From 67182ae1c42206e516f7efb292b745e826497b24 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Sun, 10 Aug 2008 18:35:38 -0700 Subject: rcu, debug: detect stalled grace periods this is a diagnostic patch for Classic RCU. The approach is to record a timestamp at the beginning of the grace period (in rcu_start_batch()), then have rcu_check_callbacks() complain if: 1. it is running on a CPU that has holding up grace periods for a long time (say one second). This will identify the culprit assuming that the culprit has not disabled hardware irqs, instruction execution, or some such. 2. it is running on a CPU that is not holding up grace periods, but grace periods have been held up for an even longer time (say two seconds). It is enabled via the default-off CONFIG_DEBUG_RCU_STALL kernel parameter. Rather than exponential backoff, it backs off to once per 30 seconds. My feeling upon thinking on it was that if you have stalled RCU grace periods for that long, a few extra printk() messages are probably the least of your worries... Signed-off-by: Paul E. McKenney Cc: Peter Zijlstra Cc: Yinghai Lu Cc: David Witbrodt Signed-off-by: Ingo Molnar --- lib/Kconfig.debug | 13 +++++++++++++ 1 file changed, 13 insertions(+) (limited to 'lib') diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug index e1d4764435e..2fb6d90bf1e 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 + 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 -- cgit v1.2.3-70-g09d2 From 2133b5d7ff531bc15a923db4a6a50bf96c561be9 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Thu, 2 Oct 2008 16:06:39 -0700 Subject: rcu: RCU-based detection of stalled CPUs for Classic RCU 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 to detect infinite loops in kernel code, 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, replacing the stall debug patch that is already in tip/core/rcu (commit 67182ae1c4). The changes from the patch in tip/core/rcu include making the config variable name match that in treercu, changing from seconds to jiffies to avoid spurious warnings, and printing a boot message when this feature is enabled. Signed-off-by: Paul E. McKenney Signed-off-by: Ingo Molnar --- include/linux/rcuclassic.h | 12 +++- kernel/rcuclassic.c | 166 +++++++++++++++++++++++---------------------- lib/Kconfig.debug | 2 +- 3 files changed, 96 insertions(+), 84 deletions(-) (limited to 'lib') diff --git a/include/linux/rcuclassic.h b/include/linux/rcuclassic.h index 29bf528c7dc..5f89b62e698 100644 --- a/include/linux/rcuclassic.h +++ b/include/linux/rcuclassic.h @@ -40,15 +40,21 @@ #include #include +#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 { long cur; /* Current batch number. */ long completed; /* Number of the last completed batch */ long pending; /* Number of the last pending batch */ -#ifdef CONFIG_DEBUG_RCU_STALL - unsigned long gp_check; /* Time grace period should end, in seconds. */ -#endif /* #ifdef CONFIG_DEBUG_RCU_STALL */ +#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 */ int signaled; diff --git a/kernel/rcuclassic.c b/kernel/rcuclassic.c index ed15128ca2c..0d07e6e5157 100644 --- a/kernel/rcuclassic.c +++ b/kernel/rcuclassic.c @@ -164,6 +164,87 @@ static void __call_rcu(struct rcu_head *head, struct rcu_ctrlblk *rcp, } } +#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) { + 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. */ + 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. @@ -293,84 +374,6 @@ static void rcu_do_batch(struct rcu_data *rdp) * period (if necessary). */ -#ifdef CONFIG_DEBUG_RCU_STALL - -static inline void record_gp_check_time(struct rcu_ctrlblk *rcp) -{ - rcp->gp_check = get_seconds() + 3; -} - -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->gp_check; - if (delta < 2L || cpus_empty(rcp->cpumask)) { - spin_unlock(&rcp->lock); - return; - } - rcp->gp_check = get_seconds() + 30; - spin_unlock_irqrestore(&rcp->lock, flags); - - /* OK, time to rat on our buddy... */ - - printk(KERN_ERR "RCU detected CPU stalls:"); - for_each_cpu_mask(cpu, rcp->cpumask) - printk(" %d", cpu); - printk(" (detected by %d, t=%lu/%lu)\n", - smp_processor_id(), get_seconds(), rcp->gp_check); -} - -static void print_cpu_stall(struct rcu_ctrlblk *rcp) -{ - unsigned long flags; - - printk(KERN_ERR "RCU detected CPU %d stall (t=%lu/%lu)\n", - smp_processor_id(), get_seconds(), rcp->gp_check); - dump_stack(); - spin_lock_irqsave(&rcp->lock, flags); - if ((long)(get_seconds() - rcp->gp_check) >= 0L) - rcp->gp_check = get_seconds() + 30; - spin_unlock_irqrestore(&rcp->lock, flags); -} - -static void check_cpu_stall(struct rcu_ctrlblk *rcp, struct rcu_data *rdp) -{ - long delta; - - delta = get_seconds() - rcp->gp_check; - if (cpu_isset(smp_processor_id(), rcp->cpumask) && delta >= 0L) { - - /* We haven't checked in, so go dump stack. */ - - print_cpu_stall(rcp); - - } else { - if (!cpus_empty(rcp->cpumask) && delta >= 2L) { - /* They had two seconds to dump stack, so complain. */ - print_other_cpu_stall(rcp); - } - } -} - -#else /* #ifdef CONFIG_DEBUG_RCU_STALL */ - -static inline void record_gp_check_time(struct rcu_ctrlblk *rcp) -{ -} - -static inline void -check_cpu_stall(struct rcu_ctrlblk *rcp, struct rcu_data *rdp) -{ -} - -#endif /* #else #ifdef CONFIG_DEBUG_RCU_STALL */ - /* * Register a new batch of callbacks, and start it up if there is currently no * active batch and the batch to be registered has not already occurred. @@ -381,7 +384,7 @@ static void rcu_start_batch(struct rcu_ctrlblk *rcp) if (rcp->cur != rcp->pending && rcp->completed == rcp->cur) { rcp->cur++; - record_gp_check_time(rcp); + record_gp_stall_check_time(rcp); /* * Accessing nohz_cpu_mask before incrementing rcp->cur needs a @@ -603,7 +606,7 @@ 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, rdp); + check_cpu_stall(rcp); if (rdp->nxtlist) { long completed_snap = ACCESS_ONCE(rcp->completed); @@ -769,6 +772,9 @@ static struct notifier_block __cpuinitdata rcu_nb = { */ void __init __rcu_init(void) { +#ifdef CONFIG_RCU_CPU_STALL_DETECTOR + printk(KERN_INFO "RCU-based detection of stalled CPUs is enabled.\n"); +#endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */ rcu_cpu_notify(&rcu_nb, CPU_UP_PREPARE, (void *)(long)smp_processor_id()); /* Register notifier for non-boot CPUs */ diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug index ccede1aeab3..9fee969dd60 100644 --- a/lib/Kconfig.debug +++ b/lib/Kconfig.debug @@ -597,7 +597,7 @@ 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 +config RCU_CPU_STALL_DETECTOR bool "Check for stalled CPUs delaying RCU grace periods" depends on CLASSIC_RCU default n -- cgit v1.2.3-70-g09d2