timer/trace: Improve timer tracing

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 the timer_start trace point and on the now in the 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>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: fweisbec@gmail.com
Cc: peterz@infradead.org
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: https://lkml.kernel.org/r/20190321120921.16463-5-anna-maria@linutronix.de

diff --git a/include/trace/events/timer.h b/include/trace/events/timer.h
index da975d69c45362ea61589b061383dba4b7b10c76..b7a904825e7df8a3dff0262b466d257c258407d1 100644 (file)
--- 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 baseclk=%lu",
+                 __entry->timer, __entry->function, __entry->now,
+                 __entry->baseclk)
 );
 
 /**

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 8d7918ae4d0c3bf31ecdd88edf60f2bfe4052c9b..a9b1bbc2d88d942e4ee6b3ddc7ee77c6e5a747bb 100644 (file)
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1293,7 +1293,9 @@ 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(struct timer_list *timer,
+                         void (*fn)(struct timer_list *),
+                         unsigned long baseclk)
 {
        int count = preempt_count();
 
@@ -1316,7 +1318,7 @@ 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 +1339,13 @@ 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 only for tracing. 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(timer, fn, baseclk);
                        raw_spin_lock(&base->lock);
                } else {
                        raw_spin_unlock_irq(&base->lock);
-                       call_timer_fn(timer, fn);
+                       call_timer_fn(timer, fn, baseclk);
                        raw_spin_lock_irq(&base->lock);
                }
        }