Re: cond_resched() and RCU CPU stall warnings

[Peter Zijlstra]

> sched,rcu: Make cond_resched() report RCU quiescent states
> 
> Given a CPU running a loop containing cond_resched(), with no
> other tasks runnable on that CPU, RCU will eventually report RCU
> CPU stall warnings due to lack of quiescent states.  Fortunately,
> every call to cond_resched() is a perfectly good quiescent state.
> Unfortunately, invoking rcu_note_context_switch() is a bit heavyweight
> for cond_resched(), especially given the need to disable preemption,
> and, for RCU-preempt, interrupts as well.
> 
> This commit therefore maintains a per-CPU counter that causes
> cond_resched(), cond_resched_lock(), and cond_resched_softirq() to call
> rcu_note_context_switch(), but only about once per 256 invocations.
> This ratio was chosen in keeping with the relative time constants of
> RCU grace periods.
> 
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 3cea28c64ebe..8d64878111ea 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -44,6 +44,7 @@
>  #include <linux/debugobjects.h>
>  #include <linux/bug.h>
>  #include <linux/compiler.h>
> +#include <linux/percpu.h>
>  #include <asm/barrier.h>
>  
>  extern int rcu_expedited; /* for sysctl */
> @@ -287,6 +288,41 @@ bool __rcu_is_watching(void);
>  #endif /* #if defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_RCU_TRACE) || defined(CONFIG_SMP) */
>  
>  /*
> + * Hooks for cond_resched() and friends to avoid RCU CPU stall warnings.
> + */
> +
> +#define RCU_COND_RESCHED_LIM 256	/* ms vs. 100s of ms. */
> +DECLARE_PER_CPU(int, rcu_cond_resched_count);
> +void rcu_resched(void);
> +
> +/*
> + * Is it time to report RCU quiescent states?
> + *
> + * Note unsynchronized access to rcu_cond_resched_count.  Yes, we might
> + * increment some random CPU's count, and possibly also load the result from
> + * yet another CPU's count.  We might even clobber some other CPU's attempt
> + * to zero its counter.  This is all OK because the goal is not precision,
> + * but rather reasonable amortization of rcu_note_context_switch() overhead
> + * and extremely high probability of avoiding RCU CPU stall warnings.
> + * Note that this function has to be preempted in just the wrong place,
> + * many thousands of times in a row, for anything bad to happen.
> + */
> +static inline bool rcu_should_resched(void)
> +{
> +	return __this_cpu_inc_return(rcu_cond_resched_count) >=
> +	       RCU_COND_RESCHED_LIM;
> +}
> +
> +/*
> + * Report quiscent states to RCU if it is time to do so.
> + */
> +static inline void rcu_cond_resched(void)
> +{
> +	if (unlikely(rcu_should_resched()))
> +		rcu_resched();
> +}
> +
> +/*
>   * Infrastructure to implement the synchronize_() primitives in
>   * TREE_RCU and rcu_barrier_() primitives in TINY_RCU.
>   */
> diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> index 4c0a9b0af469..ed7a0d72562c 100644
> --- a/kernel/rcu/update.c
> +++ b/kernel/rcu/update.c
> @@ -338,3 +338,21 @@ static int __init check_cpu_stall_init(void)
>  early_initcall(check_cpu_stall_init);
>  
>  #endif /* #ifdef CONFIG_RCU_STALL_COMMON */
> +
> +/*
> + * Hooks for cond_resched() and friends to avoid RCU CPU stall warnings.
> + */
> +
> +DEFINE_PER_CPU(int, rcu_cond_resched_count);
> +
> +/*
> + * Report a set of RCU quiescent states, for use by cond_resched()
> + * and friends.  Out of line due to being called infrequently.
> + */
> +void rcu_resched(void)
> +{
> +	preempt_disable();
> +	__this_cpu_write(rcu_cond_resched_count, 0);

Somehow I've got the urge to write:

  __this_cpu_add(rcu_cond_resched_count, -RCU_COND_RESCHED_LIM);

But the immediate write is saver, and I suppose it doesn't matter at
all.

> +	rcu_note_context_switch(smp_processor_id());
> +	preempt_enable();
> +}
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index b46131ef6aab..1f53e8871dd2 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4071,6 +4071,7 @@ static void __cond_resched(void)
>  
>  int __sched _cond_resched(void)
>  {
> +	rcu_cond_resched();
>  	if (should_resched()) {
>  		__cond_resched();
>  		return 1;
> @@ -4089,6 +4090,7 @@ EXPORT_SYMBOL(_cond_resched);
>   */
>  int __cond_resched_lock(spinlock_t *lock)
>  {
> +	bool need_rcu_resched = rcu_should_resched();
>  	int resched = should_resched();
>  	int ret = 0;
>  

I suppose you also need:

-	if (spin_needbreak(lock) || resched)
+	if (spin_needbreak(lock) || resched || need_rcu_resched)

> @@ -4098,6 +4100,8 @@ int __cond_resched_lock(spinlock_t *lock)
>  		spin_unlock(lock);
>  		if (resched)
>  			__cond_resched();
> +		else if (unlikely(need_rcu_resched))
> +			rcu_resched();
>  		else
>  			cpu_relax();
>  		ret = 1;
> @@ -4111,6 +4115,7 @@ int __sched __cond_resched_softirq(void)
>  {
>  	BUG_ON(!in_softirq());
>  
> +	rcu_cond_resched();  /* BH disabled OK, just recording QSes. */
>  	if (should_resched()) {
>  		local_bh_enable();
>  		__cond_resched();

At which point I'm wondering; do you also want __cond_resched_softirq()
to advance rcu_bh? I'm forever forgetting the exact rcu_bh semantics
though.
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