[RFC 1/3] tick-sched: Remove last_tick and calculate next tick from now

["Joel Fernandes (Google)" <joel@joelfernandes.org>]

During tick restart, we use last_tick and forward it past now.

Since we are forwarding past now, we can simply use now as a reference
instead of last_tick. This patch removes last_tick and does so.

This patch potentially does more mul/imul than the existing code,
as sometimes forwarding past now need not be done if last_tick > now.
However, the patch is a cleanup which reduces LOC and reduces the size
of struct tick_sched.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 kernel/time/tick-sched.c | 7 ++-----
 kernel/time/tick-sched.h | 1 -
 kernel/time/timer_list.c | 1 -
 3 files changed, 2 insertions(+), 7 deletions(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 71a792cd8936..52a4eda664cf 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -837,11 +837,9 @@ EXPORT_SYMBOL_GPL(get_cpu_iowait_time_us);
 
 static void tick_nohz_restart(struct tick_sched *ts, ktime_t now)
 {
+	/* Set the time to expire on the next tick and not some far away future. */
 	hrtimer_cancel(&ts->sched_timer);
-	hrtimer_set_expires(&ts->sched_timer, ts->last_tick);
-
-	/* Forward the time to expire in the future */
-	hrtimer_forward(&ts->sched_timer, now, TICK_NSEC);
+	hrtimer_set_expires(&ts->sched_timer, DIV_ROUND_UP_ULL(now, TICK_NSEC) * TICK_NSEC);
 
 	if (tick_sched_flag_test(ts, TS_FLAG_HIGHRES)) {
 		hrtimer_start_expires(&ts->sched_timer,
@@ -1043,7 +1041,6 @@ static void tick_nohz_stop_tick(struct tick_sched *ts, int cpu)
 		calc_load_nohz_start();
 		quiet_vmstat();
 
-		ts->last_tick = hrtimer_get_expires(&ts->sched_timer);
 		tick_sched_flag_set(ts, TS_FLAG_STOPPED);
 		trace_tick_stop(1, TICK_DEP_MASK_NONE);
 	}
diff --git a/kernel/time/tick-sched.h b/kernel/time/tick-sched.h
index b4a7822f495d..7210cc473855 100644
--- a/kernel/time/tick-sched.h
+++ b/kernel/time/tick-sched.h
@@ -71,7 +71,6 @@ struct tick_sched {
 
 	/* Tick handling */
 	struct hrtimer			sched_timer;
-	ktime_t				last_tick;
 	ktime_t				next_tick;
 	unsigned long			idle_jiffies;
 	ktime_t				idle_waketime;
diff --git a/kernel/time/timer_list.c b/kernel/time/timer_list.c
index 1c311c46da50..26688a3b8ea8 100644
--- a/kernel/time/timer_list.c
+++ b/kernel/time/timer_list.c
@@ -154,7 +154,6 @@ static void print_cpu(struct seq_file *m, int cpu, u64 now)
 		struct tick_sched *ts = tick_get_tick_sched(cpu);
 		P_flag(nohz, TS_FLAG_NOHZ);
 		P_flag(highres, TS_FLAG_HIGHRES);
-		P_ns(last_tick);
 		P_flag(tick_stopped, TS_FLAG_STOPPED);
 		P(idle_jiffies);
 		P(idle_calls);
-- 
2.47.0.277.g8800431eea-goog