linux-kernel - [PATCH rcu-dev] rcu/trace: Add tracing for how segcb list changes

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Message-Id: <20201114193132.2660464-1-joel@joelfernandes.org>
Date:   Sat, 14 Nov 2020 14:31:32 -0500
From:   "Joel Fernandes (Google)" <joel@...lfernandes.org>
To:     linux-kernel@...r.kernel.org
Cc:     "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Ingo Molnar <mingo@...hat.com>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Marco Elver <elver@...gle.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "Paul E. McKenney" <paulmck@...nel.org>, rcu@...r.kernel.org,
        Steven Rostedt <rostedt@...dmis.org>,
        "Uladzislau Rezki (Sony)" <urezki@...il.com>
Subject: [PATCH rcu-dev] rcu/trace: Add tracing for how segcb list changes

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. The overhead is minimal as
each segment's length is now stored in the respective segment.

Reviewed-by: Frederic Weisbecker <frederic@...nel.org>
Reviewed-by: Neeraj Upadhyay <neeraju@...eaurora.org>
Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>

---
 include/trace/events/rcu.h | 26 ++++++++++++++++++++++++++
 kernel/rcu/tree.c          |  9 +++++++++
 2 files changed, 35 insertions(+)

diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
index 155b5cb43cfd..5fc29400e1a2 100644
--- a/include/trace/events/rcu.h
+++ b/include/trace/events/rcu.h
@@ -505,6 +505,32 @@ TRACE_EVENT_RCU(rcu_callback,
 		  __entry->qlen)
 );
 
+TRACE_EVENT_RCU(rcu_segcb_stats,
+
+		TP_PROTO(struct rcu_segcblist *rs, const char *ctx),
+
+		TP_ARGS(rs, ctx),
+
+		TP_STRUCT__entry(
+			__field(const char *, ctx)
+			__array(unsigned long, gp_seq, RCU_CBLIST_NSEGS)
+			__array(long, seglen, RCU_CBLIST_NSEGS)
+		),
+
+		TP_fast_assign(
+			__entry->ctx = ctx;
+			memcpy(__entry->seglen, rs->seglen, RCU_CBLIST_NSEGS * sizeof(long));
+			memcpy(__entry->gp_seq, rs->gp_seq, RCU_CBLIST_NSEGS * sizeof(unsigned long));
+
+		),
+
+		TP_printk("%s seglen: (DONE=%ld, WAIT=%ld, NEXT_READY=%ld, NEXT=%ld) "
+			  "gp_seq: (DONE=%lu, WAIT=%lu, NEXT_READY=%lu, NEXT=%lu)", __entry->ctx,
+			  __entry->seglen[0], __entry->seglen[1], __entry->seglen[2], __entry->seglen[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
  * kvfree() form.  The first argument is the RCU type, the second argument
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 413831b48648..b96d26d0d44a 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1497,6 +1497,8 @@ static bool rcu_accelerate_cbs(struct rcu_node *rnp, struct rcu_data *rdp)
 	if (!rcu_segcblist_pend_cbs(&rdp->cblist))
 		return false;
 
+	trace_rcu_segcb_stats(&rdp->cblist, TPS("SegCbPreAcc"));
+
 	/*
 	 * Callbacks are often registered with incomplete grace-period
 	 * information.  Something about the fact that getting exact
@@ -1517,6 +1519,8 @@ static bool rcu_accelerate_cbs(struct rcu_node *rnp, struct rcu_data *rdp)
 	else
 		trace_rcu_grace_period(rcu_state.name, gp_seq_req, TPS("AccReadyCB"));
 
+	trace_rcu_segcb_stats(&rdp->cblist, TPS("SegCbPostAcc"));
+
 	return ret;
 }
 
@@ -2473,11 +2477,14 @@ static void rcu_do_batch(struct rcu_data *rdp)
 	rcu_segcblist_extract_done_cbs(&rdp->cblist, &rcl);
 	if (offloaded)
 		rdp->qlen_last_fqs_check = rcu_segcblist_n_cbs(&rdp->cblist);
+
+	trace_rcu_segcb_stats(&rdp->cblist, TPS("SegCbDequeued"));
 	rcu_nocb_unlock_irqrestore(rdp, flags);
 
 	/* Invoke callbacks. */
 	tick_dep_set_task(current, TICK_DEP_BIT_RCU);
 	rhp = rcu_cblist_dequeue(&rcl);
+
 	for (; rhp; rhp = rcu_cblist_dequeue(&rcl)) {
 		rcu_callback_t f;
 
@@ -2989,6 +2996,8 @@ __call_rcu(struct rcu_head *head, rcu_callback_t func)
 		trace_rcu_callback(rcu_state.name, head,
 				   rcu_segcblist_n_cbs(&rdp->cblist));
 
+	trace_rcu_segcb_stats(&rdp->cblist, TPS("SegCBQueued"));
+
 	/* Go handle any RCU core processing required. */
 	if (unlikely(rcu_segcblist_is_offloaded(&rdp->cblist))) {
 		__call_rcu_nocb_wake(rdp, was_alldone, flags); /* unlocks */
-- 
2.29.2.299.gdc1121823c-goog