[PATCH v6 4/4] tracing: Histogram for delayed hrtimer offsets

[Binoy Jayan]

Generate a histogram of the latencies of delayed timer offsets in
nanoseconds. It shows the latency captured due to a delayed timer expire
event. It happens for example when a timer misses its deadline due to
disabled interrupts. A process if scheduled as a result of the timer
expiration suffers this latency.

The following filter(s) may be used

'hist:key=common_pid.execname:val=toffset,hitcount'
'hist:key=cpu,tcomm:val=toffset:sort=tcomm'
'hist:key=comm,tcomm:sort=comm,tcomm'

Signed-off-by: Binoy Jayan <binoy.jayan@xxxxxxxxxx>
---
 include/linux/hrtimer.h      |  3 +++
 include/trace/events/timer.h | 25 +++++++++++++++++++++
 kernel/time/Kconfig          |  7 ++++++
 kernel/time/hrtimer.c        | 52 ++++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 87 insertions(+)

diff --git a/include/linux/hrtimer.h b/include/linux/hrtimer.h
index 5e00f80..9146842 100644
--- a/include/linux/hrtimer.h
+++ b/include/linux/hrtimer.h
@@ -104,6 +104,9 @@ struct hrtimer {
 	struct hrtimer_clock_base	*base;
 	u8				state;
 	u8				is_rel;
+#ifdef CONFIG_DELAYED_TIMER_OFFSETS_HIST
+	ktime_t				praecox;
+#endif
 #ifdef CONFIG_TIMER_STATS
 	int				start_pid;
 	void				*start_site;
diff --git a/include/trace/events/timer.h b/include/trace/events/timer.h
index 28c5da6..ee45aed 100644
--- a/include/trace/events/timer.h
+++ b/include/trace/events/timer.h
@@ -382,6 +382,31 @@ TRACE_EVENT(tick_stop,
 );
 #endif
 
+#ifdef CONFIG_DELAYED_TIMER_OFFSETS_HIST
+TRACE_EVENT(latency_hrtimer_interrupt,
+
+	TP_PROTO(long long toffset, struct task_struct *task),
+
+	TP_ARGS(toffset, task),
+
+	TP_STRUCT__entry(
+		__field(long long,	toffset)
+		__array(char,		tcomm,	TASK_COMM_LEN)
+		__field(int,		tprio)
+	),
+
+	TP_fast_assign(
+		__entry->toffset = toffset;
+		memcpy(__entry->tcomm, task != NULL ? task->comm : "<none>",
+			task != NULL ? TASK_COMM_LEN : 7);
+		__entry->tprio  = task != NULL ? task->prio : -1;
+	),
+
+	TP_printk("toffset=%lld thread=%s[%d]",
+		__entry->toffset, __entry->tcomm, __entry->tprio)
+);
+#endif
+
 #endif /*  _TRACE_TIMER_H */
 
 /* This part must be outside protection */
diff --git a/kernel/time/Kconfig b/kernel/time/Kconfig
index 4008d9f..8ff19dd 100644
--- a/kernel/time/Kconfig
+++ b/kernel/time/Kconfig
@@ -193,5 +193,12 @@ config HIGH_RES_TIMERS
 	  hardware is not capable then this option only increases
 	  the size of the kernel image.
 
+config DELAYED_TIMER_OFFSETS_HIST
+	depends on HIGH_RES_TIMERS
+	select GENERIC_TRACER
+	bool "Delayed Timer Offsets Histogram"
+	help
+	  Generate a histogram of delayed timer offsets in nanoseconds.
+
 endmenu
 endif
diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index 9ba7c82..432d49a 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -56,6 +56,8 @@
 
 #include "tick-internal.h"
 
+static enum hrtimer_restart hrtimer_wakeup(struct hrtimer *timer);
+
 /*
  * The timer bases:
  *
@@ -960,6 +962,52 @@ static inline ktime_t hrtimer_update_lowres(struct hrtimer *timer, ktime_t tim,
 	return tim;
 }
 
+#ifdef CONFIG_DELAYED_TIMER_OFFSETS_HIST
+static inline void latency_hrtimer_timing_start(struct hrtimer *timer,
+					 struct hrtimer_clock_base *new_base,
+					 ktime_t tim)
+{
+	if (unlikely(trace_latency_hrtimer_interrupt_enabled())) {
+		ktime_t now = new_base->get_time();
+
+		if (ktime_to_ns(tim) < ktime_to_ns(now))
+			timer->praecox = now;
+		else
+			timer->praecox = ktime_set(0, 0);
+	}
+}
+
+static inline void latency_hrtimer_timing_stop(struct hrtimer *timer,
+						ktime_t basenow)
+{
+	long latency;
+	struct task_struct *task;
+
+	if (likely(!trace_latency_hrtimer_interrupt_enabled()))
+		return;
+
+	latency = ktime_to_ns(ktime_sub(basenow,
+			      ktime_to_ns(timer->praecox) ?
+			      timer->praecox : hrtimer_get_expires(timer)));
+	task = timer->function == hrtimer_wakeup ?
+			container_of(timer, struct hrtimer_sleeper,
+				     timer)->task : NULL;
+	if (latency > 0)
+		trace_latency_hrtimer_interrupt((u64) latency, task);
+}
+#else
+static inline void latency_hrtimer_timing_start(struct hrtimer *timer,
+					 struct hrtimer_clock_base *new_base,
+					 ktime_t tim)
+{
+}
+static inline void latency_hrtimer_timing_stop(struct hrtimer *timer,
+						ktime_t basenow)
+{
+}
+
+#endif
+
 /**
  * hrtimer_start_range_ns - (re)start an hrtimer on the current CPU
  * @timer:	the timer to be added
@@ -992,6 +1040,8 @@ void hrtimer_start_range_ns(struct hrtimer *timer, ktime_t tim,
 
 	timer_stats_hrtimer_set_start_info(timer);
 
+	latency_hrtimer_timing_start(timer, new_base, tim);
+
 	leftmost = enqueue_hrtimer(timer, new_base);
 	if (!leftmost)
 		goto unlock;
@@ -1284,6 +1334,8 @@ static void __hrtimer_run_queues(struct hrtimer_cpu_base *cpu_base, ktime_t now)
 
 			timer = container_of(node, struct hrtimer, node);
 
+			latency_hrtimer_timing_stop(timer, basenow);
+
 			/*
 			 * The immediate goal for using the softexpires is
 			 * minimizing wakeups, not running timers at the
-- 
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