[PATCH 2/7] rcu/trace: Add tracing for how segcb list changes

["Joel Fernandes (Google)" <joel@joelfernandes.org>]

Track how the segcb list changes before/after acceleration, during
queuing and during dequeuing.

This has proved useful to discover an optimization to avoid unwanted GP
requests when there are no callbacks accelerated.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 include/trace/events/rcu.h | 25 +++++++++++++++++++++++++
 kernel/rcu/rcu_segcblist.c | 37 +++++++++++++++++++++++++++++++++++++
 kernel/rcu/rcu_segcblist.h |  7 +++++++
 kernel/rcu/tree.c          | 24 ++++++++++++++++++++++++
 4 files changed, 93 insertions(+)

diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
index 02dcd119f3263..a6d49864dcc27 100644
--- a/include/trace/events/rcu.h
+++ b/include/trace/events/rcu.h
@@ -507,6 +507,31 @@ TRACE_EVENT_RCU(rcu_callback,
 		  __entry->qlen)
 );
 
+TRACE_EVENT_RCU(rcu_segcb,
+
+		TP_PROTO(const char *ctx, int *cb_count, unsigned long *gp_seq),
+
+		TP_ARGS(ctx, cb_count, gp_seq),
+
+		TP_STRUCT__entry(
+			__field(const char *, ctx)
+			__array(int, cb_count, 4)
+			__array(unsigned long, gp_seq, 4)
+		),
+
+		TP_fast_assign(
+			__entry->ctx = ctx;
+			memcpy(__entry->cb_count, cb_count, 4 * sizeof(int));
+			memcpy(__entry->gp_seq, gp_seq, 4 * sizeof(unsigned long));
+		),
+
+		TP_printk("%s cb_count: (DONE=%d, WAIT=%d, NEXT_READY=%d, NEXT=%d) "
+			  "gp_seq: (DONE=%lu, WAIT=%lu, NEXT_READY=%lu, NEXT=%lu)", __entry->ctx,
+			  __entry->cb_count[0], __entry->cb_count[1], __entry->cb_count[2], __entry->cb_count[3],
+			  __entry->gp_seq[0], __entry->gp_seq[1], __entry->gp_seq[2], __entry->gp_seq[3])
+
+);
+
 /*
  * Tracepoint for the registration of a single RCU callback of the special
  * kfree() form.  The first argument is the RCU type, the second argument
diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
index 4782cf17bf4f9..036d4abac7c5a 100644
--- a/kernel/rcu/rcu_segcblist.c
+++ b/kernel/rcu/rcu_segcblist.c
@@ -316,6 +316,43 @@ void rcu_segcblist_extract_done_cbs(struct rcu_segcblist *rsclp,
 			WRITE_ONCE(rsclp->tails[i], &rsclp->head);
 }
 
+/*
+ * Return how many CBs each segment along with their gp_seq values.
+ *
+ * This function is O(N) where N is the number of callbacks. Only used from
+ * tracing code which is usually disabled in production.
+ */
+#ifdef CONFIG_RCU_TRACE
+void rcu_segcblist_countseq(struct rcu_segcblist *rsclp,
+			 int cbcount[RCU_CBLIST_NSEGS],
+			 unsigned long gpseq[RCU_CBLIST_NSEGS])
+{
+	struct rcu_head **cur_tail, *h;
+	int i, c;
+
+	for (i = 0; i < RCU_CBLIST_NSEGS; i++)
+		cbcount[i] = 0;
+
+	cur_tail = &(rsclp->head);
+
+	for (i = 0; i < RCU_CBLIST_NSEGS; i++) {
+		c = 0;
+		// List empty?
+		if (rsclp->tails[i] != cur_tail) {
+			// The loop skips the last node
+			c = 1;
+			for (h = *cur_tail; h->next != *(rsclp->tails[i]); h = h->next) {
+				c++;
+			}
+		}
+
+		cbcount[i] = c;
+		gpseq[i] = rsclp->gp_seq[i];
+		cur_tail = rsclp->tails[i];
+	}
+}
+#endif
+
 /*
  * Extract only those callbacks still pending (not yet ready to be
  * invoked) from the specified rcu_segcblist structure and place them in
diff --git a/kernel/rcu/rcu_segcblist.h b/kernel/rcu/rcu_segcblist.h
index 5c293afc07b8e..0a8dbac438529 100644
--- a/kernel/rcu/rcu_segcblist.h
+++ b/kernel/rcu/rcu_segcblist.h
@@ -104,3 +104,10 @@ void rcu_segcblist_advance(struct rcu_segcblist *rsclp, unsigned long seq);
 bool rcu_segcblist_accelerate(struct rcu_segcblist *rsclp, unsigned long seq);
 void rcu_segcblist_merge(struct rcu_segcblist *dst_rsclp,
 			 struct rcu_segcblist *src_rsclp);
+#ifdef CONFIG_RCU_TRACE
+void rcu_segcblist_countseq(struct rcu_segcblist *rsclp,
+			 int cbcount[RCU_CBLIST_NSEGS],
+			 unsigned long gpseq[RCU_CBLIST_NSEGS]);
+#else
+#define rcu_segcblist_countseq(...)
+#endif
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index ebce14e470d0e..c61af6a33fbfd 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1407,6 +1407,8 @@ static bool rcu_accelerate_cbs(struct rcu_node *rnp, struct rcu_data *rdp)
 {
 	unsigned long gp_seq_req;
 	bool ret = false;
+	int cbs[RCU_CBLIST_NSEGS];
+	unsigned long gps[RCU_CBLIST_NSEGS];
 
 	rcu_lockdep_assert_cblist_protected(rdp);
 	raw_lockdep_assert_held_rcu_node(rnp);
@@ -1415,6 +1417,10 @@ static bool rcu_accelerate_cbs(struct rcu_node *rnp, struct rcu_data *rdp)
 	if (!rcu_segcblist_pend_cbs(&rdp->cblist))
 		return false;
 
+	/* Count CBs for tracing. */
+	rcu_segcblist_countseq(&rdp->cblist, cbs, gps);
+	trace_rcu_segcb("SegCbPreAcc", cbs, gps);
+
 	/*
 	 * Callbacks are often registered with incomplete grace-period
 	 * information.  Something about the fact that getting exact
@@ -1434,6 +1440,11 @@ static bool rcu_accelerate_cbs(struct rcu_node *rnp, struct rcu_data *rdp)
 		trace_rcu_grace_period(rcu_state.name, rdp->gp_seq, TPS("AccWaitCB"));
 	else
 		trace_rcu_grace_period(rcu_state.name, rdp->gp_seq, TPS("AccReadyCB"));
+
+	/* Count CBs for tracing. */
+	rcu_segcblist_countseq(&rdp->cblist, cbs, gps);
+	trace_rcu_segcb("SegCbPostAcc", cbs, gps);
+
 	return ret;
 }
 
@@ -2316,6 +2327,8 @@ static void rcu_do_batch(struct rcu_data *rdp)
 	struct rcu_cblist rcl = RCU_CBLIST_INITIALIZER(rcl);
 	long bl, count;
 	long pending, tlimit = 0;
+	int cbs[RCU_CBLIST_NSEGS];
+	unsigned long gps[RCU_CBLIST_NSEGS];
 
 	/* If no callbacks are ready, just return. */
 	if (!rcu_segcblist_ready_cbs(&rdp->cblist)) {
@@ -2350,6 +2363,11 @@ static void rcu_do_batch(struct rcu_data *rdp)
 	/* Invoke callbacks. */
 	tick_dep_set_task(current, TICK_DEP_BIT_RCU);
 	rhp = rcu_cblist_dequeue(&rcl);
+
+	/* Count CBs for tracing. */
+	rcu_segcblist_countseq(&rdp->cblist, cbs, gps);
+	trace_rcu_segcb("SegCbDequeued", cbs, gps);
+
 	for (; rhp; rhp = rcu_cblist_dequeue(&rcl)) {
 		rcu_callback_t f;
 
@@ -2808,6 +2826,8 @@ __call_rcu(struct rcu_head *head, rcu_callback_t func)
 	unsigned long flags;
 	struct rcu_data *rdp;
 	bool was_alldone;
+	int cbs[RCU_CBLIST_NSEGS];
+	unsigned long gps[RCU_CBLIST_NSEGS];
 
 	/* Misaligned rcu_head! */
 	WARN_ON_ONCE((unsigned long)head & (sizeof(void *) - 1));
@@ -2852,6 +2872,10 @@ __call_rcu(struct rcu_head *head, rcu_callback_t func)
 		trace_rcu_callback(rcu_state.name, head,
 				   rcu_segcblist_n_cbs(&rdp->cblist));
 
+	/* Count CBs for tracing. */
+	rcu_segcblist_countseq(&rdp->cblist, cbs, gps);
+	trace_rcu_segcb("SegCBQueued", cbs, gps);
+
 	/* Go handle any RCU core processing required. */
 	if (IS_ENABLED(CONFIG_RCU_NOCB_CPU) &&
 	    unlikely(rcu_segcblist_is_offloaded(&rdp->cblist))) {
-- 
2.27.0.111.gc72c7da667-goog