srcu: Exact tracking of srcu_data structures containing callbacks

The current Tree SRCU implementation schedules a workqueue for every
srcu_data covered by a given leaf srcu_node structure having callbacks,
even if only one of those srcu_data structures actually contains
callbacks.  This is clearly inefficient for workloads that don't feature
callbacks everywhere all the time.  This commit therefore adds an array
of masks that are used by the leaf srcu_node structures to track exactly
which srcu_data structures contain callbacks.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Mike Galbraith <efault@gmx.de>

1 files changed, 23 insertions, 6 deletions

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 9ecf0acc18eb..1c2c1004b3b1 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -66,8 +66,12 @@ static void init_srcu_struct_nodes(struct srcu_struct *sp, bool is_static)
 	/* Each pass through this loop initializes one srcu_node structure. */
 	rcu_for_each_node_breadth_first(sp, snp) {
 		spin_lock_init(&snp->lock);
-		for (i = 0; i < ARRAY_SIZE(snp->srcu_have_cbs); i++)
+		WARN_ON_ONCE(ARRAY_SIZE(snp->srcu_have_cbs) !=
+			     ARRAY_SIZE(snp->srcu_data_have_cbs));
+		for (i = 0; i < ARRAY_SIZE(snp->srcu_have_cbs); i++) {
 			snp->srcu_have_cbs[i] = 0;
+			snp->srcu_data_have_cbs[i] = 0;
+		}
 		snp->grplo = -1;
 		snp->grphi = -1;
 		if (snp == &sp->node[0]) {
@@ -107,6 +111,7 @@ static void init_srcu_struct_nodes(struct srcu_struct *sp, bool is_static)
 		sdp->cpu = cpu;
 		INIT_DELAYED_WORK(&sdp->work, srcu_invoke_callbacks);
 		sdp->sp = sp;
+		sdp->grpmask = 1 << (cpu - sdp->mynode->grplo);
 		if (is_static)
 			continue;
 
@@ -434,16 +439,21 @@ static void srcu_schedule_cbs_sdp(struct srcu_data *sdp, unsigned long delay)
 
 /*
  * Schedule callback invocation for all srcu_data structures associated
- * with the specified srcu_node structure, if possible, on the corresponding
- * CPUs.
+ * with the specified srcu_node structure that have callbacks for the
+ * just-completed grace period, the one corresponding to idx.  If possible,
+ * schedule this invocation on the corresponding CPUs.
  */
-static void srcu_schedule_cbs_snp(struct srcu_struct *sp, struct srcu_node *snp)
+static void srcu_schedule_cbs_snp(struct srcu_struct *sp, struct srcu_node *snp,
+				  unsigned long mask)
 {
 	int cpu;
 
-	for (cpu = snp->grplo; cpu <= snp->grphi; cpu++)
+	for (cpu = snp->grplo; cpu <= snp->grphi; cpu++) {
+		if (!(mask & (1 << (cpu - snp->grplo))))
+			continue;
 		srcu_schedule_cbs_sdp(per_cpu_ptr(sp->sda, cpu),
 				      atomic_read(&sp->srcu_exp_cnt) ? 0 : SRCU_INTERVAL);
+	}
 }
 
 /*
@@ -461,6 +471,7 @@ static void srcu_gp_end(struct srcu_struct *sp)
 	unsigned long gpseq;
 	int idx;
 	int idxnext;
+	unsigned long mask;
 	struct srcu_node *snp;
 
 	/* Prevent more than one additional grace period. */
@@ -486,10 +497,12 @@ static void srcu_gp_end(struct srcu_struct *sp)
 			cbs = snp->srcu_have_cbs[idx] == gpseq;
 		snp->srcu_have_cbs[idx] = gpseq;
 		rcu_seq_set_state(&snp->srcu_have_cbs[idx], 1);
+		mask = snp->srcu_data_have_cbs[idx];
+		snp->srcu_data_have_cbs[idx] = 0;
 		spin_unlock_irq(&snp->lock);
 		if (cbs) {
 			smp_mb(); /* GP end before CB invocation. */
-			srcu_schedule_cbs_snp(sp, snp);
+			srcu_schedule_cbs_snp(sp, snp, mask);
 		}
 	}
 
@@ -536,6 +549,8 @@ static void srcu_funnel_gp_start(struct srcu_struct *sp,
 		spin_lock_irqsave(&snp->lock, flags);
 		if (ULONG_CMP_GE(snp->srcu_have_cbs[idx], s)) {
 			snp_seq = snp->srcu_have_cbs[idx];
+			if (snp == sdp->mynode && snp_seq == s)
+				snp->srcu_data_have_cbs[idx] |= sdp->grpmask;
 			spin_unlock_irqrestore(&snp->lock, flags);
 			if (snp == sdp->mynode && snp_seq != s) {
 				smp_mb(); /* CBs after GP! */
@@ -544,6 +559,8 @@ static void srcu_funnel_gp_start(struct srcu_struct *sp,
 			return;
 		}
 		snp->srcu_have_cbs[idx] = s;
+		if (snp == sdp->mynode)
+			snp->srcu_data_have_cbs[idx] |= sdp->grpmask;
 		spin_unlock_irqrestore(&snp->lock, flags);
 	}