Merge branch 'urgent.2014.06.23a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu into core/urgent

Pull RCU fixes from Paul E. McKenney:

" This series includes the following:

   1.    Export a pair of debug-object interfaces for RCU that will
         allow the slab allocators to avoid a recursion bug located
         by Sasha Levin.  Strictly speaking, this is not a regression,
         but it would be good to enable the fix.

   2.    Address a serious performance regression on an open/close
         micro-benchmark located by Dave Hansen.  The offending commit
         is ac1bea85781e (Make cond_resched() report RCU quiescent states).  "

Signed-off-by: Ingo Molnar <mingo@kernel.org>

7 files changed, 137 insertions, 92 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 884904975d0b..7ffecb5fd004 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -2790,6 +2790,12 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
                         leaf rcu_node structure.  Useful for very large
                         systems.
 
+        rcutree.jiffies_till_sched_qs= [KNL]
+                        Set required age in jiffies for a
+                        given grace period before RCU starts
+                        soliciting quiescent-state help from
+                        rcu_note_context_switch().
+
         rcutree.jiffies_till_first_fqs= [KNL]
                         Set delay from grace-period initialization to
                         first attempt to force quiescent states.
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 5a75d19aa661..6a94cc8b1ca0 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -44,7 +44,6 @@
 #include <linux/debugobjects.h>
 #include <linux/bug.h>
 #include <linux/compiler.h>
-#include <linux/percpu.h>
 #include <asm/barrier.h>
 
 extern int rcu_expedited; /* for sysctl */
@@ -300,41 +299,6 @@ bool __rcu_is_watching(void);
 #endif /* #if defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_RCU_TRACE) || defined(CONFIG_SMP) */
 
 /*
- * Hooks for cond_resched() and friends to avoid RCU CPU stall warnings.
- */
-
-#define RCU_COND_RESCHED_LIM 256        /* ms vs. 100s of ms. */
-DECLARE_PER_CPU(int, rcu_cond_resched_count);
-void rcu_resched(void);
-
-/*
- * Is it time to report RCU quiescent states?
- *
- * Note unsynchronized access to rcu_cond_resched_count.  Yes, we might
- * increment some random CPU's count, and possibly also load the result from
- * yet another CPU's count.  We might even clobber some other CPU's attempt
- * to zero its counter.  This is all OK because the goal is not precision,
- * but rather reasonable amortization of rcu_note_context_switch() overhead
- * and extremely high probability of avoiding RCU CPU stall warnings.
- * Note that this function has to be preempted in just the wrong place,
- * many thousands of times in a row, for anything bad to happen.
- */
-static inline bool rcu_should_resched(void)
-{
-        return raw_cpu_inc_return(rcu_cond_resched_count) >=
-               RCU_COND_RESCHED_LIM;
-}
-
-/*
- * Report quiscent states to RCU if it is time to do so.
- */
-static inline void rcu_cond_resched(void)
-{
-        if (unlikely(rcu_should_resched()))
-                rcu_resched();
-}
-
-/*
  * Infrastructure to implement the synchronize_() primitives in
  * TREE_RCU and rcu_barrier_() primitives in TINY_RCU.
  */
@@ -358,9 +322,19 @@ void wait_rcu_gp(call_rcu_func_t crf);
  * initialization.
  */
 #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD
+void init_rcu_head(struct rcu_head *head);
+void destroy_rcu_head(struct rcu_head *head);
 void init_rcu_head_on_stack(struct rcu_head *head);
 void destroy_rcu_head_on_stack(struct rcu_head *head);
 #else /* !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
+static inline void init_rcu_head(struct rcu_head *head)
+{
+}
+
+static inline void destroy_rcu_head(struct rcu_head *head)
+{
+}
+
 static inline void init_rcu_head_on_stack(struct rcu_head *head)
 {
 }
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index f1ba77363fbb..625d0b0cd75a 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -206,6 +206,70 @@ void rcu_bh_qs(int cpu)
         rdp->passed_quiesce = 1;
 }
 
+static DEFINE_PER_CPU(int, rcu_sched_qs_mask);
+
+static DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {
+        .dynticks_nesting = DYNTICK_TASK_EXIT_IDLE,
+        .dynticks = ATOMIC_INIT(1),
+#ifdef CONFIG_NO_HZ_FULL_SYSIDLE
+        .dynticks_idle_nesting = DYNTICK_TASK_NEST_VALUE,
+        .dynticks_idle = ATOMIC_INIT(1),
+#endif /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
+};
+
+/*
+ * Let the RCU core know that this CPU has gone through the scheduler,
+ * which is a quiescent state.  This is called when the need for a
+ * quiescent state is urgent, so we burn an atomic operation and full
+ * memory barriers to let the RCU core know about it, regardless of what
+ * this CPU might (or might not) do in the near future.
+ *
+ * We inform the RCU core by emulating a zero-duration dyntick-idle
+ * period, which we in turn do by incrementing the ->dynticks counter
+ * by two.
+ */
+static void rcu_momentary_dyntick_idle(void)
+{
+        unsigned long flags;
+        struct rcu_data *rdp;
+        struct rcu_dynticks *rdtp;
+        int resched_mask;
+        struct rcu_state *rsp;
+
+        local_irq_save(flags);
+
+        /*
+         * Yes, we can lose flag-setting operations.  This is OK, because
+         * the flag will be set again after some delay.
+         */
+        resched_mask = raw_cpu_read(rcu_sched_qs_mask);
+        raw_cpu_write(rcu_sched_qs_mask, 0);
+
+        /* Find the flavor that needs a quiescent state. */
+        for_each_rcu_flavor(rsp) {
+                rdp = raw_cpu_ptr(rsp->rda);
+                if (!(resched_mask & rsp->flavor_mask))
+                        continue;
+                smp_mb(); /* rcu_sched_qs_mask before cond_resched_completed. */
+                if (ACCESS_ONCE(rdp->mynode->completed) !=
+                    ACCESS_ONCE(rdp->cond_resched_completed))
+                        continue;
+
+                /*
+                 * Pretend to be momentarily idle for the quiescent state.
+                 * This allows the grace-period kthread to record the
+                 * quiescent state, with no need for this CPU to do anything
+                 * further.
+                 */
+                rdtp = this_cpu_ptr(&rcu_dynticks);
+                smp_mb__before_atomic(); /* Earlier stuff before QS. */
+                atomic_add(2, &rdtp->dynticks);  /* QS. */
+                smp_mb__after_atomic(); /* Later stuff after QS. */
+                break;
+        }
+        local_irq_restore(flags);
+}
+
 /*
  * Note a context switch.  This is a quiescent state for RCU-sched,
  * and requires special handling for preemptible RCU.
@@ -216,19 +280,12 @@ void rcu_note_context_switch(int cpu)
         trace_rcu_utilization(TPS("Start context switch"));
         rcu_sched_qs(cpu);
         rcu_preempt_note_context_switch(cpu);
+        if (unlikely(raw_cpu_read(rcu_sched_qs_mask)))
+                rcu_momentary_dyntick_idle();
         trace_rcu_utilization(TPS("End context switch"));
 }
 EXPORT_SYMBOL_GPL(rcu_note_context_switch);
 
-static DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {
-        .dynticks_nesting = DYNTICK_TASK_EXIT_IDLE,
-        .dynticks = ATOMIC_INIT(1),
-#ifdef CONFIG_NO_HZ_FULL_SYSIDLE
-        .dynticks_idle_nesting = DYNTICK_TASK_NEST_VALUE,
-        .dynticks_idle = ATOMIC_INIT(1),
-#endif /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
-};
-
 static long blimit = 10;        /* Maximum callbacks per rcu_do_batch. */
 static long qhimark = 10000;    /* If this many pending, ignore blimit. */
 static long qlowmark = 100;     /* Once only this many pending, use blimit. */
@@ -243,6 +300,13 @@ static ulong jiffies_till_next_fqs = ULONG_MAX;
 module_param(jiffies_till_first_fqs, ulong, 0644);
 module_param(jiffies_till_next_fqs, ulong, 0644);
 
+/*
+ * How long the grace period must be before we start recruiting
+ * quiescent-state help from rcu_note_context_switch().
+ */
+static ulong jiffies_till_sched_qs = HZ / 20;
+module_param(jiffies_till_sched_qs, ulong, 0644);
+
 static bool rcu_start_gp_advanced(struct rcu_state *rsp, struct rcu_node *rnp,
                                   struct rcu_data *rdp);
 static void force_qs_rnp(struct rcu_state *rsp,
@@ -853,6 +917,7 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp,
                                     bool *isidle, unsigned long *maxj)
 {
         unsigned int curr;
+        int *rcrmp;
         unsigned int snap;
 
         curr = (unsigned int)atomic_add_return(0, &rdp->dynticks->dynticks);
@@ -893,27 +958,43 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp,
         }
 
         /*
-         * There is a possibility that a CPU in adaptive-ticks state
-         * might run in the kernel with the scheduling-clock tick disabled
-         * for an extended time period.  Invoke rcu_kick_nohz_cpu() to
-         * force the CPU to restart the scheduling-clock tick in this
-         * CPU is in this state.
-         */
-        rcu_kick_nohz_cpu(rdp->cpu);
-
-        /*
-         * Alternatively, the CPU might be running in the kernel
-         * for an extended period of time without a quiescent state.
-         * Attempt to force the CPU through the scheduler to gain the
-         * needed quiescent state, but only if the grace period has gone
-         * on for an uncommonly long time.  If there are many stuck CPUs,
-         * we will beat on the first one until it gets unstuck, then move
-         * to the next.  Only do this for the primary flavor of RCU.
+         * A CPU running for an extended time within the kernel can
+         * delay RCU grace periods.  When the CPU is in NO_HZ_FULL mode,
+         * even context-switching back and forth between a pair of
+         * in-kernel CPU-bound tasks cannot advance grace periods.
+         * So if the grace period is old enough, make the CPU pay attention.
+         * Note that the unsynchronized assignments to the per-CPU
+         * rcu_sched_qs_mask variable are safe.  Yes, setting of
+         * bits can be lost, but they will be set again on the next
+         * force-quiescent-state pass.  So lost bit sets do not result
+         * in incorrect behavior, merely in a grace period lasting
+         * a few jiffies longer than it might otherwise.  Because
+         * there are at most four threads involved, and because the
+         * updates are only once every few jiffies, the probability of
+         * lossage (and thus of slight grace-period extension) is
+         * quite low.
+         *
+         * Note that if the jiffies_till_sched_qs boot/sysfs parameter
+         * is set too high, we override with half of the RCU CPU stall
+         * warning delay.
          */
-        if (rdp->rsp == rcu_state_p &&
+        rcrmp = &per_cpu(rcu_sched_qs_mask, rdp->cpu);
+        if (ULONG_CMP_GE(jiffies,
+                         rdp->rsp->gp_start + jiffies_till_sched_qs) ||
             ULONG_CMP_GE(jiffies, rdp->rsp->jiffies_resched)) {
-                rdp->rsp->jiffies_resched += 5;
-                resched_cpu(rdp->cpu);
+                if (!(ACCESS_ONCE(*rcrmp) & rdp->rsp->flavor_mask)) {
+                        ACCESS_ONCE(rdp->cond_resched_completed) =
+                                ACCESS_ONCE(rdp->mynode->completed);
+                        smp_mb(); /* ->cond_resched_completed before *rcrmp. */
+                        ACCESS_ONCE(*rcrmp) =
+                                ACCESS_ONCE(*rcrmp) + rdp->rsp->flavor_mask;
+                        resched_cpu(rdp->cpu);  /* Force CPU into scheduler. */
+                        rdp->rsp->jiffies_resched += 5; /* Enable beating. */
+                } else if (ULONG_CMP_GE(jiffies, rdp->rsp->jiffies_resched)) {
+                        /* Time to beat on that CPU again! */
+                        resched_cpu(rdp->cpu);  /* Force CPU into scheduler. */
+                        rdp->rsp->jiffies_resched += 5; /* Re-enable beating. */
+                }
         }
 
         return 0;
@@ -3491,6 +3572,7 @@ static void __init rcu_init_one(struct rcu_state *rsp,
                                "rcu_node_fqs_1",
                                "rcu_node_fqs_2",
                                "rcu_node_fqs_3" };  /* Match MAX_RCU_LVLS */
+        static u8 fl_mask = 0x1;
         int cpustride = 1;
         int i;
         int j;
@@ -3509,6 +3591,8 @@ static void __init rcu_init_one(struct rcu_state *rsp,
         for (i = 1; i < rcu_num_lvls; i++)
                 rsp->level[i] = rsp->level[i - 1] + rsp->levelcnt[i - 1];
         rcu_init_levelspread(rsp);
+        rsp->flavor_mask = fl_mask;
+        fl_mask <<= 1;
 
         /* Initialize the elements themselves, starting from the leaves. */
 
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index bf2c1e669691..0f69a79c5b7d 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -307,6 +307,9 @@ struct rcu_data {
         /* 4) reasons this CPU needed to be kicked by force_quiescent_state */
         unsigned long dynticks_fqs;     /* Kicked due to dynticks idle. */
         unsigned long offline_fqs;      /* Kicked due to being offline. */
+        unsigned long cond_resched_completed;
+                                        /* Grace period that needs help */
+                                        /*  from cond_resched(). */
 
         /* 5) __rcu_pending() statistics. */
         unsigned long n_rcu_pending;    /* rcu_pending() calls since boot. */
@@ -392,6 +395,7 @@ struct rcu_state {
         struct rcu_node *level[RCU_NUM_LVLS];   /* Hierarchy levels. */
         u32 levelcnt[MAX_RCU_LVLS + 1];         /* # nodes in each level. */
         u8 levelspread[RCU_NUM_LVLS];           /* kids/node in each level. */
+        u8 flavor_mask;                         /* bit in flavor mask. */
         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));
@@ -563,7 +567,7 @@ static bool rcu_nocb_need_deferred_wakeup(struct rcu_data *rdp);
 static void do_nocb_deferred_wakeup(struct rcu_data *rdp);
 static void rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp);
 static void rcu_spawn_nocb_kthreads(struct rcu_state *rsp);
-static void rcu_kick_nohz_cpu(int cpu);
+static void __maybe_unused rcu_kick_nohz_cpu(int cpu);
 static bool init_nocb_callback_list(struct rcu_data *rdp);
 static void rcu_sysidle_enter(struct rcu_dynticks *rdtp, int irq);
 static void rcu_sysidle_exit(struct rcu_dynticks *rdtp, int irq);
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index cbc2c45265e2..02ac0fb186b8 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2404,7 +2404,7 @@ static bool init_nocb_callback_list(struct rcu_data *rdp)
  * if an adaptive-ticks CPU is failing to respond to the current grace
  * period and has not be idle from an RCU perspective, kick it.
  */
-static void rcu_kick_nohz_cpu(int cpu)
+static void __maybe_unused rcu_kick_nohz_cpu(int cpu)
 {
 #ifdef CONFIG_NO_HZ_FULL
         if (tick_nohz_full_cpu(cpu))
diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index a2aeb4df0f60..bc7883570530 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -200,12 +200,12 @@ void wait_rcu_gp(call_rcu_func_t crf)
 EXPORT_SYMBOL_GPL(wait_rcu_gp);
 
 #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD
-static inline void debug_init_rcu_head(struct rcu_head *head)
+void init_rcu_head(struct rcu_head *head)
 {
         debug_object_init(head, &rcuhead_debug_descr);
 }
 
-static inline void debug_rcu_head_free(struct rcu_head *head)
+void destroy_rcu_head(struct rcu_head *head)
 {
         debug_object_free(head, &rcuhead_debug_descr);
 }
@@ -350,21 +350,3 @@ static int __init check_cpu_stall_init(void)
 early_initcall(check_cpu_stall_init);
 
 #endif /* #ifdef CONFIG_RCU_STALL_COMMON */
-
-/*
- * Hooks for cond_resched() and friends to avoid RCU CPU stall warnings.
- */
-
-DEFINE_PER_CPU(int, rcu_cond_resched_count);
-
-/*
- * Report a set of RCU quiescent states, for use by cond_resched()
- * and friends.  Out of line due to being called infrequently.
- */
-void rcu_resched(void)
-{
-        preempt_disable();
-        __this_cpu_write(rcu_cond_resched_count, 0);
-        rcu_note_context_switch(smp_processor_id());
-        preempt_enable();
-}
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 3bdf01b494fe..bc1638b33449 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4147,7 +4147,6 @@ static void __cond_resched(void)
 
 int __sched _cond_resched(void)
 {
-        rcu_cond_resched();
         if (should_resched()) {
                 __cond_resched();
                 return 1;
@@ -4166,18 +4165,15 @@ EXPORT_SYMBOL(_cond_resched);
  */
 int __cond_resched_lock(spinlock_t *lock)
 {
-        bool need_rcu_resched = rcu_should_resched();
         int resched = should_resched();
         int ret = 0;
 
         lockdep_assert_held(lock);
 
-        if (spin_needbreak(lock) || resched || need_rcu_resched) {
+        if (spin_needbreak(lock) || resched) {
                 spin_unlock(lock);
                 if (resched)
                         __cond_resched();
-                else if (unlikely(need_rcu_resched))
-                        rcu_resched();
                 else
                         cpu_relax();
                 ret = 1;
@@ -4191,7 +4187,6 @@ int __sched __cond_resched_softirq(void)
 {
         BUG_ON(!in_softirq());
 
-        rcu_cond_resched();  /* BH disabled OK, just recording QSes. */
         if (should_resched()) {
                 local_bh_enable();
                 __cond_resched();