preempt/dynamic: Provide cond_resched() and might_resched() static calls

Provide static calls to control cond_resched() (called in !CONFIG_PREEMPT)
and might_resched() (called in CONFIG_PREEMPT_VOLUNTARY) to that we
can override their behaviour when preempt= is overriden.

Since the default behaviour is full preemption, both their calls are
ignored when preempt= isn't passed.

  [fweisbec: branch might_resched() directly to __cond_resched(), only
             define static calls when PREEMPT_DYNAMIC]
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lkml.kernel.org/r/20210118141223.123667-6-frederic@kernel.org

include/linux/kernel.h

@@ -15,7 +15,7 @@
 #include <linux/typecheck.h>
 #include <linux/printk.h>
 #include <linux/build_bug.h>
-
+#include <linux/static_call_types.h>
 #include <asm/byteorder.h>
 
 #include <uapi/linux/kernel.h>
@@ -81,11 +81,26 @@ struct pt_regs;
 struct user;
 
 #ifdef CONFIG_PREEMPT_VOLUNTARY
-extern int _cond_resched(void);
-# define might_resched() _cond_resched()
+
+extern int __cond_resched(void);
+# define might_resched() __cond_resched()
+
+#elif defined(CONFIG_PREEMPT_DYNAMIC)
+
+extern int __cond_resched(void);
+
+DECLARE_STATIC_CALL(might_resched, __cond_resched);
+
+static __always_inline void might_resched(void)
+{
+	static_call(might_resched)();
+}
+
 #else
+
 # define might_resched() do { } while (0)
-#endif
+
+#endif /* CONFIG_PREEMPT_* */
 
 #ifdef CONFIG_DEBUG_ATOMIC_SLEEP
 extern void ___might_sleep(const char *file, int line, int preempt_offset);

include/linux/sched.h

@@ -1871,11 +1871,32 @@ static inline int test_tsk_need_resched(struct task_struct *tsk)
  * value indicates whether a reschedule was done in fact.
  * cond_resched_lock() will drop the spinlock before scheduling,
  */
-#ifndef CONFIG_PREEMPTION
-extern int _cond_resched(void);
+#if !defined(CONFIG_PREEMPTION) || defined(CONFIG_PREEMPT_DYNAMIC)
+extern int __cond_resched(void);
+
+#ifdef CONFIG_PREEMPT_DYNAMIC
+
+DECLARE_STATIC_CALL(cond_resched, __cond_resched);
+
+static __always_inline int _cond_resched(void)
+{
+	return static_call(cond_resched)();
+}
+
 #else
+
+static inline int _cond_resched(void)
+{
+	return __cond_resched();
+}
+
+#endif /* CONFIG_PREEMPT_DYNAMIC */
+
+#else
+
 static inline int _cond_resched(void) { return 0; }
-#endif
+
+#endif /* !defined(CONFIG_PREEMPTION) || defined(CONFIG_PREEMPT_DYNAMIC) */
 
 #define cond_resched() ({			\
 	___might_sleep(__FILE__, __LINE__, 0);	\

kernel/sched/core.c

@@ -6785,17 +6785,27 @@ SYSCALL_DEFINE0(sched_yield)
 	return 0;
 }
 
-#ifndef CONFIG_PREEMPTION
-int __sched _cond_resched(void)
+#if !defined(CONFIG_PREEMPTION) || defined(CONFIG_PREEMPT_DYNAMIC)
+int __sched __cond_resched(void)
 {
 	if (should_resched(0)) {
 		preempt_schedule_common();
 		return 1;
 	}
+#ifndef CONFIG_PREEMPT_RCU
 	rcu_all_qs();
+#endif
 	return 0;
 }
-EXPORT_SYMBOL(_cond_resched);
+EXPORT_SYMBOL(__cond_resched);
+#endif
+
+#ifdef CONFIG_PREEMPT_DYNAMIC
+DEFINE_STATIC_CALL_RET0(cond_resched, __cond_resched);
+EXPORT_STATIC_CALL(cond_resched);
+
+DEFINE_STATIC_CALL_RET0(might_resched, __cond_resched);
+EXPORT_STATIC_CALL(might_resched);
 #endif
 
 /*