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Date: Fri, 03 Oct 2008 18:12:28 +0800
From: Lai Jiangshan <laijs@...fujitsu.com>
To: paulmck@...ux.vnet.ibm.com
CC: Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
akpm@...ux-foundation.org, mathieu.desnoyers@...ymtl.ca,
manfred@...orfullife.com, dipankar@...ibm.com, niv@...ibm.com,
dvhltc@...ibm.com, josht@...ux.vnet.ibm.com
Subject: Re: [PATCH,RFC] RCU-based detection of stalled CPUs for Classic RCU
I found a magic number in it.
Paul E. McKenney wrote:
> 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@...ux.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?
> + 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.
> + 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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