[RFC][PATCH 2/3] time/tick: Introduce a dyn_hz boot option

[John Stultz <jstultz@google.com>]

Introduce dyn_hz= option, which allows the actual timer tick to
be scaled down at boot time.

This allows kerenls to be built with HZ=1000 but systems to
effectively run as if HZ=100 if specified.

The system will still run with the configured HZ value, but
ticks will just arrive "late".

The valid values are between 100 and the build time CONFIG_HZ
value.

Cc: Anna-Maria Behnsen <anna-maria@linutronix.de>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: Yury Norov <yury.norov@gmail.com>
Cc: Bitao Hu <yaoma@linux.alibaba.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: kernel-team@android.com
Signed-off-by: John Stultz <jstultz@google.com>
---
 include/linux/tick.h      | 11 +++++++++--
 kernel/Kconfig.hz         |  9 +++++++++
 kernel/time/tick-common.c | 30 ++++++++++++++++++++++++++++++
 kernel/time/tick-sched.c  |  4 ++--
 4 files changed, 50 insertions(+), 4 deletions(-)

diff --git a/include/linux/tick.h b/include/linux/tick.h
index b8ddc8e631a3c..734ee1e08c9ef 100644
--- a/include/linux/tick.h
+++ b/include/linux/tick.h
@@ -14,6 +14,13 @@
 #include <linux/rcupdate.h>
 #include <linux/static_key.h>
 
+#ifdef CONFIG_DYN_HZ
+extern long long dyn_tick_nsec;
+#define DYN_TICK_NSEC (dyn_tick_nsec)
+#else
+#define DYN_TICK_NSEC TICK_NSEC
+#endif
+
 #ifdef CONFIG_GENERIC_CLOCKEVENTS
 extern void __init tick_init(void);
 /* Should be core only, but ARM BL switcher requires it */
@@ -153,11 +160,11 @@ static inline bool tick_nohz_idle_got_tick(void) { return false; }
 static inline ktime_t tick_nohz_get_next_hrtimer(void)
 {
 	/* Next wake up is the tick period, assume it starts now */
-	return ktime_add(ktime_get(), TICK_NSEC);
+	return ktime_add(ktime_get(), DYN_TICK_NSEC);
 }
 static inline ktime_t tick_nohz_get_sleep_length(ktime_t *delta_next)
 {
-	*delta_next = TICK_NSEC;
+	*delta_next = DYN_TICK_NSEC;
 	return *delta_next;
 }
 static inline u64 get_cpu_idle_time_us(int cpu, u64 *unused) { return -1; }
diff --git a/kernel/Kconfig.hz b/kernel/Kconfig.hz
index 38ef6d06888ef..76714317674c5 100644
--- a/kernel/Kconfig.hz
+++ b/kernel/Kconfig.hz
@@ -55,5 +55,14 @@ config HZ
 	default 300 if HZ_300
 	default 1000 if HZ_1000
 
+config DYN_HZ
+	bool "Support dynamic HZ via boot argument"
+	default n
+	help
+	 Allow for the tick rate to be set at boot time via the dynhz=
+	 boot argument. This allows the tick rate to be lower then the
+	 build time configured HZ value.
+	 If you are unsure, say no.
+
 config SCHED_HRTICK
 	def_bool HIGH_RES_TIMERS
diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c
index ae5c5befdc58b..75fd9dadb8273 100644
--- a/kernel/time/tick-common.c
+++ b/kernel/time/tick-common.c
@@ -80,6 +80,30 @@ int tick_is_oneshot_available(void)
 	return tick_broadcast_oneshot_available();
 }
 
+#ifdef CONFIG_DYN_HZ
+long long dyn_tick_nsec = TICK_NSEC;
+
+static int __init set_dyn_hz(char *str)
+{
+	int ret, dyn_hz;
+
+	ret = kstrtoint(str, 0, &dyn_hz);
+	if (ret)
+		return ret;
+	if (dyn_hz > HZ || dyn_hz < 100)
+		dyn_hz = HZ;
+	dyn_tick_nsec = TICK_NSEC * HZ / dyn_hz;
+	return 1;
+}
+#else /* !CONFIG_DYN_HZ */
+static int __init set_dyn_hz(char *str)
+{
+	pr_warn("CONFIG_DYN_HZ not enabled, ignoring dyn_hz boot argument\n");
+	return -1;
+}
+#endif /* CONFIG_DYN_HZ */
+__setup("dyn_hz=", set_dyn_hz);
+
 /*
  * Periodic tick
  */
@@ -575,4 +599,10 @@ void __init tick_init(void)
 {
 	tick_broadcast_init();
 	tick_nohz_init();
+	if (DYN_TICK_NSEC != TICK_NSEC) {
+		long long dynhz = TICK_NSEC * HZ;
+
+		do_div(dynhz, DYN_TICK_NSEC);
+		pr_info("dynHZ in use! HZ=%ld dynHZ=%lld\n", (long)HZ, dynhz);
+	}
 }
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 983790923aee9..040ad70c31d38 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -309,7 +309,7 @@ static enum hrtimer_restart tick_nohz_handler(struct hrtimer *timer)
 	if (unlikely(tick_sched_flag_test(ts, TS_FLAG_STOPPED)))
 		return HRTIMER_NORESTART;
 
-	hrtimer_forward(timer, now, TICK_NSEC);
+	hrtimer_forward(timer, now, DYN_TICK_NSEC);
 
 	return HRTIMER_RESTART;
 }
@@ -842,7 +842,7 @@ static void tick_nohz_restart(struct tick_sched *ts, ktime_t now)
 	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_forward(&ts->sched_timer, now, DYN_TICK_NSEC);
 
 	if (tick_sched_flag_test(ts, TS_FLAG_HIGHRES)) {
 		hrtimer_start_expires(&ts->sched_timer,
-- 
2.48.1.262.g85cc9f2d1e-goog