[PATCH 4/4] trace/timer: Improve timer tracing

[Anna-Maria Gleixner]

Timers are added to the timer wheel off by one. This is required in
case a timer is queued directly before incrementing jiffies to prevent
early timer expiry.

When reading a timer trace and relying only on the expiry time of the
timer in timer_start trace point and on the now in timer_expiry_entry
trace point, it seems that the timer fires late. With the current
timer_expiry_entry trace point information only now=jiffies is printed
but not the value of base->clk. This makes it impossible to draw a
conclusion to the index of base->clk and makes it impossible to
examine timer problems without additional trace points.

Therefore add the base->clk value to the timer_expire_entry trace
point, to be able to calculate the index the timer base is located at
during collecting expired timers.

Signed-off-by: Anna-Maria Gleixner <anna-maria@xxxxxxxxxxxxx>
---
 include/trace/events/timer.h | 11 +++++++----
 kernel/time/timer.c          | 17 +++++++++++++----
 2 files changed, 20 insertions(+), 8 deletions(-)

diff --git a/include/trace/events/timer.h b/include/trace/events/timer.h
index da975d69c453..dade735657ef 100644
--- a/include/trace/events/timer.h
+++ b/include/trace/events/timer.h
@@ -89,24 +89,27 @@ TRACE_EVENT(timer_start,
  */
 TRACE_EVENT(timer_expire_entry,
 
-	TP_PROTO(struct timer_list *timer),
+	TP_PROTO(struct timer_list *timer, unsigned long baseclk),
 
-	TP_ARGS(timer),
+	TP_ARGS(timer, baseclk),
 
 	TP_STRUCT__entry(
 		__field( void *,	timer	)
 		__field( unsigned long,	now	)
 		__field( void *,	function)
+		__field( unsigned long,	baseclk	)
 	),
 
 	TP_fast_assign(
 		__entry->timer		= timer;
 		__entry->now		= jiffies;
 		__entry->function	= timer->function;
+		__entry->baseclk	= baseclk;
 	),
 
-	TP_printk("timer=%p function=%ps now=%lu",
-		  __entry->timer, __entry->function, __entry->now)
+	TP_printk("timer=%p function=%ps now=%lu base->clk=%lu",
+		  __entry->timer, __entry->function, __entry->now,
+		  __entry->baseclk)
 );
 
 /**
diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 8d7918ae4d0c..c0233c1a4ccb 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1293,7 +1293,8 @@ int del_timer_sync(struct timer_list *timer)
 EXPORT_SYMBOL(del_timer_sync);
 #endif
 
-static void call_timer_fn(struct timer_list *timer, void (*fn)(struct timer_list *))
+static void call_timer_fn(unsigned long baseclk, struct timer_list *timer,
+			  void (*fn)(struct timer_list *))
 {
 	int count = preempt_count();
 
@@ -1316,7 +1317,7 @@ static void call_timer_fn(struct timer_list *timer, void (*fn)(struct timer_list
 	 */
 	lock_map_acquire(&lockdep_map);
 
-	trace_timer_expire_entry(timer);
+	trace_timer_expire_entry(timer, baseclk);
 	fn(timer);
 	trace_timer_expire_exit(timer);
 
@@ -1337,6 +1338,14 @@ static void call_timer_fn(struct timer_list *timer, void (*fn)(struct timer_list
 
 static void expire_timers(struct timer_base *base, struct hlist_head *head)
 {
+	/*
+	 * this value is required for tracing only
+	 *
+	 * base->clk was incremented directly before expire_timers was
+	 * called. But expiry is related to the old base->clk value.
+	 */
+	unsigned long baseclk = base->clk - 1;
+
 	while (!hlist_empty(head)) {
 		struct timer_list *timer;
 		void (*fn)(struct timer_list *);
@@ -1350,11 +1359,11 @@ static void expire_timers(struct timer_base *base, struct hlist_head *head)
 
 		if (timer->flags & TIMER_IRQSAFE) {
 			raw_spin_unlock(&base->lock);
-			call_timer_fn(timer, fn);
+			call_timer_fn(baseclk, timer, fn);
 			raw_spin_lock(&base->lock);
 		} else {
 			raw_spin_unlock_irq(&base->lock);
-			call_timer_fn(timer, fn);
+			call_timer_fn(baseclk, timer, fn);
 			raw_spin_lock_irq(&base->lock);
 		}
 	}
-- 
2.20.1