu_segcblist_empty(&rdp->cblist) && /* No early-boot CBs? */
-	    !init_nocb_callback_list(rdp))
+	    !rcu_segcblist_is_offloaded(&rdp->cblist))
 		rcu_segcblist_init(&rdp->cblist);  /* Re-enable callbacks. */
 	rdp->dynticks_nesting = 1;	/* CPU not up, no tearing. */
 	rcu_dynticks_eqs_online();

kernel/rcu/tree.h

@@ -438,7 +438,6 @@ static void __init rcu_spawn_nocb_kthreads(void);
 #ifdef CONFIG_RCU_NOCB_CPU
 static void __init rcu_organize_nocb_kthreads(void);
 #endif /* #ifdef CONFIG_RCU_NOCB_CPU */
-static bool init_nocb_callback_list(struct rcu_data *rdp);
 static unsigned long rcu_get_n_cbs_nocb_cpu(struct rcu_data *rdp);
 static void rcu_bind_gp_kthread(void);
 static bool rcu_nohz_full_cpu(void);

kernel/rcu/tree_plugin.h

@@ -2007,6 +2007,7 @@ void __init rcu_init_nohz(void)
 {
 	int cpu;
 	bool need_rcu_nocb_mask = false;
+	struct rcu_data *rdp;
 
 #if defined(CONFIG_NO_HZ_FULL)
 	if (tick_nohz_full_running && cpumask_weight(tick_nohz_full_mask))
@@ -2040,8 +2041,12 @@ void __init rcu_init_nohz(void)
 	if (rcu_nocb_poll)
 		pr_info("\tPoll for callbacks from no-CBs CPUs.\n");
 
-	for_each_cpu(cpu, rcu_nocb_mask)
+	for_each_cpu(cpu, rcu_nocb_mask) {
-		init_nocb_callback_list(per_cpu_ptr(&rcu_data, cpu));
+		rdp = per_cpu_ptr(&rcu_data, cpu);
+		if (rcu_segcblist_empty(&rdp->cblist))
+			rcu_segcblist_init(&rdp->cblist);
+		rcu_segcblist_offload(&rdp->cblist);
+	}
 	rcu_organize_nocb_kthreads();
 }
 
@@ -2167,27 +2172,6 @@ static void __init rcu_organize_nocb_kthreads(void)
 	}
 }
 
-/* Prevent __call_rcu() from enqueuing callbacks on no-CBs CPUs */
-static bool init_nocb_callback_list(struct rcu_data *rdp)
-{
-	if (!rcu_is_nocb_cpu(rdp->cpu))
-		return false;
-
-	/* If there are early-boot callbacks, move them to nocb lists. */
-	if (!rcu_segcblist_empty(&rdp->cblist)) {
-		rdp->nocb_head = rcu_segcblist_head(&rdp->cblist);
-		rdp->nocb_tail = rcu_segcblist_tail(&rdp->cblist);
-		atomic_long_set(&rdp->nocb_q_count,
-				rcu_segcblist_n_cbs(&rdp->cblist));
-		atomic_long_set(&rdp->nocb_q_count_lazy,
-				rcu_segcblist_n_lazy_cbs(&rdp->cblist));
-	}
-	rcu_segcblist_init(&rdp->cblist);
-	rcu_segcblist_disable(&rdp->cblist);
-	rcu_segcblist_offload(&rdp->cblist);
-	return true;
-}
-
 /*
  * Bind the current task to the offloaded CPUs.  If there are no offloaded
  * CPUs, leave the task unbound.  Splat if the bind attempt fails.
@@ -2263,11 +2247,6 @@ static void __init rcu_spawn_nocb_kthreads(void)
 {
 }
 
-static bool init_nocb_callback_list(struct rcu_data *rdp)
-{
-	return false;
-}
-
 static unsigned long rcu_get_n_cbs_nocb_cpu(struct rcu_data *rdp)
 {
 	return 0;