rcu: Remove restrictions on no-CBs CPUs

Currently, CPU 0 is constrained to not be a no-CBs CPU, and furthermore
at least one no-CBs CPU must remain online at any given time.  These
restrictions are problematic in some situations, such as cases where
all CPUs must run a real-time workload that needs to be insulated from
OS jitter and latencies due to RCU callback invocation.  This commit
therefore provides no-CBs CPUs a (very crude and energy-inefficient)
way to start and to wait for grace periods independently of the normal
RCU callback mechanisms.  This approach allows any or all of the CPUs to
be designated as no-CBs CPUs, and allows any proper subset of the CPUs
(whether no-CBs CPUs or not) to be offlined.

This commit also provides a fix for a locking bug spotted by Xie
ChanglongX <changlongx.xie@intel.com>.

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

1 files changed, 3 insertions, 9 deletions

diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index c896b5045d9d..7af39f4aaac4 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -326,6 +326,7 @@ struct rcu_data {
         int nocb_p_count_lazy;          /*  (approximate). */
         wait_queue_head_t nocb_wq;      /* For nocb kthreads to sleep on. */
         struct task_struct *nocb_kthread;
+        bool nocb_needs_gp;
 #endif /* #ifdef CONFIG_RCU_NOCB_CPU */
 
         int cpu;
@@ -375,12 +376,6 @@ struct rcu_state {
         struct rcu_data __percpu *rda;          /* pointer of percu rcu_data. */
         void (*call)(struct rcu_head *head,     /* call_rcu() flavor. */
                      void (*func)(struct rcu_head *head));
-#ifdef CONFIG_RCU_NOCB_CPU
-        void (*call_remote)(struct rcu_head *head,
-                     void (*func)(struct rcu_head *head));
-                                                /* call_rcu() flavor, but for */
-                                                /*  placing on remote CPU. */
-#endif /* #ifdef CONFIG_RCU_NOCB_CPU */
 
         /* The following fields are guarded by the root rcu_node's lock. */
 
@@ -529,16 +524,15 @@ static void print_cpu_stall_info(struct rcu_state *rsp, int cpu);
 static void print_cpu_stall_info_end(void);
 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_data *rdp);
 static bool is_nocb_cpu(int cpu);
 static bool __call_rcu_nocb(struct rcu_data *rdp, struct rcu_head *rhp,
                             bool lazy);
 static bool rcu_nocb_adopt_orphan_cbs(struct rcu_state *rsp,
                                       struct rcu_data *rdp);
-static bool nocb_cpu_expendable(int cpu);
 static void rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp);
 static void rcu_spawn_nocb_kthreads(struct rcu_state *rsp);
-static void init_nocb_callback_list(struct rcu_data *rdp);
-static void __init rcu_init_nocb(void);
+static bool init_nocb_callback_list(struct rcu_data *rdp);
 
 #endif /* #ifndef RCU_TREE_NONCORE */