Messages in this thread Patch in this message | | | Date | Wed, 1 Oct 2008 17:36:28 -0700 | From | "Paul E. McKenney" <> | Subject | [PATCH,RFC] RCU-based detection of stalled CPUs for Classic RCU |
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Hello!
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.
One current shortcoming: on some systems, stalls are detected during early boot, when we normally would not care about them. My thought is to add a call from late initialization to suppress stall detection until the system is well along its way to being booted, but thought I should check to see if there might already be something for this purpose.
(Currently against 2.6.27-rc8, FYI.)
Thoughts?
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..9b62e9a 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 /* for rcp->seconds_stall */ +#define RCU_SECONDS_TILL_STALL_RECHECK 30 /* for rcp->seconds_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 seconds_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..c092ba9 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->seconds_stall = get_seconds() + 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 = get_seconds() - rcp->seconds_stall; + if (delta < 2 || rcp->cur != rcp->completed) { + spin_unlock_irqrestore(&rcp->lock, flags); + return; + } + rcp->seconds_stall = get_seconds() + 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(), get_seconds(), + jiffies - rcp->gp_start); + dump_stack(); + spin_lock_irqsave(&rcp->lock, flags); + if ((long)(get_seconds() - rcp->seconds_stall) >= 0) + rcp->seconds_stall = + get_seconds() + 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 = get_seconds() - rcp->seconds_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. */ + 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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