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Date: Thu, 18 Jun 2020 16:29:50 -0400
From: "Joel Fernandes (Google)" <joel@...lfernandes.org>
To: linux-kernel@...r.kernel.org
Cc: "Joel Fernandes (Google)" <joel@...lfernandes.org>,
Davidlohr Bueso <dave@...olabs.net>,
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 2/7] 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.
Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.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
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