linux-kernel - [RFC][PATCH 1/3] cpuidle: Inject tick boundary state

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Message-ID: <20230728145808.835742568@infradead.org>
Date:   Fri, 28 Jul 2023 16:55:16 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     anna-maria@...utronix.de, rafael@...nel.org, tglx@...utronix.de,
        frederic@...nel.org, gautham.shenoy@....com
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        daniel.lezcano@...aro.org, linux-pm@...r.kernel.org,
        mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, vschneid@...hat.com
Subject: [RFC][PATCH 1/3] cpuidle: Inject tick boundary state

In order to facilitate governors that track history in idle-state
buckets (TEO) making a useful decision about NOHZ, make sure we have a
bucket that counts tick-and-longer.

In order to be inclusive of the tick itself -- after all, if we do not
disable NOHZ we'll sleep for a full tick, the actual boundary should
be just short of a full tick.

IOW, when registering the idle-states, add one that is always
disabled, just to have a bucket.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 drivers/cpuidle/cpuidle.h |    2 +
 drivers/cpuidle/driver.c  |   48 +++++++++++++++++++++++++++++++++++++++++++++-
 include/linux/cpuidle.h   |    2 -
 3 files changed, 50 insertions(+), 2 deletions(-)

--- a/drivers/cpuidle/cpuidle.h
+++ b/drivers/cpuidle/cpuidle.h
@@ -72,4 +72,6 @@ static inline void cpuidle_coupled_unreg
 }
 #endif
 
+#define SHORT_TICK_NSEC (TICK_NSEC - TICK_NSEC/32)
+
 #endif /* __DRIVER_CPUIDLE_H */
--- a/drivers/cpuidle/driver.c
+++ b/drivers/cpuidle/driver.c
@@ -147,13 +147,37 @@ static void cpuidle_setup_broadcast_time
 		tick_broadcast_disable();
 }
 
+static int tick_enter(struct cpuidle_device *dev,
+		      struct cpuidle_driver *drv,
+		      int index)
+{
+	return -ENODEV;
+}
+
+static void __cpuidle_state_init_tick(struct cpuidle_state *s)
+{
+	strcpy(s->name, "TICK");
+	strcpy(s->desc, "(no-op)");
+
+	s->target_residency_ns = SHORT_TICK_NSEC;
+	s->target_residency = div_u64(SHORT_TICK_NSEC, NSEC_PER_USEC);
+
+	s->exit_latency_ns = 0;
+	s->exit_latency = 0;
+
+	s->flags |= CPUIDLE_FLAG_UNUSABLE;
+
+	s->enter = tick_enter;
+	s->enter_s2idle = tick_enter;
+}
+
 /**
  * __cpuidle_driver_init - initialize the driver's internal data
  * @drv: a valid pointer to a struct cpuidle_driver
  */
 static void __cpuidle_driver_init(struct cpuidle_driver *drv)
 {
-	int i;
+	int tick = 0, i;
 
 	/*
 	 * Use all possible CPUs as the default, because if the kernel boots
@@ -163,6 +187,9 @@ static void __cpuidle_driver_init(struct
 	if (!drv->cpumask)
 		drv->cpumask = (struct cpumask *)cpu_possible_mask;
 
+	if (WARN_ON_ONCE(drv->state_count >= CPUIDLE_STATE_MAX-2))
+		tick = 1;
+
 	for (i = 0; i < drv->state_count; i++) {
 		struct cpuidle_state *s = &drv->states[i];
 
@@ -192,6 +219,25 @@ static void __cpuidle_driver_init(struct
 			s->exit_latency_ns =  0;
 		else
 			s->exit_latency = div_u64(s->exit_latency_ns, NSEC_PER_USEC);
+
+		if (!tick && s->target_residency_ns >= SHORT_TICK_NSEC) {
+			tick = 1;
+
+			if (s->target_residency_ns == SHORT_TICK_NSEC)
+				continue;
+
+			memmove(&drv->states[i+1], &drv->states[i],
+				sizeof(struct cpuidle_state) * (CPUIDLE_STATE_MAX - i - 1));
+			__cpuidle_state_init_tick(s);
+			drv->state_count++;
+			i++;
+		}
+	}
+
+	if (!tick) {
+		struct cpuidle_state *s = &drv->states[i];
+		__cpuidle_state_init_tick(s);
+		drv->state_count++;
 	}
 }
 
--- a/include/linux/cpuidle.h
+++ b/include/linux/cpuidle.h
@@ -16,7 +16,7 @@
 #include <linux/hrtimer.h>
 #include <linux/context_tracking.h>
 
-#define CPUIDLE_STATE_MAX	10
+#define CPUIDLE_STATE_MAX	16
 #define CPUIDLE_NAME_LEN	16
 #define CPUIDLE_DESC_LEN	32