[v4,bpf-next,10/14] rcu: Export rcu_request_urgent_qs_task()

Commit Message

Alexei Starovoitov July 6, 2023, 3:34 a.m. UTC

From: "Paul E. McKenney" <[e-mail]>

If a CPU is executing a long series of non-sleeping system calls,
RCU grace periods can be delayed for on the order of a couple hundred
milliseconds.  This is normally not a problem, but if each system call
does a call_rcu(), those callbacks can stack up.  RCU will eventually
notice this callback storm, but use of rcu_request_urgent_qs_task()
allows the code invoking call_rcu() to give RCU a heads up.

This function is not for general use, not yet, anyway.

Reported-by: Alexei Starovoitov <[e-mail]>
Signed-off-by: Paul E. McKenney <[e-mail]>
Signed-off-by: Alexei Starovoitov <[e-mail]>
---
 include/linux/rcutiny.h | 2 ++
 include/linux/rcutree.h | 1 +
 kernel/rcu/rcu.h        | 2 --
 3 files changed, 3 insertions(+), 2 deletions(-)

Patch

diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index 7f17acf29dda..7b949292908a 100644
--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -138,6 +138,8 @@  static inline int rcu_needs_cpu(void)
 	return 0;
 }
 
+static inline void rcu_request_urgent_qs_task(struct task_struct *t) { }
+
 /*
  * Take advantage of the fact that there is only one CPU, which
  * allows us to ignore virtualization-based context switches.
diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
index 56bccb5a8fde..126f6b418f6a 100644
--- a/include/linux/rcutree.h
+++ b/include/linux/rcutree.h
@@ -21,6 +21,7 @@  void rcu_softirq_qs(void);
 void rcu_note_context_switch(bool preempt);
 int rcu_needs_cpu(void);
 void rcu_cpu_stall_reset(void);
+void rcu_request_urgent_qs_task(struct task_struct *t);
 
 /*
  * Note a virtualization-based context switch.  This is simply a
diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index 98c1544cf572..f95cfb5bf2ee 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -493,7 +493,6 @@  static inline void rcu_expedite_gp(void) { }
 static inline void rcu_unexpedite_gp(void) { }
 static inline void rcu_async_hurry(void) { }
 static inline void rcu_async_relax(void) { }
-static inline void rcu_request_urgent_qs_task(struct task_struct *t) { }
 #else /* #ifdef CONFIG_TINY_RCU */
 bool rcu_gp_is_normal(void);     /* Internal RCU use. */
 bool rcu_gp_is_expedited(void);  /* Internal RCU use. */
@@ -508,7 +507,6 @@  void show_rcu_tasks_gp_kthreads(void);
 #else /* #ifdef CONFIG_TASKS_RCU_GENERIC */
 static inline void show_rcu_tasks_gp_kthreads(void) {}
 #endif /* #else #ifdef CONFIG_TASKS_RCU_GENERIC */
-void rcu_request_urgent_qs_task(struct task_struct *t);
 #endif /* #else #ifdef CONFIG_TINY_RCU */
 
 #define RCU_SCHEDULER_INACTIVE	0