[PATCH v3 0/4] rcu: Add RCU stall diagnosis information

[PATCH v3 0/4] rcu: Add RCU stall diagnosis information

[Zhen Lei]

v2 --> v3:
1. Fix the return type of kstat_cpu_irqs_sum()
2. Add Kconfig option CONFIG_RCU_CPU_STALL_DEEP_DEBUG and boot parameter
   rcupdate.rcu_cpu_stall_deep_debug.
3. Add comments and normalize local variable name


v1 --> v2:
1. Fixed a bug in the code. If the rcu stall is detected by another CPU,
   kcpustat_this_cpu cannot be used.
@@ -451,7 +451,7 @@ static void print_cpu_stat_info(int cpu)
        if (r->gp_seq != rdp->gp_seq)
                return;

-       cpustat = kcpustat_this_cpu->cpustat;
+       cpustat = kcpustat_cpu(cpu).cpustat;
2. Move the start point of statistics from rcu_stall_kick_kthreads() to
   rcu_implicit_dynticks_qs(), removing the dependency on irq_work.

v1:
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.

Zhen Lei (4):
  genirq: Fix the return type of kstat_cpu_irqs_sum()
  sched: Add helper kstat_cpu_softirqs_sum()
  sched: Add helper nr_context_switches_cpu()
  rcu: Add RCU stall diagnosis information

 .../admin-guide/kernel-parameters.txt         |  3 ++
 include/linux/kernel_stat.h                   | 14 ++++++++-
 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 ++
 kernel/sched/core.c                           |  5 +++
 9 files changed, 98 insertions(+), 1 deletion(-)

-- 
2.25.1

[PATCH v3 1/4] genirq: Fix the return type of kstat_cpu_irqs_sum()

[Zhen Lei]

The type of member irqs_sum is unsigned long, so kstat_cpu_irqs_sum()
should also return unsigned long. Otherwise, the value maybe truncated.

Fixes: f2c66cd8eedd ("/proc/stat: scalability of irq num per cpu")
Reported-by: Elliott, Robert (Servers) <elliott@hpe.com>
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
---
 include/linux/kernel_stat.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/kernel_stat.h b/include/linux/kernel_stat.h
index ddb5a358fd829f4..90e2fdc17d79ff8 100644
--- a/include/linux/kernel_stat.h
+++ b/include/linux/kernel_stat.h
@@ -75,7 +75,7 @@ extern unsigned int kstat_irqs_usr(unsigned int irq);
 /*
  * Number of interrupts per cpu, since bootup
  */
-static inline unsigned int kstat_cpu_irqs_sum(unsigned int cpu)
+static inline unsigned long kstat_cpu_irqs_sum(unsigned int cpu)
 {
 	return kstat_cpu(cpu).irqs_sum;
 }
-- 
2.25.1

[PATCH v3 2/4] sched: Add helper kstat_cpu_softirqs_sum()

[Zhen Lei]

Similar to kstat_cpu_irqs_sum(), it counts the sum of all software
interrupts on a specified CPU.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
---
 include/linux/kernel_stat.h | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/include/linux/kernel_stat.h b/include/linux/kernel_stat.h
index 90e2fdc17d79ff8..898076e173a928a 100644
--- a/include/linux/kernel_stat.h
+++ b/include/linux/kernel_stat.h
@@ -67,6 +67,17 @@ static inline unsigned int kstat_softirqs_cpu(unsigned int irq, int cpu)
        return kstat_cpu(cpu).softirqs[irq];
 }
 
+static inline unsigned int kstat_cpu_softirqs_sum(int cpu)
+{
+	int i;
+	unsigned int sum = 0;
+
+	for (i = 0; i < NR_SOFTIRQS; i++)
+		sum += kstat_softirqs_cpu(i, cpu);
+
+	return sum;
+}
+
 /*
  * Number of interrupts per specific IRQ source, since bootup
  */
-- 
2.25.1

[PATCH v3 3/4] sched: Add helper nr_context_switches_cpu()

[Zhen Lei]

Returns the number of context switches on the specified CPU, which can be
used to diagnose rcu stall.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
---
 include/linux/kernel_stat.h | 1 +
 kernel/sched/core.c         | 5 +++++
 2 files changed, 6 insertions(+)

diff --git a/include/linux/kernel_stat.h b/include/linux/kernel_stat.h
index 898076e173a928a..9935f7ecbfb9e31 100644
--- a/include/linux/kernel_stat.h
+++ b/include/linux/kernel_stat.h
@@ -52,6 +52,7 @@ DECLARE_PER_CPU(struct kernel_cpustat, kernel_cpustat);
 #define kstat_cpu(cpu) per_cpu(kstat, cpu)
 #define kcpustat_cpu(cpu) per_cpu(kernel_cpustat, cpu)
 
+extern unsigned long long nr_context_switches_cpu(int cpu);
 extern unsigned long long nr_context_switches(void);
 
 extern unsigned int kstat_irqs_cpu(unsigned int irq, int cpu);
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 06e268c15f14ae4..380c521170d68f3 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5234,6 +5234,11 @@ bool single_task_running(void)
 }
 EXPORT_SYMBOL(single_task_running);
 
+unsigned long long nr_context_switches_cpu(int cpu)
+{
+	return cpu_rq(cpu)->nr_switches;
+}
+
 unsigned long long nr_context_switches(void)
 {
 	int i;
-- 
2.25.1

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

[Zhen Lei]

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@huawei.com>
---
 .../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

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

[Paul E. McKenney]

On Thu, Nov 03, 2022 at 08:35:15PM +0800, Zhen Lei wrote:
> 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@huawei.com>

Much better, thank you!

How about rcu_cpu_stall_cputime instead of rcu_cpu_stall_deep_debug and
RCU_CPU_STALL_CPUTIME instead of RCU_CPU_STALL_DEEP_DEBUG?

Just to forestall things like rcu_cpu_stall_deeper_debug, to say nothing
of rcu_cpu_stall_deepest_debug or rcu_cpu_stall_abyssal_debug.  ;-)

							Thanx, Paul

> ---
>  .../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
> 

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

[Leizhen (ThunderTown)]

On 2022/11/4 1:28, Paul E. McKenney wrote:
> On Thu, Nov 03, 2022 at 08:35:15PM +0800, Zhen Lei wrote:
>> 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@huawei.com>
> 
> Much better, thank you!
> 
> How about rcu_cpu_stall_cputime instead of rcu_cpu_stall_deep_debug and
> RCU_CPU_STALL_CPUTIME instead of RCU_CPU_STALL_DEEP_DEBUG?

Very well. I'll update and post v5 today.

> 
> Just to forestall things like rcu_cpu_stall_deeper_debug, to say nothing
> of rcu_cpu_stall_deepest_debug or rcu_cpu_stall_abyssal_debug.  ;-)

Ha ha! Right.

> 
> 							Thanx, Paul
> 
>> ---
>>  .../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
>>
> .
> 

-- 
Regards,
  Zhen Lei