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Subject[PATCH 4/4] trace/timer: Improve timer tracing
Date
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@linutronix.de>
---
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
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