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rcu: Print scheduling-clock information on RCU CPU stall-warning mess…
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…ages

There have been situations where RCU CPU stall warnings were caused by
issues in scheduling-clock timer initialization.  To make it easier to
track these down, this commit causes the RCU CPU stall-warning messages
to print out the number of scheduling-clock interrupts taken in the
current grace period for each stalled CPU.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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Paul E. McKenney authored and Paul E. McKenney committed Feb 21, 2012
1 parent 13cfcca commit a858af2
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Showing 4 changed files with 194 additions and 14 deletions.
33 changes: 22 additions & 11 deletions kernel/rcutree.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -689,21 +689,16 @@ static void print_other_cpu_stall(struct rcu_state *rsp)
return;
}
rsp->jiffies_stall = jiffies + 3 * jiffies_till_stall_check() + 3;

/*
* Now rat on any tasks that got kicked up to the root rcu_node
* due to CPU offlining.
*/
ndetected = rcu_print_task_stall(rnp);
raw_spin_unlock_irqrestore(&rnp->lock, flags);

/*
* OK, time to rat on our buddy...
* See Documentation/RCU/stallwarn.txt for info on how to debug
* RCU CPU stall warnings.
*/
printk(KERN_ERR "INFO: %s detected stalls on CPUs/tasks: {",
printk(KERN_ERR "INFO: %s detected stalls on CPUs/tasks:",
rsp->name);
print_cpu_stall_info_begin();
rcu_for_each_leaf_node(rsp, rnp) {
raw_spin_lock_irqsave(&rnp->lock, flags);
ndetected += rcu_print_task_stall(rnp);
Expand All @@ -712,11 +707,22 @@ static void print_other_cpu_stall(struct rcu_state *rsp)
continue;
for (cpu = 0; cpu <= rnp->grphi - rnp->grplo; cpu++)
if (rnp->qsmask & (1UL << cpu)) {
printk(" %d", rnp->grplo + cpu);
print_cpu_stall_info(rsp, rnp->grplo + cpu);
ndetected++;
}
}
printk("} (detected by %d, t=%ld jiffies)\n",

/*
* Now rat on any tasks that got kicked up to the root rcu_node
* due to CPU offlining.
*/
rnp = rcu_get_root(rsp);
raw_spin_lock_irqsave(&rnp->lock, flags);
ndetected = rcu_print_task_stall(rnp);
raw_spin_unlock_irqrestore(&rnp->lock, flags);

print_cpu_stall_info_end();
printk(KERN_CONT "(detected by %d, t=%ld jiffies)\n",
smp_processor_id(), (long)(jiffies - rsp->gp_start));
if (ndetected == 0)
printk(KERN_ERR "INFO: Stall ended before state dump start\n");
Expand All @@ -740,8 +746,11 @@ static void print_cpu_stall(struct rcu_state *rsp)
* See Documentation/RCU/stallwarn.txt for info on how to debug
* RCU CPU stall warnings.
*/
printk(KERN_ERR "INFO: %s detected stall on CPU %d (t=%lu jiffies)\n",
rsp->name, smp_processor_id(), jiffies - rsp->gp_start);
printk(KERN_ERR "INFO: %s self-detected stall on CPU", rsp->name);
print_cpu_stall_info_begin();
print_cpu_stall_info(rsp, smp_processor_id());
print_cpu_stall_info_end();
printk(KERN_CONT " (t=%lu jiffies)\n", jiffies - rsp->gp_start);
if (!trigger_all_cpu_backtrace())
dump_stack();

Expand Down Expand Up @@ -831,6 +840,7 @@ static void __note_new_gpnum(struct rcu_state *rsp, struct rcu_node *rnp, struct
rdp->passed_quiesce = 0;
} else
rdp->qs_pending = 0;
zero_cpu_stall_ticks(rdp);
}
}

Expand Down Expand Up @@ -1496,6 +1506,7 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
void rcu_check_callbacks(int cpu, int user)
{
trace_rcu_utilization("Start scheduler-tick");
increment_cpu_stall_ticks();
if (user || rcu_is_cpu_rrupt_from_idle()) {

/*
Expand Down
11 changes: 11 additions & 0 deletions kernel/rcutree.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -239,6 +239,12 @@ struct rcu_data {
bool preemptible; /* Preemptible RCU? */
struct rcu_node *mynode; /* This CPU's leaf of hierarchy */
unsigned long grpmask; /* Mask to apply to leaf qsmask. */
#ifdef CONFIG_RCU_CPU_STALL_INFO
unsigned long ticks_this_gp; /* The number of scheduling-clock */
/* ticks this CPU has handled */
/* during and after the last grace */
/* period it is aware of. */
#endif /* #ifdef CONFIG_RCU_CPU_STALL_INFO */

/* 2) batch handling */
/*
Expand Down Expand Up @@ -466,5 +472,10 @@ static void __cpuinit rcu_prepare_kthreads(int cpu);
static void rcu_prepare_for_idle_init(int cpu);
static void rcu_cleanup_after_idle(int cpu);
static void rcu_prepare_for_idle(int cpu);
static void print_cpu_stall_info_begin(void);
static void print_cpu_stall_info(struct rcu_state *rsp, int cpu);
static void print_cpu_stall_info_end(void);
static void zero_cpu_stall_ticks(struct rcu_data *rdp);
static void increment_cpu_stall_ticks(void);

#endif /* #ifndef RCU_TREE_NONCORE */
150 changes: 147 additions & 3 deletions kernel/rcutree_plugin.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -63,7 +63,10 @@ static void __init rcu_bootup_announce_oddness(void)
printk(KERN_INFO "\tRCU torture testing starts during boot.\n");
#endif
#if defined(CONFIG_TREE_PREEMPT_RCU) && !defined(CONFIG_RCU_CPU_STALL_VERBOSE)
printk(KERN_INFO "\tVerbose stalled-CPUs detection is disabled.\n");
printk(KERN_INFO "\tDump stacks of tasks blocking RCU-preempt GP.\n");
#endif
#if defined(CONFIG_RCU_CPU_STALL_INFO)
printk(KERN_INFO "\tAdditional per-CPU info printed with stalls.\n");
#endif
#if NUM_RCU_LVL_4 != 0
printk(KERN_INFO "\tExperimental four-level hierarchy is enabled.\n");
Expand Down Expand Up @@ -490,6 +493,31 @@ static void rcu_print_detail_task_stall(struct rcu_state *rsp)

#endif /* #else #ifdef CONFIG_RCU_CPU_STALL_VERBOSE */

#ifdef CONFIG_RCU_CPU_STALL_INFO

static void rcu_print_task_stall_begin(struct rcu_node *rnp)
{
printk(KERN_ERR "\tTasks blocked on level-%d rcu_node (CPUs %d-%d):",
rnp->level, rnp->grplo, rnp->grphi);
}

static void rcu_print_task_stall_end(void)
{
printk(KERN_CONT "\n");
}

#else /* #ifdef CONFIG_RCU_CPU_STALL_INFO */

static void rcu_print_task_stall_begin(struct rcu_node *rnp)
{
}

static void rcu_print_task_stall_end(void)
{
}

#endif /* #else #ifdef CONFIG_RCU_CPU_STALL_INFO */

/*
* Scan the current list of tasks blocked within RCU read-side critical
* sections, printing out the tid of each.
Expand All @@ -501,12 +529,14 @@ static int rcu_print_task_stall(struct rcu_node *rnp)

if (!rcu_preempt_blocked_readers_cgp(rnp))
return 0;
rcu_print_task_stall_begin(rnp);
t = list_entry(rnp->gp_tasks,
struct task_struct, rcu_node_entry);
list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
printk(" P%d", t->pid);
printk(KERN_CONT " P%d", t->pid);
ndetected++;
}
rcu_print_task_stall_end();
return ndetected;
}

Expand Down Expand Up @@ -2004,7 +2034,7 @@ static void rcu_cleanup_after_idle(int cpu)
}

/*
* Do the idle-entry grace-period work, which, because CONFIG_RCU_FAST_NO_HZ=y,
* Do the idle-entry grace-period work, which, because CONFIG_RCU_FAST_NO_HZ=n,
* is nothing.
*/
static void rcu_prepare_for_idle(int cpu)
Expand Down Expand Up @@ -2273,3 +2303,117 @@ static void rcu_prepare_for_idle(int cpu)
}

#endif /* #else #if !defined(CONFIG_RCU_FAST_NO_HZ) */

#ifdef CONFIG_RCU_CPU_STALL_INFO

#ifdef CONFIG_RCU_FAST_NO_HZ

static void print_cpu_stall_fast_no_hz(char *cp, int cpu)
{
struct hrtimer *hrtp = &per_cpu(rcu_idle_gp_timer, cpu);

sprintf(cp, "drain=%d %c timer=%lld",
per_cpu(rcu_dyntick_drain, cpu),
per_cpu(rcu_dyntick_holdoff, cpu) == jiffies ? 'H' : '.',
hrtimer_active(hrtp)
? ktime_to_us(hrtimer_get_remaining(hrtp))
: -1);
}

#else /* #ifdef CONFIG_RCU_FAST_NO_HZ */

static void print_cpu_stall_fast_no_hz(char *cp, int cpu)
{
}

#endif /* #else #ifdef CONFIG_RCU_FAST_NO_HZ */

/* Initiate the stall-info list. */
static void print_cpu_stall_info_begin(void)
{
printk(KERN_CONT "\n");
}

/*
* Print out diagnostic information for the specified stalled CPU.
*
* If the specified CPU is aware of the current RCU grace period
* (flavor specified by rsp), then print the number of scheduling
* clock interrupts the CPU has taken during the time that it has
* been aware. Otherwise, print the number of RCU grace periods
* that this CPU is ignorant of, for example, "1" if the CPU was
* aware of the previous grace period.
*
* Also print out idle and (if CONFIG_RCU_FAST_NO_HZ) idle-entry info.
*/
static void print_cpu_stall_info(struct rcu_state *rsp, int cpu)
{
char fast_no_hz[72];
struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
struct rcu_dynticks *rdtp = rdp->dynticks;
char *ticks_title;
unsigned long ticks_value;

if (rsp->gpnum == rdp->gpnum) {
ticks_title = "ticks this GP";
ticks_value = rdp->ticks_this_gp;
} else {
ticks_title = "GPs behind";
ticks_value = rsp->gpnum - rdp->gpnum;
}
print_cpu_stall_fast_no_hz(fast_no_hz, cpu);
printk(KERN_ERR "\t%d: (%lu %s) idle=%03x/%llx/%d %s\n",
cpu, ticks_value, ticks_title,
atomic_read(&rdtp->dynticks) & 0xfff,
rdtp->dynticks_nesting, rdtp->dynticks_nmi_nesting,
fast_no_hz);
}

/* Terminate the stall-info list. */
static void print_cpu_stall_info_end(void)
{
printk(KERN_ERR "\t");
}

/* Zero ->ticks_this_gp for all flavors of RCU. */
static void zero_cpu_stall_ticks(struct rcu_data *rdp)
{
rdp->ticks_this_gp = 0;
}

/* Increment ->ticks_this_gp for all flavors of RCU. */
static void increment_cpu_stall_ticks(void)
{
__get_cpu_var(rcu_sched_data).ticks_this_gp++;
__get_cpu_var(rcu_bh_data).ticks_this_gp++;
#ifdef CONFIG_TREE_PREEMPT_RCU
__get_cpu_var(rcu_preempt_data).ticks_this_gp++;
#endif /* #ifdef CONFIG_TREE_PREEMPT_RCU */
}

#else /* #ifdef CONFIG_RCU_CPU_STALL_INFO */

static void print_cpu_stall_info_begin(void)
{
printk(KERN_CONT " {");
}

static void print_cpu_stall_info(struct rcu_state *rsp, int cpu)
{
printk(KERN_CONT " %d", cpu);
}

static void print_cpu_stall_info_end(void)
{
printk(KERN_CONT "} ");
}

static void zero_cpu_stall_ticks(struct rcu_data *rdp)
{
}

static void increment_cpu_stall_ticks(void)
{
}

#endif /* #else #ifdef CONFIG_RCU_CPU_STALL_INFO */
14 changes: 14 additions & 0 deletions lib/Kconfig.debug
View file
Original file line number Diff line number Diff line change
Expand Up @@ -927,6 +927,20 @@ config RCU_CPU_STALL_VERBOSE

Say Y if you want to enable such checks.

config RCU_CPU_STALL_INFO
bool "Print additional diagnostics on RCU CPU stall"
depends on (TREE_RCU || TREE_PREEMPT_RCU) && DEBUG_KERNEL
default n
help
For each stalled CPU that is aware of the current RCU grace
period, print out additional per-CPU diagnostic information
regarding scheduling-clock ticks, idle state, and,
for RCU_FAST_NO_HZ kernels, idle-entry state.

Say N if you are unsure.

Say Y if you want to enable such diagnostics.

config RCU_TRACE
bool "Enable tracing for RCU"
depends on DEBUG_KERNEL
Expand Down

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