rcu: Eliminate RCU_FAST_NO_HZ grace-period hang

With the new implementation of RCU_FAST_NO_HZ, it was possible to hang
RCU grace periods as follows:

o	CPU 0 attempts to go idle, cycles several times through the
	rcu_prepare_for_idle() loop, then goes dyntick-idle when
	RCU needs nothing more from it, while still having at least
	on RCU callback pending.

o	CPU 1 goes idle with no callbacks.

Both CPUs can then stay in dyntick-idle mode indefinitely, preventing
the RCU grace period from ever completing, possibly hanging the system.

This commit therefore prevents CPUs that have RCU callbacks from entering
dyntick-idle mode.  This approach also eliminates the need for the
end-of-grace-period IPIs used previously.

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

4 files changed, 2 insertions, 82 deletions

diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
index debe453c9623..8dd6fcb94946 100644
--- a/include/trace/events/rcu.h
+++ b/include/trace/events/rcu.h
@@ -287,7 +287,6 @@ TRACE_EVENT(rcu_dyntick,
  *
  *      "No callbacks": Nothing to do, no callbacks on this CPU.
  *      "In holdoff": Nothing to do, holding off after unsuccessful attempt.
- *      "Dyntick with callbacks": Callbacks remain, but RCU doesn't need CPU.
  *      "Begin holdoff": Attempt failed, don't retry until next jiffy.
  *      "More callbacks": Still more callbacks, try again to clear them out.
  *      "Callbacks drained": All callbacks processed, off to dyntick idle!
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 7fb8b0e60811..13fab4a9f9fb 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1086,7 +1086,6 @@ static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags)
          * callbacks are waiting on the grace period that just now
          * completed.
          */
-        rcu_schedule_wake_gp_end();
         if (*rdp->nxttail[RCU_WAIT_TAIL] == NULL) {
                 raw_spin_unlock(&rnp->lock);     /* irqs remain disabled. */
 
@@ -1672,7 +1671,6 @@ static void rcu_process_callbacks(struct softirq_action *unused)
                                 &__get_cpu_var(rcu_sched_data));
         __rcu_process_callbacks(&rcu_bh_state, &__get_cpu_var(rcu_bh_data));
         rcu_preempt_process_callbacks();
-        rcu_wake_cpus_for_gp_end();
         trace_rcu_utilization("End RCU core");
 }
 
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index ea32405177c9..70d8a557090f 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -88,7 +88,6 @@ struct rcu_dynticks {
                                     /* Process level is worth LLONG_MAX/2. */
         int dynticks_nmi_nesting;   /* Track NMI nesting level. */
         atomic_t dynticks;          /* Even value for idle, else odd. */
-        int wake_gp_end;            /* A GP ended, need to wake up CPUs. */
 };
 
 /* RCU's kthread states for tracing. */
@@ -469,7 +468,5 @@ static void rcu_yield(void (*f)(unsigned long), unsigned long arg);
 static void rcu_cpu_kthread_setrt(int cpu, int to_rt);
 static void __cpuinit rcu_prepare_kthreads(int cpu);
 static void rcu_prepare_for_idle(int cpu);
-static void rcu_wake_cpus_for_gp_end(void);
-static void rcu_schedule_wake_gp_end(void);
 
 #endif /* #ifndef RCU_TREE_NONCORE */
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index c4daf1e19e01..3d84dbc113d6 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -1964,28 +1964,11 @@ static void rcu_prepare_for_idle(int cpu)
 {
 }
 
-/*
- * CPUs are never putting themselves to sleep with callbacks pending,
- * so there is no need to awaken them.
- */
-static void rcu_wake_cpus_for_gp_end(void)
-{
-}
-
-/*
- * CPUs are never putting themselves to sleep with callbacks pending,
- * so there is no need to schedule the act of awakening them.
- */
-static void rcu_schedule_wake_gp_end(void)
-{
-}
-
 #else /* #if !defined(CONFIG_RCU_FAST_NO_HZ) */
 
 #define RCU_NEEDS_CPU_FLUSHES 5
 static DEFINE_PER_CPU(int, rcu_dyntick_drain);
 static DEFINE_PER_CPU(unsigned long, rcu_dyntick_holdoff);
-static DEFINE_PER_CPU(bool, rcu_awake_at_gp_end);
 
 /*
  * Allow the CPU to enter dyntick-idle mode if either: (1) There are no
@@ -2032,26 +2015,16 @@ static void rcu_prepare_for_idle(int cpu)
         local_irq_save(flags);
 
         /*
-         * If there are no callbacks on this CPU or if RCU has no further
-         * need for this CPU at the moment, enter dyntick-idle mode.
-         * Also reset state so as to not prejudice later attempts.
+         * If there are no callbacks on this CPU, enter dyntick-idle mode.
+         * Also reset state to avoid prejudicing later attempts.
          */
         if (!rcu_cpu_has_callbacks(cpu)) {
                 per_cpu(rcu_dyntick_holdoff, cpu) = jiffies - 1;
                 per_cpu(rcu_dyntick_drain, cpu) = 0;
-                per_cpu(rcu_awake_at_gp_end, cpu) = 0;
                 local_irq_restore(flags);
                 trace_rcu_prep_idle("No callbacks");
                 return;
         }
-        if (!rcu_pending(cpu)) {
-                per_cpu(rcu_dyntick_holdoff, cpu) = jiffies - 1;
-                per_cpu(rcu_dyntick_drain, cpu) = 0;
-                per_cpu(rcu_awake_at_gp_end, cpu) = 1;
-                local_irq_restore(flags);
-                trace_rcu_prep_idle("Dyntick with callbacks");
-                return;  /* Nothing to do immediately. */
-        }
 
         /*
          * If in holdoff mode, just return.  We will presumably have
@@ -2067,7 +2040,6 @@ static void rcu_prepare_for_idle(int cpu)
         if (per_cpu(rcu_dyntick_drain, cpu) <= 0) {
                 /* First time through, initialize the counter. */
                 per_cpu(rcu_dyntick_drain, cpu) = RCU_NEEDS_CPU_FLUSHES;
-                per_cpu(rcu_awake_at_gp_end, cpu) = 0;
         } else if (--per_cpu(rcu_dyntick_drain, cpu) <= 0) {
                 /* We have hit the limit, so time to give up. */
                 per_cpu(rcu_dyntick_holdoff, cpu) = jiffies;
@@ -2113,50 +2085,4 @@ static void rcu_prepare_for_idle(int cpu)
         }
 }
 
-/*
- * Wake up a CPU by invoking the RCU core.  Intended for use by
- * rcu_wake_cpus_for_gp_end(), which passes this function to
- * smp_call_function_single().
- */
-static void rcu_wake_cpu(void *unused)
-{
-        trace_rcu_prep_idle("CPU awakened at GP end");
-        invoke_rcu_core();
-}
-
-/*
- * If an RCU grace period ended recently, scan the rcu_awake_at_gp_end
- * per-CPU variables, and wake up any CPUs that requested a wakeup.
- */
-static void rcu_wake_cpus_for_gp_end(void)
-{
-        int cpu;
-        struct rcu_dynticks *rdtp = &__get_cpu_var(rcu_dynticks);
-
-        if (!rdtp->wake_gp_end)
-                return;
-        rdtp->wake_gp_end = 0;
-        for_each_online_cpu(cpu) {
-                if (per_cpu(rcu_awake_at_gp_end, cpu)) {
-                        per_cpu(rcu_awake_at_gp_end, cpu) = 0;
-                        smp_call_function_single(cpu, rcu_wake_cpu, NULL, 0);
-                }
-        }
-}
-
-/*
- * A grace period has just ended, and so we will need to awaken CPUs
- * that now have work to do.  But we cannot send IPIs with interrupts
- * disabled, so just set a flag so that this will happen upon exit
- * from RCU core processing.
- */
-static void rcu_schedule_wake_gp_end(void)
-{
-        struct rcu_dynticks *rdtp = &__get_cpu_var(rcu_dynticks);
-
-        rdtp->wake_gp_end = 1;
-}
-
-/* @@@ need tracing as well. */
-
 #endif /* #else #if !defined(CONFIG_RCU_FAST_NO_HZ) */