[PATCH v3 4/4] rcu: Add RCU stall diagnosis information

From: Zhen Lei
Date: Thu Nov 03 2022 - 08:37:34 EST

In some extreme cases, such as the I/O pressure test, the CPU usage may
be 100%, causing RCU stall. In this case, the printed information about
current is not useful. Displays the number and usage of hard interrupts,
soft interrupts, and context switches that are generated within half of
the CPU stall timeout, can help us make a general judgment. In other
cases, we can preliminarily determine whether an infinite loop occurs
when local_irq, local_bh or preempt is disabled.

For example:
rcu: INFO: rcu_preempt self-detected stall on CPU
rcu: 0-....: (1250 ticks this GP) <omitted>
rcu: hardirqs softirqs csw/system
rcu: number: 624 45 0
rcu: cputime: 69 1 2425 ==> 2500(ms)

The example above shows that the number of hard and soft interrupts is
small, there is zero context switching, and the system takes up a lot of
time. We can quickly conclude that the current task is infinitely looped
with preempt_disable().

The impact on system performance is negligible because snapshot is
recorded only one time after 1/2 CPU stall timeout.

This enhanced debugging information is suppressed by default and can be
enabled by CONFIG_RCU_CPU_STALL_DEEP_DEBUG=y or
rcupdate.rcu_cpu_stall_deep_debug=1.

Signed-off-by: Zhen Lei <thunder.leizhen@xxxxxxxxxx>
---
.../admin-guide/kernel-parameters.txt | 3 ++
kernel/rcu/Kconfig.debug | 10 ++++++
kernel/rcu/rcu.h | 1 +
kernel/rcu/tree.c | 16 ++++++++++
kernel/rcu/tree.h | 17 ++++++++++
kernel/rcu/tree_stall.h | 31 +++++++++++++++++++
kernel/rcu/update.c | 2 ++
7 files changed, 80 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index a465d5242774af8..fc619e9e8d056da 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5082,6 +5082,9 @@
rcupdate.rcu_cpu_stall_timeout to be used (after
conversion from seconds to milliseconds).

+ rcupdate.rcu_cpu_stall_deep_debug= [KNL]
+ Provide additional rcu stall debug information.
+
rcupdate.rcu_expedited= [KNL]
Use expedited grace-period primitives, for
example, synchronize_rcu_expedited() instead
diff --git a/kernel/rcu/Kconfig.debug b/kernel/rcu/Kconfig.debug
index 1b0c41d490f0588..7b18763c574cb13 100644
--- a/kernel/rcu/Kconfig.debug
+++ b/kernel/rcu/Kconfig.debug
@@ -95,6 +95,16 @@ config RCU_EXP_CPU_STALL_TIMEOUT
says to use the RCU_CPU_STALL_TIMEOUT value converted from
seconds to milliseconds.

+config RCU_CPU_STALL_DEEP_DEBUG
+ bool "Provide additional rcu stall debug information"
+ depends on RCU_STALL_COMMON
+ default n
+ help
+ Statistics during the period from RCU_CPU_STALL_TIMEOUT/2 to
+ RCU_CPU_STALL_TIMEOUT, such as the number of (hard interrupts, soft
+ interrupts, task switches) and the cputime of (hard interrupts, soft
+ interrupts, kerenl tasks) are added to the rcu stall report.
+
config RCU_TRACE
bool "Enable tracing for RCU"
depends on DEBUG_KERNEL
diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index be5979da07f5985..2445b0c58348400 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -224,6 +224,7 @@ extern int rcu_cpu_stall_ftrace_dump;
extern int rcu_cpu_stall_suppress;
extern int rcu_cpu_stall_timeout;
extern int rcu_exp_cpu_stall_timeout;
+extern int rcu_cpu_stall_deep_debug;
int rcu_jiffies_till_stall_check(void);
int rcu_exp_jiffies_till_stall_check(void);

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 93416afebd59c7a..a90d2e2e5bbd99e 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -931,6 +931,22 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
rdp->rcu_iw_gp_seq = rnp->gp_seq;
irq_work_queue_on(&rdp->rcu_iw, rdp->cpu);
}
+
+ if (rcu_cpu_stall_deep_debug && rdp->snap_record.gp_seq != rdp->gp_seq) {
+ u64 *cpustat;
+ struct rcu_snap_record *rsrp;
+
+ cpustat = kcpustat_cpu(rdp->cpu).cpustat;
+
+ rsrp = &rdp->snap_record;
+ rsrp->cputime_irq = cpustat[CPUTIME_IRQ];
+ rsrp->cputime_softirq = cpustat[CPUTIME_SOFTIRQ];
+ rsrp->cputime_system = cpustat[CPUTIME_SYSTEM];
+ rsrp->nr_hardirqs = kstat_cpu_irqs_sum(rdp->cpu);
+ rsrp->nr_softirqs = kstat_cpu_softirqs_sum(rdp->cpu);
+ rsrp->nr_csw = nr_context_switches_cpu(rdp->cpu);
+ rsrp->gp_seq = rdp->gp_seq;
+ }
}

return 0;
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index d4a97e40ea9c3e2..44d49819a31f050 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -158,6 +158,22 @@ union rcu_noqs {
u16 s; /* Set of bits, aggregate OR here. */
};

+/*
+ * Record the snapshot of the core stats at 1/2 rcu stall timeout. The member
+ * gp_seq is used to ensure that all members are updated only once during the
+ * second half period. The snapshot is taken only if this gp_seq is not equal
+ * to rdp->gp_seq.
+ */
+struct rcu_snap_record {
+ unsigned long gp_seq; /* Track rdp->gp_seq counter */
+ u64 cputime_irq; /* Accumulated cputime of hard irqs */
+ u64 cputime_softirq;/* Accumulated cputime of soft irqs */
+ u64 cputime_system; /* Accumulated cputime of kernel tasks */
+ unsigned long nr_hardirqs; /* Accumulated number of hard irqs */
+ unsigned int nr_softirqs; /* Accumulated number of soft irqs */
+ unsigned long long nr_csw; /* Accumulated number of task switches */
+};
+
/* Per-CPU data for read-copy update. */
struct rcu_data {
/* 1) quiescent-state and grace-period handling : */
@@ -262,6 +278,7 @@ struct rcu_data {
short rcu_onl_gp_flags; /* ->gp_flags at last online. */
unsigned long last_fqs_resched; /* Time of last rcu_resched(). */
unsigned long last_sched_clock; /* Jiffies of last rcu_sched_clock_irq(). */
+ struct rcu_snap_record snap_record; /* Snapshot of core stats at 1/2 rcu stall timeout */

int cpu;
};
diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index 5653560573e22d6..5b258bcdb25ba04 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -428,6 +428,35 @@ static bool rcu_is_rcuc_kthread_starving(struct rcu_data *rdp, unsigned long *jp
return j > 2 * HZ;
}

+static void print_cpu_stat_info(int cpu)
+{
+ u64 *cpustat;
+ unsigned long half_timeout;
+ struct rcu_snap_record *rsrp;
+ struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
+
+ if (!rcu_cpu_stall_deep_debug)
+ return;
+
+ rsrp = &rdp->snap_record;
+ if (rsrp->gp_seq != rdp->gp_seq)
+ return;
+
+ cpustat = kcpustat_cpu(cpu).cpustat;
+ half_timeout = rcu_jiffies_till_stall_check() / 2;
+
+ pr_err(" hardirqs softirqs csw/system\n");
+ pr_err(" number: %8ld %10d %12lld\n",
+ kstat_cpu_irqs_sum(cpu) - rsrp->nr_hardirqs,
+ kstat_cpu_softirqs_sum(cpu) - rsrp->nr_softirqs,
+ nr_context_switches_cpu(cpu) - rsrp->nr_csw);
+ pr_err("cputime: %8lld %10lld %12lld ==> %lld(ms)\n",
+ div_u64(cpustat[CPUTIME_IRQ] - rsrp->cputime_irq, NSEC_PER_MSEC),
+ div_u64(cpustat[CPUTIME_SOFTIRQ] - rsrp->cputime_softirq, NSEC_PER_MSEC),
+ div_u64(cpustat[CPUTIME_SYSTEM] - rsrp->cputime_system, NSEC_PER_MSEC),
+ jiffies64_to_msecs(half_timeout));
+}
+
/*
* Print out diagnostic information for the specified stalled CPU.
*
@@ -484,6 +513,8 @@ static void print_cpu_stall_info(int cpu)
data_race(rcu_state.n_force_qs) - rcu_state.n_force_qs_gpstart,
rcuc_starved ? buf : "",
falsepositive ? " (false positive?)" : "");
+
+ print_cpu_stat_info(cpu);
}

/* Complain about starvation of grace-period kthread. */
diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index 738842c4886b235..9c68c88a700f185 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -508,6 +508,8 @@ int rcu_cpu_stall_timeout __read_mostly = CONFIG_RCU_CPU_STALL_TIMEOUT;
module_param(rcu_cpu_stall_timeout, int, 0644);
int rcu_exp_cpu_stall_timeout __read_mostly = CONFIG_RCU_EXP_CPU_STALL_TIMEOUT;
module_param(rcu_exp_cpu_stall_timeout, int, 0644);
+int rcu_cpu_stall_deep_debug __read_mostly = IS_ENABLED(CONFIG_RCU_CPU_STALL_DEEP_DEBUG);
+module_param(rcu_cpu_stall_deep_debug, int, 0644);
#endif /* #ifdef CONFIG_RCU_STALL_COMMON */

// Suppress boot-time RCU CPU stall warnings and rcutorture writer stall
--
2.25.1