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

diff --git a/include/linux/preempt.h b/include/linux/preempt.h
index ce76f1a45722529832a5eaefc077356cc0124c9f..ca86235ac15c010346d371515036650825c30a2e 100644 (file)
--- a/include/linux/preempt.h
+++ b/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 */

diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
index fe782cd77388592b0c1b4197ae0d1e20b837b553..09f06d8964cf0c33e67853b9a4f67a05c63911fd 100644 (file)
--- a/kernel/Kconfig.preempt
+++ b/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

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 0cd05e36b6b676c0143c1227baa960852bfe212e..df6a34d27d2b448502cfe3c5815b538aa576e842 100644 (file)
--- a/kernel/sched/core.c
+++ b/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);
@@ -7374,6 +7411,7 @@ 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
@@ -7381,6 +7419,7 @@ EXPORT_SYMBOL(__cond_resched_rwlock_write);
  *   preempt_schedule           <- NOP
  *   preempt_schedule_notrace   <- NOP
  *   irqentry_exit_cond_resched <- NOP
+ *   dynamic_preempt_lazy       <- false
  *
  * FULL:
  *   cond_resched               <- RET0
@@ -7388,6 +7427,15 @@ EXPORT_SYMBOL(__cond_resched_rwlock_write);
  *   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 {
@@ -7395,6 +7443,7 @@ enum {
        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: */
 

diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index f4035c7a0fa1df365222300f443f87a6fe02dd71..44a49f90b05fee0c6b58ded36bc162c5dd2c96ee 100644 (file)
--- a/kernel/sched/debug.c
+++ b/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]);

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6512258dc71f3a51b9911ae90cafed50653e5c33..3356315d7e641e252b3156b851391766c229d2ce 100644 (file)
--- a/kernel/sched/fair.c
+++ b/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)

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index e51bf5a344d34ca9521a440f6916a751aa93b35a..090dd4b38fa2950f2d93a6734aaa163f2fec6ec7 100644 (file)
--- a/kernel/sched/sched.h
+++ b/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);