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rcu: Export rcu_request_urgent_qs_task()

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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 <ast@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230706033447.54696-11-alexei.starovoitov@gmail.com
This commit is contained in:
Paul E. McKenney 2023-07-05 20:34:43 -07:00 committed by Daniel Borkmann
parent 04fabf00b4
commit 43a89baecf
3 changed files with 3 additions and 2 deletions
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2
include/linux/rcutiny.h
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@ -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.

1
include/linux/rcutree.h
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@ -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

2
kernel/rcu/rcu.h
View File

@ -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