I found a magic number in it.

Paul E. McKenney wrote:
quoted text
> Hello again!
> 
> This patch adds stalled-CPU detection to Classic RCU.  This capability 
> is enabled by a new config variable CONFIG_RCU_CPU_STALL_DETECTOR, 
> which defaults disabled.  This is a debugging feature, not something 
> that non-kernel-hackers would be expected to care about.  This feature 
> can detect looping CPUs in !PREEMPT builds and looping CPUs with 
> preemption disabled in PREEMPT builds.  This is essentially a port of 
> this functionality from the treercu patch.
> 
> This version uses jiffies rather than get_seconds(), which eliminates
> the spurious boot-time CPU stall warnings seen on some systems with
> the previous patch.
> 
> This is still against v2.6.27-rc8 -- I will do a version against tip
> this evening (Pacific Time) when I get back to the combination of better
> bandwidth and AC power.
> 
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> ---
> 
>  include/linux/rcuclassic.h |    9 ++++
>  kernel/rcuclassic.c        |   88 +++++++++++++++++++++++++++++++++++++++++++++
>  lib/Kconfig.debug          |   13 ++++++
>  3 files changed, 110 insertions(+)
> 
> diff --git a/include/linux/rcuclassic.h b/include/linux/rcuclassic.h
> index 4ab8436..cab055b 100644
> --- a/include/linux/rcuclassic.h
> +++ b/include/linux/rcuclassic.h
> @@ -40,6 +40,10 @@
>  #include <linux/cpumask.h>
>  #include <linux/seqlock.h>
>  
> +#ifdef CONFIG_RCU_CPU_STALL_DETECTOR
> +#define RCU_SECONDS_TILL_STALL_CHECK	3 * HZ	/* for rcp->jiffies_stall */
> +#define RCU_SECONDS_TILL_STALL_RECHECK	30 * HZ	/* for rcp->jiffies_stall */
> +#endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
>  
>  /* Global control variables for rcupdate callback mechanism. */
>  struct rcu_ctrlblk {
> @@ -52,6 +56,11 @@ struct rcu_ctrlblk {
>  	spinlock_t	lock	____cacheline_internodealigned_in_smp;
>  	cpumask_t	cpumask; /* CPUs that need to switch in order    */
>  				 /* for current batch to proceed.        */
> +#ifdef CONFIG_RCU_CPU_STALL_DETECTOR
> +	unsigned long gp_start;	 /* Time at which GP started in jiffies. */
> +	unsigned long jiffies_stall;
> +				 /* Time at which to check for CPU stalls. */
> +#endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
>  } ____cacheline_internodealigned_in_smp;
>  
>  /* Is batch a before batch b ? */
> diff --git a/kernel/rcuclassic.c b/kernel/rcuclassic.c
> index aad93cd..a299876 100644
> --- a/kernel/rcuclassic.c
> +++ b/kernel/rcuclassic.c
> @@ -118,6 +118,87 @@ static inline void force_quiescent_state(struct rcu_data *rdp,
>  }
>  #endif
>  
> +#ifdef CONFIG_RCU_CPU_STALL_DETECTOR
> +
> +static void record_gp_stall_check_time(struct rcu_ctrlblk *rcp)
> +{
> +	rcp->gp_start = jiffies;
> +	rcp->jiffies_stall = jiffies + RCU_SECONDS_TILL_STALL_CHECK;
> +}
> +
> +static void print_other_cpu_stall(struct rcu_ctrlblk *rcp)
> +{
> +	int cpu;
> +	long delta;
> +	unsigned long flags;
> +
> +	/* Only let one CPU complain about others per time interval. */
> +
> +	spin_lock_irqsave(&rcp->lock, flags);
> +	delta = jiffies - rcp->jiffies_stall;
> +	if (delta < 2 || rcp->cur != rcp->completed) {

Is it (2 * HZ)?
should it be defined as macro?

quoted text
> +		spin_unlock_irqrestore(&rcp->lock, flags);
> +		return;
> +	}
> +	rcp->jiffies_stall = jiffies + RCU_SECONDS_TILL_STALL_RECHECK;
> +	spin_unlock_irqrestore(&rcp->lock, flags);
> +
> +	/* OK, time to rat on our buddy... */
> +
> +	printk(KERN_ERR "RCU detected CPU stalls:");
> +	for_each_possible_cpu(cpu) {
> +		if (cpu_isset(cpu, rcp->cpumask))
> +			printk(" %d", cpu);
> +	}
> +	printk(" (detected by %d, t=%ld jiffies)\n",
> +	       smp_processor_id(), (long)(jiffies - rcp->gp_start));
> +}
> +
> +static void print_cpu_stall(struct rcu_ctrlblk *rcp)
> +{
> +	unsigned long flags;
> +
> +	printk(KERN_ERR "RCU detected CPU %d stall (t=%lu/%lu jiffies)\n",
> +			smp_processor_id(), jiffies,
> +			jiffies - rcp->gp_start);
> +	dump_stack();
> +	spin_lock_irqsave(&rcp->lock, flags);
> +	if ((long)(jiffies - rcp->jiffies_stall) >= 0)
> +		rcp->jiffies_stall =
> +			jiffies + RCU_SECONDS_TILL_STALL_RECHECK;
> +	spin_unlock_irqrestore(&rcp->lock, flags);
> +	set_need_resched();  /* kick ourselves to get things going. */
> +}
> +
> +static void check_cpu_stall(struct rcu_ctrlblk *rcp)
> +{
> +	long delta;
> +
> +	delta = jiffies - rcp->jiffies_stall;
> +	if (cpu_isset(smp_processor_id(), rcp->cpumask) && delta >= 0) {
> +		
> +		/* We haven't checked in, so go dump stack. */
> +		print_cpu_stall(rcp);
> +
> +	} else if (rcp->cur != rcp->completed && delta >= 2) {
> +
> +		/* They had two seconds to dump stack, so complain. */

appear here again! and it's inconsistent with comment.

quoted text
> +		print_other_cpu_stall(rcp);
> +	}
> +}
> +
> +#else /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
> +
> +static void record_gp_stall_check_time(struct rcu_ctrlblk *rcp)
> +{
> +}
> +
> +static void check_cpu_stall(struct rcu_ctrlblk *rcp, struct rcu_data *rdp)
> +{
> +}
> +
> +#endif /* #else #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
> +
>  /**
>   * call_rcu - Queue an RCU callback for invocation after a grace period.
>   * @head: structure to be used for queueing the RCU updates.
> @@ -285,6 +366,7 @@ static void rcu_start_batch(struct rcu_ctrlblk *rcp)
>  		 */
>  		smp_wmb();
>  		rcp->cur++;
> +		record_gp_stall_check_time(rcp);
>  
>  		/*
>  		 * Accessing nohz_cpu_mask before incrementing rcp->cur needs a
> @@ -468,6 +550,9 @@ static void rcu_process_callbacks(struct softirq_action *unused)
>  
>  static int __rcu_pending(struct rcu_ctrlblk *rcp, struct rcu_data *rdp)
>  {
> +	/* Check for CPU stalls, if enabled. */
> +	check_cpu_stall(rcp);
> +
>  	/* This cpu has pending rcu entries and the grace period
>  	 * for them has completed.
>  	 */
> @@ -558,6 +643,9 @@ void rcu_check_callbacks(int cpu, int user)
>  static void rcu_init_percpu_data(int cpu, struct rcu_ctrlblk *rcp,
>  						struct rcu_data *rdp)
>  {
> +#ifdef CONFIG_DEBUG_RCU_STALL
> +	printk(KERN_INFO "RCU-based detection of stalled CPUs is enabled.\n");
> +#endif /* #ifdef CONFIG_DEBUG_RCU_STALL */
>  	memset(rdp, 0, sizeof(*rdp));
>  	rdp->curtail = &rdp->curlist;
>  	rdp->nxttail = &rdp->nxtlist;
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 0b50481..9fee969 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -597,6 +597,19 @@ config RCU_TORTURE_TEST_RUNNABLE
>  	  Say N here if you want the RCU torture tests to start only
>  	  after being manually enabled via /proc.
>  
> +config RCU_CPU_STALL_DETECTOR
> +	bool "Check for stalled CPUs delaying RCU grace periods"
> +	depends on CLASSIC_RCU
> +	default n
> +	help
> +	  This option causes RCU to printk information on which
> +	  CPUs are delaying the current grace period, but only when
> +	  the grace period extends for excessive time periods.
> +
> +	  Say Y if you want RCU to perform such checks.
> +
> +	  Say N if you are unsure.
> +
>  config KPROBES_SANITY_TEST
>  	bool "Kprobes sanity tests"
>  	depends on DEBUG_KERNEL
> 
> 
> 


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