rcu: Eliminate flavor scan in rcu_momentary_dyntick_idle()

The rcu_momentary_dyntick_idle() function scans the RCU flavors, checking
that one of them still needs a quiescent state before doing an expensive
atomic operation on the ->dynticks counter.  However, this check reduces
overhead only after a rare race condition, and increases complexity.  This
commit therefore removes the scan and the mechanism enabling the scan.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Documentation/RCU/Design/Data-Structures/Data-Structures.html

@@ -1104,7 +1104,7 @@ Its fields are as follows:
   1   int dynticks_nesting;
   2   int dynticks_nmi_nesting;
   3   atomic_t dynticks;
-  4   int rcu_sched_qs_mask;
+  4   bool rcu_need_heavy_qs;
   5   unsigned long rcu_qs_ctr;
 </pre>
 
@@ -1124,7 +1124,7 @@ CPU's transitions to and from dyntick-idle mode, so that this counter
 has an even value when the CPU is in dyntick-idle mode and an odd
 value otherwise.
 
-</p><p>The <tt>-&gt;rcu_sched_qs_mask</tt> field is used
+</p><p>The <tt>-&gt;rcu_need_heavy_qs</tt> field is used
 to record the fact that the RCU core code would really like to
 see a quiescent state from the corresponding CPU, so much so that
 it is willing to call for heavy-weight dyntick-counter operations.

kernel/rcu/tree.c

@@ -443,44 +443,14 @@ bool rcu_eqs_special_set(int cpu)
  * 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.
+ * We inform the RCU core by emulating a zero-duration dyntick-idle period.
  *
  * The caller must have disabled interrupts.
  */
 static void rcu_momentary_dyntick_idle(void)
 {
-	struct rcu_data *rdp;
-	int resched_mask;
-	struct rcu_state *rsp;
-
-	/*
-	 * 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_dynticks.rcu_sched_qs_mask);
-	raw_cpu_write(rcu_dynticks.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 (READ_ONCE(rdp->mynode->completed) !=
-		    READ_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.
-		 */
-		rcu_dynticks_momentary_idle();
-		break;
-	}
+	raw_cpu_write(rcu_dynticks.rcu_need_heavy_qs, false);
+	rcu_dynticks_momentary_idle();
 }
 
 /*
@@ -494,7 +464,7 @@ void rcu_note_context_switch(void)
 	trace_rcu_utilization(TPS("Start context switch"));
 	rcu_sched_qs();
 	rcu_preempt_note_context_switch();
-	if (unlikely(raw_cpu_read(rcu_dynticks.rcu_sched_qs_mask)))
+	if (unlikely(raw_cpu_read(rcu_dynticks.rcu_need_heavy_qs)))
 		rcu_momentary_dyntick_idle();
 	trace_rcu_utilization(TPS("End context switch"));
 	barrier(); /* Avoid RCU read-side critical sections leaking up. */
@@ -519,7 +489,7 @@ void rcu_all_qs(void)
 	unsigned long flags;
 
 	barrier(); /* Avoid RCU read-side critical sections leaking down. */
-	if (unlikely(raw_cpu_read(rcu_dynticks.rcu_sched_qs_mask))) {
+	if (unlikely(raw_cpu_read(rcu_dynticks.rcu_need_heavy_qs))) {
 		local_irq_save(flags);
 		rcu_momentary_dyntick_idle();
 		local_irq_restore(flags);
@@ -1275,7 +1245,7 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp,
 				    bool *isidle, unsigned long *maxj)
 {
 	unsigned long jtsq;
-	int *rcrmp;
+	bool *rnhqp;
 	unsigned long rjtsc;
 	struct rcu_node *rnp;
 
@@ -1332,7 +1302,7 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp,
 	 * 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
+	 * rcu_need_heavy_qs 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
@@ -1346,16 +1316,11 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp,
 	 * is set too high, we override with half of the RCU CPU stall
 	 * warning delay.
 	 */
-	rcrmp = &per_cpu(rcu_dynticks.rcu_sched_qs_mask, rdp->cpu);
-	if (time_after(jiffies, rdp->rsp->gp_start + jtsq) ||
-	    time_after(jiffies, rdp->rsp->jiffies_resched)) {
-		if (!(READ_ONCE(*rcrmp) & rdp->rsp->flavor_mask)) {
-			WRITE_ONCE(rdp->cond_resched_completed,
-				   READ_ONCE(rdp->mynode->completed));
-			smp_mb(); /* ->cond_resched_completed before *rcrmp. */
-			WRITE_ONCE(*rcrmp,
-				   READ_ONCE(*rcrmp) + rdp->rsp->flavor_mask);
-		}
+	rnhqp = &per_cpu(rcu_dynticks.rcu_need_heavy_qs, rdp->cpu);
+	if (!READ_ONCE(*rnhqp) &&
+	    (time_after(jiffies, rdp->rsp->gp_start + jtsq) ||
+	     time_after(jiffies, rdp->rsp->jiffies_resched))) {
+		WRITE_ONCE(*rnhqp, true);
 		rdp->rsp->jiffies_resched += 5; /* Re-enable beating. */
 	}
 
@@ -4169,7 +4134,6 @@ static void __init rcu_init_one(struct rcu_state *rsp)
 	static const char * const fqs[] = RCU_FQS_NAME_INIT;
 	static struct lock_class_key rcu_node_class[RCU_NUM_LVLS];
 	static struct lock_class_key rcu_fqs_class[RCU_NUM_LVLS];
-	static u8 fl_mask = 0x1;
 
 	int levelcnt[RCU_NUM_LVLS];		/* # nodes in each level. */
 	int levelspread[RCU_NUM_LVLS];		/* kids/node in each level. */
@@ -4191,8 +4155,6 @@ 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] + levelcnt[i - 1];
 	rcu_init_levelspread(levelspread, levelcnt);
-	rsp->flavor_mask = fl_mask;
-	fl_mask <<= 1;
 
 	/* Initialize the elements themselves, starting from the leaves. */
 

kernel/rcu/tree.h

@@ -113,7 +113,7 @@ 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 rcu_sched_qs_mask;      /* GP old, need heavy quiescent state. */
+	bool rcu_need_heavy_qs;      /* GP old, need heavy quiescent state. */
 	unsigned long rcu_qs_ctr;   /* Light universal quiescent state ctr. */
 #ifdef CONFIG_NO_HZ_FULL_SYSIDLE
 	long long dynticks_idle_nesting;
@@ -484,7 +484,6 @@ struct rcu_state {
 	struct rcu_node *level[RCU_NUM_LVLS + 1];
 						/* Hierarchy levels (+1 to */
 						/*  shut bogus gcc warning) */
-	u8 flavor_mask;				/* bit in flavor mask. */
 	struct rcu_data __percpu *rda;		/* pointer of percu rcu_data. */
 	call_rcu_func_t call;			/* call_rcu() flavor. */
 	int ncpus;				/* # CPUs seen so far. */