[RFC 3/3] tick-sched: Replace jiffie readout with idle_entrytime

[Joel Fernandes (Google)]

This solves the issue where jiffies can be stale and inaccurate.

Putting some prints, I see that basemono can be quite stale:
tick_nohz_next_event: basemono=18692000000 basemono_from_idle_entrytime=18695000000

Since we have 'now' in ts->idle_entrytime, we can just use that. It is
more accurate, cleaner, reduces lines of code and reduces any lock
contention with the seq locks.

I was also concerned about issue where jiffies is not updated for a long
time, and then we receive a non-tick interrupt in the future. Relying on
stale jiffies value and using that as base can be inaccurate to determine
whether next event occurs within next tick. Fix that.

XXX: Need to fix issue in idle accounting which does 'jiffies -
idle_entrytime'. If idle_entrytime is more current than jiffies, it
could cause negative values. I could replace jiffies with idle_exittime
in this computation potentially to fix that.

Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
---
 kernel/time/tick-sched.c | 27 +++++++--------------------
 1 file changed, 7 insertions(+), 20 deletions(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 4aa64266f2b0..22a4f96d9585 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -860,24 +860,6 @@ static inline bool local_timer_softirq_pending(void)
 	return local_softirq_pending() & BIT(TIMER_SOFTIRQ);
 }
 
-/*
- * Read jiffies and the time when jiffies were updated last
- */
-u64 get_jiffies_update(unsigned long *basej)
-{
-	unsigned long basejiff;
-	unsigned int seq;
-	u64 basemono;
-
-	do {
-		seq = read_seqcount_begin(&jiffies_seq);
-		basemono = last_jiffies_update;
-		basejiff = jiffies;
-	} while (read_seqcount_retry(&jiffies_seq, seq));
-	*basej = basejiff;
-	return basemono;
-}
-
 /**
  * tick_nohz_next_event() - return the clock monotonic based next event
  * @ts:		pointer to tick_sched struct
@@ -887,14 +869,19 @@ u64 get_jiffies_update(unsigned long *basej)
  * *%0		- When the next event is a maximum of TICK_NSEC in the future
  *		  and the tick is not stopped yet
  * *%next_event	- Next event based on clock monotonic
+ *
+ * Note: ts->idle_entrytime is updated with 'now' via tick_nohz_idle_enter().
  */
 static ktime_t tick_nohz_next_event(struct tick_sched *ts, int cpu)
 {
-	u64 basemono, next_tick, delta, expires, delta_hr, next_hr_wo;
+	u64 basemono, next_tick, delta, expires, delta_hr, next_hr_wo, boot_ticks;
 	unsigned long basejiff;
 	int tick_cpu;
 
-	basemono = get_jiffies_update(&basejiff);
+	boot_ticks = DIV_ROUND_DOWN_ULL(ts->idle_entrytime, TICK_NSEC);
+	basejiff = boot_ticks + INITIAL_JIFFIES;
+	basemono = boot_ticks * TICK_NSEC;
+
 	ts->last_jiffies = basejiff;
 	ts->timer_expires_base = basemono;
 
-- 
2.47.0.277.g8800431eea-goog