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rcu: RCU-based detection of stalled CPUs for Classic RCU
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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 67182ae).

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 <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
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@paulmck
paulmck authored and Ingo Molnar committed Oct 3, 2008
1 parent b5259d9 commit 2133b5d
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Showing 3 changed files with 96 additions and 84 deletions.
12 changes: 9 additions & 3 deletions include/linux/rcuclassic.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -40,15 +40,21 @@
#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 {
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;

Expand Down
166 changes: 86 additions & 80 deletions kernel/rcuclassic.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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.
Expand Down Expand Up @@ -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.
Expand All @@ -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
Expand Down Expand Up @@ -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);
Expand Down Expand Up @@ -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 */
Expand Down
2 changes: 1 addition & 1 deletion lib/Kconfig.debug
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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
Expand Down

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