sched: Add Lazy preemption model

Change fair to use resched_curr_lazy(), which, when the lazy
preemption model is selected, will set TIF_NEED_RESCHED_LAZY.

This LAZY bit will be promoted to the full NEED_RESCHED bit on tick.
As such, the average delay between setting LAZY and actually
rescheduling will be TICK_NSEC/2.

In short, Lazy preemption will delay preemption for fair class but
will function as Full preemption for all the other classes, most
notably the realtime (RR/FIFO/DEADLINE) classes.

The goal is to bridge the performance gap with Voluntary, such that we
might eventually remove that option entirely.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lkml.kernel.org/r/20241007075055.331243614@infradead.org

include/linux/preempt.h

@@ -486,6 +486,7 @@ DEFINE_LOCK_GUARD_0(migrate, migrate_disable(), migrate_enable())
 extern bool preempt_model_none(void);
 extern bool preempt_model_voluntary(void);
 extern bool preempt_model_full(void);
+extern bool preempt_model_lazy(void);
 
 #else
 
@@ -502,6 +503,11 @@ static inline bool preempt_model_full(void)
 	return IS_ENABLED(CONFIG_PREEMPT);
 }
 
+static inline bool preempt_model_lazy(void)
+{
+	return IS_ENABLED(CONFIG_PREEMPT_LAZY);
+}
+
 #endif
 
 static inline bool preempt_model_rt(void)
@@ -519,7 +525,7 @@ static inline bool preempt_model_rt(void)
  */
 static inline bool preempt_model_preemptible(void)
 {
-	return preempt_model_full() || preempt_model_rt();
+	return preempt_model_full() || preempt_model_lazy() || preempt_model_rt();
 }
 
 #endif /* __LINUX_PREEMPT_H */

kernel/Kconfig.preempt

@@ -11,6 +11,9 @@ config PREEMPT_BUILD
 	select PREEMPTION
 	select UNINLINE_SPIN_UNLOCK if !ARCH_INLINE_SPIN_UNLOCK
 
+config ARCH_HAS_PREEMPT_LAZY
+	bool
+
 choice
 	prompt "Preemption Model"
 	default PREEMPT_NONE
@@ -67,6 +70,18 @@ config PREEMPT
 	  embedded system with latency requirements in the milliseconds
 	  range.
 
+config PREEMPT_LAZY
+	bool "Scheduler controlled preemption model"
+	depends on !ARCH_NO_PREEMPT
+	depends on ARCH_HAS_PREEMPT_LAZY
+	select PREEMPT_BUILD
+	help
+	  This option provides a scheduler driven preemption model that
+	  is fundamentally similar to full preemption, but is less
+	  eager to preempt SCHED_NORMAL tasks in an attempt to
+	  reduce lock holder preemption and recover some of the performance
+	  gains seen from using Voluntary preemption.
+
 config PREEMPT_RT
 	bool "Fully Preemptible Kernel (Real-Time)"
 	depends on EXPERT && ARCH_SUPPORTS_RT

kernel/sched/core.c

@@ -1083,6 +1083,13 @@ static void __resched_curr(struct rq *rq, int tif)
 
 	lockdep_assert_rq_held(rq);
 
+	/*
+	 * Always immediately preempt the idle task; no point in delaying doing
+	 * actual work.
+	 */
+	if (is_idle_task(curr) && tif == TIF_NEED_RESCHED_LAZY)
+		tif = TIF_NEED_RESCHED;
+
 	if (cti->flags & ((1 << tif) | _TIF_NEED_RESCHED))
 		return;
 
@@ -1108,6 +1115,32 @@ void resched_curr(struct rq *rq)
 	__resched_curr(rq, TIF_NEED_RESCHED);
 }
 
+#ifdef CONFIG_PREEMPT_DYNAMIC
+static DEFINE_STATIC_KEY_FALSE(sk_dynamic_preempt_lazy);
+static __always_inline bool dynamic_preempt_lazy(void)
+{
+	return static_branch_unlikely(&sk_dynamic_preempt_lazy);
+}
+#else
+static __always_inline bool dynamic_preempt_lazy(void)
+{
+	return IS_ENABLED(CONFIG_PREEMPT_LAZY);
+}
+#endif
+
+static __always_inline int get_lazy_tif_bit(void)
+{
+	if (dynamic_preempt_lazy())
+		return TIF_NEED_RESCHED_LAZY;
+
+	return TIF_NEED_RESCHED;
+}
+
+void resched_curr_lazy(struct rq *rq)
+{
+	__resched_curr(rq, get_lazy_tif_bit());
+}
+
 void resched_cpu(int cpu)
 {
 	struct rq *rq = cpu_rq(cpu);
@@ -5612,6 +5645,10 @@ void sched_tick(void)
 	update_rq_clock(rq);
 	hw_pressure = arch_scale_hw_pressure(cpu_of(rq));
 	update_hw_load_avg(rq_clock_task(rq), rq, hw_pressure);
+
+	if (dynamic_preempt_lazy() && tif_test_bit(TIF_NEED_RESCHED_LAZY))
+		resched_curr(rq);
+
 	donor->sched_class->task_tick(rq, donor, 0);
 	if (sched_feat(LATENCY_WARN))
 		resched_latency = cpu_resched_latency(rq);
@@ -7374,27 +7411,39 @@ EXPORT_SYMBOL(__cond_resched_rwlock_write);
  *   preempt_schedule           <- NOP
  *   preempt_schedule_notrace   <- NOP
  *   irqentry_exit_cond_resched <- NOP
+ *   dynamic_preempt_lazy       <- false
  *
  * VOLUNTARY:
  *   cond_resched               <- __cond_resched
  *   might_resched              <- __cond_resched
  *   preempt_schedule           <- NOP
  *   preempt_schedule_notrace   <- NOP
  *   irqentry_exit_cond_resched <- NOP
+ *   dynamic_preempt_lazy       <- false
  *
  * FULL:
  *   cond_resched               <- RET0
  *   might_resched              <- RET0
  *   preempt_schedule           <- preempt_schedule
  *   preempt_schedule_notrace   <- preempt_schedule_notrace
  *   irqentry_exit_cond_resched <- irqentry_exit_cond_resched
+ *   dynamic_preempt_lazy       <- false
+ *
+ * LAZY:
+ *   cond_resched               <- RET0
+ *   might_resched              <- RET0
+ *   preempt_schedule           <- preempt_schedule
+ *   preempt_schedule_notrace   <- preempt_schedule_notrace
+ *   irqentry_exit_cond_resched <- irqentry_exit_cond_resched
+ *   dynamic_preempt_lazy       <- true
  */
 
 enum {
 	preempt_dynamic_undefined = -1,
 	preempt_dynamic_none,
 	preempt_dynamic_voluntary,
 	preempt_dynamic_full,
+	preempt_dynamic_lazy,
 };
 
 int preempt_dynamic_mode = preempt_dynamic_undefined;
@@ -7410,15 +7459,23 @@ int sched_dynamic_mode(const char *str)
 	if (!strcmp(str, "full"))
 		return preempt_dynamic_full;
 
+#ifdef CONFIG_ARCH_HAS_PREEMPT_LAZY
+	if (!strcmp(str, "lazy"))
+		return preempt_dynamic_lazy;
+#endif
+
 	return -EINVAL;
 }
 
+#define preempt_dynamic_key_enable(f)	static_key_enable(&sk_dynamic_##f.key)
+#define preempt_dynamic_key_disable(f)	static_key_disable(&sk_dynamic_##f.key)
+
 #if defined(CONFIG_HAVE_PREEMPT_DYNAMIC_CALL)
 #define preempt_dynamic_enable(f)	static_call_update(f, f##_dynamic_enabled)
 #define preempt_dynamic_disable(f)	static_call_update(f, f##_dynamic_disabled)
 #elif defined(CONFIG_HAVE_PREEMPT_DYNAMIC_KEY)
-#define preempt_dynamic_enable(f)	static_key_enable(&sk_dynamic_##f.key)
-#define preempt_dynamic_disable(f)	static_key_disable(&sk_dynamic_##f.key)
+#define preempt_dynamic_enable(f)	preempt_dynamic_key_enable(f)
+#define preempt_dynamic_disable(f)	preempt_dynamic_key_disable(f)
 #else
 #error "Unsupported PREEMPT_DYNAMIC mechanism"
 #endif
@@ -7438,6 +7495,7 @@ static void __sched_dynamic_update(int mode)
 	preempt_dynamic_enable(preempt_schedule);
 	preempt_dynamic_enable(preempt_schedule_notrace);
 	preempt_dynamic_enable(irqentry_exit_cond_resched);
+	preempt_dynamic_key_disable(preempt_lazy);
 
 	switch (mode) {
 	case preempt_dynamic_none:
@@ -7447,6 +7505,7 @@ static void __sched_dynamic_update(int mode)
 		preempt_dynamic_disable(preempt_schedule);
 		preempt_dynamic_disable(preempt_schedule_notrace);
 		preempt_dynamic_disable(irqentry_exit_cond_resched);
+		preempt_dynamic_key_disable(preempt_lazy);
 		if (mode != preempt_dynamic_mode)
 			pr_info("Dynamic Preempt: none\n");
 		break;
@@ -7458,6 +7517,7 @@ static void __sched_dynamic_update(int mode)
 		preempt_dynamic_disable(preempt_schedule);
 		preempt_dynamic_disable(preempt_schedule_notrace);
 		preempt_dynamic_disable(irqentry_exit_cond_resched);
+		preempt_dynamic_key_disable(preempt_lazy);
 		if (mode != preempt_dynamic_mode)
 			pr_info("Dynamic Preempt: voluntary\n");
 		break;
@@ -7469,9 +7529,22 @@ static void __sched_dynamic_update(int mode)
 		preempt_dynamic_enable(preempt_schedule);
 		preempt_dynamic_enable(preempt_schedule_notrace);
 		preempt_dynamic_enable(irqentry_exit_cond_resched);
+		preempt_dynamic_key_disable(preempt_lazy);
 		if (mode != preempt_dynamic_mode)
 			pr_info("Dynamic Preempt: full\n");
 		break;
+
+	case preempt_dynamic_lazy:
+		if (!klp_override)
+			preempt_dynamic_disable(cond_resched);
+		preempt_dynamic_disable(might_resched);
+		preempt_dynamic_enable(preempt_schedule);
+		preempt_dynamic_enable(preempt_schedule_notrace);
+		preempt_dynamic_enable(irqentry_exit_cond_resched);
+		preempt_dynamic_key_enable(preempt_lazy);
+		if (mode != preempt_dynamic_mode)
+			pr_info("Dynamic Preempt: lazy\n");
+		break;
 	}
 
 	preempt_dynamic_mode = mode;
@@ -7534,6 +7607,8 @@ static void __init preempt_dynamic_init(void)
 			sched_dynamic_update(preempt_dynamic_none);
 		} else if (IS_ENABLED(CONFIG_PREEMPT_VOLUNTARY)) {
 			sched_dynamic_update(preempt_dynamic_voluntary);
+		} else if (IS_ENABLED(CONFIG_PREEMPT_LAZY)) {
+			sched_dynamic_update(preempt_dynamic_lazy);
 		} else {
 			/* Default static call setting, nothing to do */
 			WARN_ON_ONCE(!IS_ENABLED(CONFIG_PREEMPT));
@@ -7554,6 +7629,7 @@ static void __init preempt_dynamic_init(void)
 PREEMPT_MODEL_ACCESSOR(none);
 PREEMPT_MODEL_ACCESSOR(voluntary);
 PREEMPT_MODEL_ACCESSOR(full);
+PREEMPT_MODEL_ACCESSOR(lazy);
 
 #else /* !CONFIG_PREEMPT_DYNAMIC: */
 

kernel/sched/debug.c

@@ -245,11 +245,12 @@ static ssize_t sched_dynamic_write(struct file *filp, const char __user *ubuf,
 static int sched_dynamic_show(struct seq_file *m, void *v)
 {
 	static const char * preempt_modes[] = {
-		"none", "voluntary", "full"
+		"none", "voluntary", "full", "lazy",
 	};
+	int j = ARRAY_SIZE(preempt_modes) - !IS_ENABLED(CONFIG_ARCH_HAS_PREEMPT_LAZY);
 	int i;
 
-	for (i = 0; i < ARRAY_SIZE(preempt_modes); i++) {
+	for (i = 0; i < j; i++) {
 		if (preempt_dynamic_mode == i)
 			seq_puts(m, "(");
 		seq_puts(m, preempt_modes[i]);

kernel/sched/fair.c

@@ -1251,7 +1251,7 @@ static void update_curr(struct cfs_rq *cfs_rq)
 		return;
 
 	if (resched || did_preempt_short(cfs_rq, curr)) {
-		resched_curr(rq);
+		resched_curr_lazy(rq);
 		clear_buddies(cfs_rq, curr);
 	}
 }
@@ -5677,7 +5677,7 @@ entity_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr, int queued)
 	 * validating it and just reschedule.
 	 */
 	if (queued) {
-		resched_curr(rq_of(cfs_rq));
+		resched_curr_lazy(rq_of(cfs_rq));
 		return;
 	}
 #endif
@@ -8829,7 +8829,7 @@ static void check_preempt_wakeup_fair(struct rq *rq, struct task_struct *p, int
 	return;
 
 preempt:
-	resched_curr(rq);
+	resched_curr_lazy(rq);
 }
 
 static struct task_struct *pick_task_fair(struct rq *rq)

kernel/sched/sched.h

@@ -2689,6 +2689,7 @@ extern void init_sched_rt_class(void);
 extern void init_sched_fair_class(void);
 
 extern void resched_curr(struct rq *rq);
+extern void resched_curr_lazy(struct rq *rq);
 extern void resched_cpu(int cpu);
 
 extern void init_rt_bandwidth(struct rt_bandwidth *rt_b, u64 period, u64 runtime);