[PATCHv3 0/2] *** Detect interrupt storm in softlockup ***

[PATCHv3 0/2] *** Detect interrupt storm in softlockup ***

[Bitao Hu]

Hi, guys.
I have implemented a low-overhead method for detecting interrupt storm
in softlockup. Please review it, all comments are welcome.

Changes from v2 to v3:

- From Liu Song, using enum instead of macro for cpu_stats, shortening
the name 'idx_to_stat' to 'stats', adding 'get_16bit_precesion' instead
of using right shift operations, and using 'struct irq_counts'.

- From kernel robot test, using '__this_cpu_read' and '__this_cpu_write'
instead of accessing to an per-cpu array directly, in order to avoid
this warning.
'sparse: incorrect type in initializer (different modifiers)'.

Changes from v1 to v2:

- From Douglas, optimize the memory of cpustats. With the maximum number
of CPUs, that's now this.
2 * 8192 * 4 + 1 * 8192 * 5 * 4 + 1 * 8192 = 237,568 bytes.

- From Liu Song, refactor the code format and add necessary comments.

- From Douglas, use interrupt counts instead of interrupt time to
determine the cause of softlockup.

- Remove the cmdline parameter added in PATCHv1.

Bitao Hu (2):
  watchdog/softlockup: low-overhead detection of interrupt storm
  watchdog/softlockup: report the most frequent interrupts

 kernel/watchdog.c | 238 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 238 insertions(+)

-- 
2.37.1 (Apple Git-137.1)

[PATCHv3 1/2] watchdog/softlockup: low-overhead detection of interrupt storm

[Bitao Hu]

The following softlockup is caused by interrupt storm, but it cannot be
identified from the call tree. Because the call tree is just a snapshot
and doesn't fully capture the behavior of the CPU during the soft lockup.
  watchdog: BUG: soft lockup - CPU#28 stuck for 23s! [fio:83921]
  ...
  Call trace:
    __do_softirq+0xa0/0x37c
    __irq_exit_rcu+0x108/0x140
    irq_exit+0x14/0x20
    __handle_domain_irq+0x84/0xe0
    gic_handle_irq+0x80/0x108
    el0_irq_naked+0x50/0x58

Therefore，I think it is necessary to report CPU utilization during the
softlockup_thresh period (report once every sample_period, for a total
of 5 reportings), like this:
  watchdog: BUG: soft lockup - CPU#28 stuck for 23s! [fio:83921]
  CPU#28 Utilization every 4s during lockup:
    #1: 0% system, 0% softirq, 100% hardirq, 0% idle
    #2: 0% system, 0% softirq, 100% hardirq, 0% idle
    #3: 0% system, 0% softirq, 100% hardirq, 0% idle
    #4: 0% system, 0% softirq, 100% hardirq, 0% idle
    #5: 0% system, 0% softirq, 100% hardirq, 0% idle
  ...

This would be helpful in determining whether an interrupt storm has
occurred or in identifying the cause of the softlockup. The criteria for
determination are as follows:
  a. If the hardirq utilization is high, then interrupt storm should be
  considered and the root cause cannot be determined from the call tree.
  b. If the softirq utilization is high, then we could analyze the call
  tree but it may cannot reflect the root cause.
  c. If the system utilization is high, then we could analyze the root
  cause from the call tree.

Signed-off-by: Bitao Hu <yaoma@linux.alibaba.com>
---
 kernel/watchdog.c | 84 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 84 insertions(+)

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 81a8862295d6..046507be4eb5 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -23,6 +23,8 @@
 #include <linux/sched/debug.h>
 #include <linux/sched/isolation.h>
 #include <linux/stop_machine.h>
+#include <linux/kernel_stat.h>
+#include <linux/math64.h>
 
 #include <asm/irq_regs.h>
 #include <linux/kvm_para.h>
@@ -441,6 +443,85 @@ static int is_softlockup(unsigned long touch_ts,
 	return 0;
 }
 
+#ifdef CONFIG_IRQ_TIME_ACCOUNTING
+#define NUM_STATS_GROUPS	5
+enum stats_per_group {
+	STATS_SYSTEM,
+	STATS_SOFTIRQ,
+	STATS_HARDIRQ,
+	STATS_IDLE,
+	NUM_STATS_PER_GROUP,
+};
+static enum cpu_usage_stat stats[NUM_STATS_PER_GROUP] = {
+	CPUTIME_SYSTEM,
+	CPUTIME_SOFTIRQ,
+	CPUTIME_IRQ,
+	CPUTIME_IDLE,
+};
+static DEFINE_PER_CPU(u16, cpustat_old[NUM_STATS_PER_GROUP]);
+static DEFINE_PER_CPU(u8, cpustat_utilization[NUM_STATS_GROUPS][NUM_STATS_PER_GROUP]);
+static DEFINE_PER_CPU(u8, cpustat_tail);
+
+/*
+ * We don't need nanosecond resolution. A granularity of 16ms is
+ * sufficient for our precision, allowing us to use u16 to store
+ * cpustats, which will roll over roughly every ~1000 seconds.
+ * 2^24 ~= 16 * 10^6
+ */
+static u16 get_16bit_precision(u64 data)
+{
+	return data >> 24LL; /* 2^24ns ~= 16.8ms */
+}
+
+static void update_cpustat(void)
+{
+	u8 i;
+	u16 old;
+	u8 utilization;
+	u8 tail = __this_cpu_read(cpustat_tail);
+	struct kernel_cpustat kcpustat;
+	u64 *cpustat = kcpustat.cpustat;
+	u16 sample_period_ms = get_16bit_precision(sample_period);
+
+	kcpustat_cpu_fetch(&kcpustat, smp_processor_id());
+	for (i = STATS_SYSTEM; i < NUM_STATS_PER_GROUP; i++) {
+		old = __this_cpu_read(cpustat_old[i]);
+		cpustat[stats[i]] = get_16bit_precision(cpustat[stats[i]]);
+		utilization = 100 * (u16)(cpustat[stats[i]] - old) / sample_period_ms;
+		__this_cpu_write(cpustat_utilization[tail][i], utilization);
+		__this_cpu_write(cpustat_old[i], cpustat[stats[i]]);
+	}
+	__this_cpu_write(cpustat_tail, (tail + 1) % NUM_STATS_GROUPS);
+}
+
+static void print_cpustat(void)
+{
+	u8 i, j;
+	u8 tail = __this_cpu_read(cpustat_tail);
+	u64 sample_period_second = sample_period;
+
+	do_div(sample_period_second, NSEC_PER_SEC);
+	/*
+	 * We do not want the "watchdog: " prefix on every line,
+	 * hence we use "printk" instead of "pr_crit".
+	 */
+	printk(KERN_CRIT "CPU#%d Utilization every %llus during lockup:\n",
+		smp_processor_id(), sample_period_second);
+	for (j = STATS_SYSTEM, i = tail; j < NUM_STATS_GROUPS;
+		j++, i = (i + 1) % NUM_STATS_GROUPS) {
+		printk(KERN_CRIT "\t#%d: %3u%% system,\t%3u%% softirq,\t"
+			"%3u%% hardirq,\t%3u%% idle\n", j+1,
+			__this_cpu_read(cpustat_utilization[i][STATS_SYSTEM]),
+			__this_cpu_read(cpustat_utilization[i][STATS_SOFTIRQ]),
+			__this_cpu_read(cpustat_utilization[i][STATS_HARDIRQ]),
+			__this_cpu_read(cpustat_utilization[i][STATS_IDLE]));
+	}
+}
+#else
+static inline void update_cpustat(void) { }
+static inline void print_cpustat(void) { }
+#endif
+
 /* watchdog detector functions */
 static DEFINE_PER_CPU(struct completion, softlockup_completion);
 static DEFINE_PER_CPU(struct cpu_stop_work, softlockup_stop_work);
@@ -504,6 +585,8 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
 	 */
 	period_ts = READ_ONCE(*this_cpu_ptr(&watchdog_report_ts));
 
+	update_cpustat();
+
 	/* Reset the interval when touched by known problematic code. */
 	if (period_ts == SOFTLOCKUP_DELAY_REPORT) {
 		if (unlikely(__this_cpu_read(softlockup_touch_sync))) {
@@ -539,6 +622,7 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
 		pr_emerg("BUG: soft lockup - CPU#%d stuck for %us! [%s:%d]\n",
 			smp_processor_id(), duration,
 			current->comm, task_pid_nr(current));
+		print_cpustat();
 		print_modules();
 		print_irqtrace_events(current);
 		if (regs)
-- 
2.37.1 (Apple Git-137.1)

[PATCHv3 2/2] watchdog/softlockup: report the most frequent interrupts

[Bitao Hu]

When the watchdog determines that the current soft lockup is due
to an interrupt storm based on CPU utilization, reporting the
most frequent interrupts could be good enough for further
troubleshooting.

Below is an example of interrupt storm. The call tree does not
provide useful information, but we can analyze which interrupt
caused the soft lockup by comparing the counts of interrupts.

[ 2987.488075] watchdog: BUG: soft lockup - CPU#9 stuck for 23s! [kworker/9:1:214]
[ 2987.488607] CPU#9 Utilization every 4s during lockup:
[ 2987.488941]  #1:   0% system,          0% softirq,   100% hardirq,     0% idle
[ 2987.489357]  #2:   0% system,          0% softirq,   100% hardirq,     0% idle
[ 2987.489771]  #3:   0% system,          0% softirq,   100% hardirq,     0% idle
[ 2987.490186]  #4:   0% system,          0% softirq,   100% hardirq,     0% idle
[ 2987.490601]  #5:   0% system,          0% softirq,   100% hardirq,     0% idle
[ 2987.491034] CPU#9 Detect HardIRQ Time exceeds 50%. Most frequent HardIRQs:
[ 2987.491493]  #1: 330985      irq#7(IPI)
[ 2987.491743]  #2: 5000        irq#10(arch_timer)
[ 2987.492039]  #3: 9           irq#91(nvme0q2)
[ 2987.492318]  #4: 3           irq#118(virtio1-output.12)
...
[ 2987.492728] Call trace:
[ 2987.492729]  __do_softirq+0xa8/0x364

Signed-off-by: Bitao Hu <yaoma@linux.alibaba.com>
---
 kernel/watchdog.c | 154 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 154 insertions(+)

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 046507be4eb5..6841ecdc3053 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -25,6 +25,9 @@
 #include <linux/stop_machine.h>
 #include <linux/kernel_stat.h>
 #include <linux/math64.h>
+#include <linux/irq.h>
+#include <linux/bitops.h>
+#include <linux/irqdesc.h>
 
 #include <asm/irq_regs.h>
 #include <linux/kvm_para.h>
@@ -431,11 +434,15 @@ void touch_softlockup_watchdog_sync(void)
 	__this_cpu_write(watchdog_report_ts, SOFTLOCKUP_DELAY_REPORT);
 }
 
+static void set_potential_softlockup(unsigned long now, unsigned long touch_ts);
+
 static int is_softlockup(unsigned long touch_ts,
 			 unsigned long period_ts,
 			 unsigned long now)
 {
 	if ((watchdog_enabled & WATCHDOG_SOFTOCKUP_ENABLED) && watchdog_thresh) {
+		/* Softlockup may occur in the current period */
+		set_potential_softlockup(now, period_ts);
 		/* Warn about unreasonable delays. */
 		if (time_after(now, period_ts + get_softlockup_thresh()))
 			return now - touch_ts;
@@ -462,6 +469,8 @@ static DEFINE_PER_CPU(u16, cpustat_old[NUM_STATS_PER_GROUP]);
 static DEFINE_PER_CPU(u8, cpustat_utilization[NUM_STATS_GROUPS][NUM_STATS_PER_GROUP]);
 static DEFINE_PER_CPU(u8, cpustat_tail);
 
+static void print_hardirq_counts(void);
+
 /*
  * We don't need nanosecond resolution. A granularity of 16ms is
  * sufficient for our precision, allowing us to use u16 to store
@@ -516,7 +525,151 @@ static void print_cpustat(void)
 			__this_cpu_read(cpustat_utilization[i][STATS_HARDIRQ]),
 			__this_cpu_read(cpustat_utilization[i][STATS_IDLE]));
 	}
+	print_hardirq_counts();
+}
+
+#define HARDIRQ_PERCENT_THRESH		50
+#define NUM_HARDIRQ_REPORT		5
+static DECLARE_BITMAP(softlockup_hardirq_cpus, CONFIG_NR_CPUS);
+static DEFINE_PER_CPU(u32 *, hardirq_counts);
+
+struct irq_counts {
+	int irq;
+	u32 counts;
+};
+
+static void find_counts_top(struct irq_counts *irq_counts, int irq, u32 counts, int range)
+{
+	unsigned int i, j;
+
+	for (i = 0; i < range; i++) {
+		if (counts > irq_counts[i].counts) {
+			for (j = range - 1; j > i; j--) {
+				irq_counts[j].counts = irq_counts[j - 1].counts;
+				irq_counts[j].irq = irq_counts[j - 1].irq;
+			}
+			irq_counts[j].counts = counts;
+			irq_counts[j].irq = irq;
+			break;
+		}
+	}
+}
+
+/*
+ * If the proportion of time spent handling irq exceeds HARDIRQ_PERCENT_THRESH%
+ * during sample_period, then it is necessary to record the counts of each irq.
+ */
+static inline bool need_record_irq_counts(int type)
+{
+	int tail = this_cpu_read(cpustat_tail);
+	u8 utilization;
+
+	if (--tail == -1)
+		tail = 4;
+	utilization = this_cpu_read(cpustat_utilization[tail][type]);
+	return utilization > HARDIRQ_PERCENT_THRESH;
+}
+
+/*
+ * Mark softlockup as potentially caused by hardirq
+ */
+static void set_potential_softlockup_hardirq(void)
+{
+	u32 i;
+	u32 *counts = __this_cpu_read(hardirq_counts);
+	int cpu = smp_processor_id();
+	struct irq_desc *desc;
+
+	if (!need_record_irq_counts(STATS_HARDIRQ))
+		return;
+
+	if (!test_bit(cpu, softlockup_hardirq_cpus)) {
+		counts = kmalloc_array(nr_irqs, sizeof(u32), GFP_ATOMIC);
+		if (!counts)
+			return;
+		for_each_irq_desc(i, desc) {
+			if (!desc)
+				continue;
+			counts[i] = desc->kstat_irqs ?
+				*this_cpu_ptr(desc->kstat_irqs) : 0;
+		}
+		__this_cpu_write(hardirq_counts, counts);
+		set_bit(cpu, softlockup_hardirq_cpus);
+	}
+}
+
+static void clear_potential_softlockup_hardirq(void)
+{
+	u32 *counts = __this_cpu_read(hardirq_counts);
+	int cpu = smp_processor_id();
+
+	if (test_bit(cpu, softlockup_hardirq_cpus)) {
+		kfree(counts);
+		counts = NULL;
+		__this_cpu_write(hardirq_counts, counts);
+		clear_bit(cpu, softlockup_hardirq_cpus);
+	}
 }
+
+/*
+ * Mark that softlockup may occur
+ */
+static void set_potential_softlockup(unsigned long now, unsigned long period_ts)
+{
+	if (time_after_eq(now, period_ts + get_softlockup_thresh() / 5))
+		set_potential_softlockup_hardirq();
+}
+
+static void clear_potential_softlockup(void)
+{
+	clear_potential_softlockup_hardirq();
+}
+
+static void print_hardirq_counts(void)
+{
+	u32 i;
+	struct irq_desc *desc;
+	u32 counts_diff;
+	u32 *counts = __this_cpu_read(hardirq_counts);
+	int cpu = smp_processor_id();
+	struct irq_counts hardirq_counts_top[NUM_HARDIRQ_REPORT] = {
+		{-1, 0}, {-1, 0}, {-1, 0}, {-1, 0},
+	};
+
+	if (test_bit(cpu, softlockup_hardirq_cpus)) {
+		/* Find the top NUM_HARDIRQ_REPORT most frequent interrupts */
+		for_each_irq_desc(i, desc) {
+			if (!desc)
+				continue;
+			counts_diff = desc->kstat_irqs ?
+				*this_cpu_ptr(desc->kstat_irqs) - counts[i] : 0;
+			find_counts_top(hardirq_counts_top, i, counts_diff,
+					NUM_HARDIRQ_REPORT);
+		}
+		/*
+		 * We do not want the "watchdog: " prefix on every line,
+		 * hence we use "printk" instead of "pr_crit".
+		 */
+		printk(KERN_CRIT "CPU#%d Detect HardIRQ Time exceeds %d%%. Most frequent HardIRQs:\n",
+			smp_processor_id(), HARDIRQ_PERCENT_THRESH);
+		for (i = 0; i < NUM_HARDIRQ_REPORT; i++) {
+			if (hardirq_counts_top[i].irq == -1)
+				break;
+			desc = irq_to_desc(hardirq_counts_top[i].irq);
+			if (desc && desc->action)
+				printk(KERN_CRIT "\t#%u: %-10u\tirq#%d(%s)\n",
+					i+1, hardirq_counts_top[i].counts,
+					hardirq_counts_top[i].irq, desc->action->name);
+			else
+				printk(KERN_CRIT "\t#%u: %-10u\tirq#%d\n",
+					i+1, hardirq_counts_top[i].counts,
+					hardirq_counts_top[i].irq);
+		}
+		if (!need_record_irq_counts(STATS_HARDIRQ))
+			clear_potential_softlockup_hardirq();
+	}
+}
+
 #else
 static inline void update_cpustat(void) { }
 static inline void print_cpustat(void) { }
@@ -537,6 +690,7 @@ static DEFINE_PER_CPU(struct cpu_stop_work, softlockup_stop_work);
 static int softlockup_fn(void *data)
 {
 	update_touch_ts();
+	clear_potential_softlockup();
 	complete(this_cpu_ptr(&softlockup_completion));
 
 	return 0;
-- 
2.37.1 (Apple Git-137.1)

Re: [PATCHv3 2/2] watchdog/softlockup: report the most frequent interrupts

[Bitao Hu]

The version is wrong, please ignore this.

Best regards,

Bitao Hu

On 2024/2/1 00:45, Bitao Hu wrote:
> When the watchdog determines that the current soft lockup is due
> to an interrupt storm based on CPU utilization, reporting the
> most frequent interrupts could be good enough for further
> troubleshooting.
> 
> Below is an example of interrupt storm. The call tree does not
> provide useful information, but we can analyze which interrupt
> caused the soft lockup by comparing the counts of interrupts.
> 
> [ 2987.488075] watchdog: BUG: soft lockup - CPU#9 stuck for 23s! [kworker/9:1:214]
> [ 2987.488607] CPU#9 Utilization every 4s during lockup:
> [ 2987.488941]  #1:   0% system,          0% softirq,   100% hardirq,     0% idle
> [ 2987.489357]  #2:   0% system,          0% softirq,   100% hardirq,     0% idle
> [ 2987.489771]  #3:   0% system,          0% softirq,   100% hardirq,     0% idle
> [ 2987.490186]  #4:   0% system,          0% softirq,   100% hardirq,     0% idle
> [ 2987.490601]  #5:   0% system,          0% softirq,   100% hardirq,     0% idle
> [ 2987.491034] CPU#9 Detect HardIRQ Time exceeds 50%. Most frequent HardIRQs:
> [ 2987.491493]  #1: 330985      irq#7(IPI)
> [ 2987.491743]  #2: 5000        irq#10(arch_timer)
> [ 2987.492039]  #3: 9           irq#91(nvme0q2)
> [ 2987.492318]  #4: 3           irq#118(virtio1-output.12)
> ...
> [ 2987.492728] Call trace:
> [ 2987.492729]  __do_softirq+0xa8/0x364
> 
> Signed-off-by: Bitao Hu <yaoma@linux.alibaba.com>
> ---
>   kernel/watchdog.c | 154 ++++++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 154 insertions(+)
> 
> diff --git a/kernel/watchdog.c b/kernel/watchdog.c
> index 046507be4eb5..6841ecdc3053 100644
> --- a/kernel/watchdog.c
> +++ b/kernel/watchdog.c
> @@ -25,6 +25,9 @@
>   #include <linux/stop_machine.h>
>   #include <linux/kernel_stat.h>
>   #include <linux/math64.h>
> +#include <linux/irq.h>
> +#include <linux/bitops.h>
> +#include <linux/irqdesc.h>
>   
>   #include <asm/irq_regs.h>
>   #include <linux/kvm_para.h>
> @@ -431,11 +434,15 @@ void touch_softlockup_watchdog_sync(void)
>   	__this_cpu_write(watchdog_report_ts, SOFTLOCKUP_DELAY_REPORT);
>   }
>   
> +static void set_potential_softlockup(unsigned long now, unsigned long touch_ts);
> +
>   static int is_softlockup(unsigned long touch_ts,
>   			 unsigned long period_ts,
>   			 unsigned long now)
>   {
>   	if ((watchdog_enabled & WATCHDOG_SOFTOCKUP_ENABLED) && watchdog_thresh) {
> +		/* Softlockup may occur in the current period */
> +		set_potential_softlockup(now, period_ts);
>   		/* Warn about unreasonable delays. */
>   		if (time_after(now, period_ts + get_softlockup_thresh()))
>   			return now - touch_ts;
> @@ -462,6 +469,8 @@ static DEFINE_PER_CPU(u16, cpustat_old[NUM_STATS_PER_GROUP]);
>   static DEFINE_PER_CPU(u8, cpustat_utilization[NUM_STATS_GROUPS][NUM_STATS_PER_GROUP]);
>   static DEFINE_PER_CPU(u8, cpustat_tail);
>   
> +static void print_hardirq_counts(void);
> +
>   /*
>    * We don't need nanosecond resolution. A granularity of 16ms is
>    * sufficient for our precision, allowing us to use u16 to store
> @@ -516,7 +525,151 @@ static void print_cpustat(void)
>   			__this_cpu_read(cpustat_utilization[i][STATS_HARDIRQ]),
>   			__this_cpu_read(cpustat_utilization[i][STATS_IDLE]));
>   	}
> +	print_hardirq_counts();
> +}
> +
> +#define HARDIRQ_PERCENT_THRESH		50
> +#define NUM_HARDIRQ_REPORT		5
> +static DECLARE_BITMAP(softlockup_hardirq_cpus, CONFIG_NR_CPUS);
> +static DEFINE_PER_CPU(u32 *, hardirq_counts);
> +
> +struct irq_counts {
> +	int irq;
> +	u32 counts;
> +};
> +
> +static void find_counts_top(struct irq_counts *irq_counts, int irq, u32 counts, int range)
> +{
> +	unsigned int i, j;
> +
> +	for (i = 0; i < range; i++) {
> +		if (counts > irq_counts[i].counts) {
> +			for (j = range - 1; j > i; j--) {
> +				irq_counts[j].counts = irq_counts[j - 1].counts;
> +				irq_counts[j].irq = irq_counts[j - 1].irq;
> +			}
> +			irq_counts[j].counts = counts;
> +			irq_counts[j].irq = irq;
> +			break;
> +		}
> +	}
> +}
> +
> +/*
> + * If the proportion of time spent handling irq exceeds HARDIRQ_PERCENT_THRESH%
> + * during sample_period, then it is necessary to record the counts of each irq.
> + */
> +static inline bool need_record_irq_counts(int type)
> +{
> +	int tail = this_cpu_read(cpustat_tail);
> +	u8 utilization;
> +
> +	if (--tail == -1)
> +		tail = 4;
> +	utilization = this_cpu_read(cpustat_utilization[tail][type]);
> +	return utilization > HARDIRQ_PERCENT_THRESH;
> +}
> +
> +/*
> + * Mark softlockup as potentially caused by hardirq
> + */
> +static void set_potential_softlockup_hardirq(void)
> +{
> +	u32 i;
> +	u32 *counts = __this_cpu_read(hardirq_counts);
> +	int cpu = smp_processor_id();
> +	struct irq_desc *desc;
> +
> +	if (!need_record_irq_counts(STATS_HARDIRQ))
> +		return;
> +
> +	if (!test_bit(cpu, softlockup_hardirq_cpus)) {
> +		counts = kmalloc_array(nr_irqs, sizeof(u32), GFP_ATOMIC);
> +		if (!counts)
> +			return;
> +		for_each_irq_desc(i, desc) {
> +			if (!desc)
> +				continue;
> +			counts[i] = desc->kstat_irqs ?
> +				*this_cpu_ptr(desc->kstat_irqs) : 0;
> +		}
> +		__this_cpu_write(hardirq_counts, counts);
> +		set_bit(cpu, softlockup_hardirq_cpus);
> +	}
> +}
> +
> +static void clear_potential_softlockup_hardirq(void)
> +{
> +	u32 *counts = __this_cpu_read(hardirq_counts);
> +	int cpu = smp_processor_id();
> +
> +	if (test_bit(cpu, softlockup_hardirq_cpus)) {
> +		kfree(counts);
> +		counts = NULL;
> +		__this_cpu_write(hardirq_counts, counts);
> +		clear_bit(cpu, softlockup_hardirq_cpus);
> +	}
>   }
> +
> +/*
> + * Mark that softlockup may occur
> + */
> +static void set_potential_softlockup(unsigned long now, unsigned long period_ts)
> +{
> +	if (time_after_eq(now, period_ts + get_softlockup_thresh() / 5))
> +		set_potential_softlockup_hardirq();
> +}
> +
> +static void clear_potential_softlockup(void)
> +{
> +	clear_potential_softlockup_hardirq();
> +}
> +
> +static void print_hardirq_counts(void)
> +{
> +	u32 i;
> +	struct irq_desc *desc;
> +	u32 counts_diff;
> +	u32 *counts = __this_cpu_read(hardirq_counts);
> +	int cpu = smp_processor_id();
> +	struct irq_counts hardirq_counts_top[NUM_HARDIRQ_REPORT] = {
> +		{-1, 0}, {-1, 0}, {-1, 0}, {-1, 0},
> +	};
> +
> +	if (test_bit(cpu, softlockup_hardirq_cpus)) {
> +		/* Find the top NUM_HARDIRQ_REPORT most frequent interrupts */
> +		for_each_irq_desc(i, desc) {
> +			if (!desc)
> +				continue;
> +			counts_diff = desc->kstat_irqs ?
> +				*this_cpu_ptr(desc->kstat_irqs) - counts[i] : 0;
> +			find_counts_top(hardirq_counts_top, i, counts_diff,
> +					NUM_HARDIRQ_REPORT);
> +		}
> +		/*
> +		 * We do not want the "watchdog: " prefix on every line,
> +		 * hence we use "printk" instead of "pr_crit".
> +		 */
> +		printk(KERN_CRIT "CPU#%d Detect HardIRQ Time exceeds %d%%. Most frequent HardIRQs:\n",
> +			smp_processor_id(), HARDIRQ_PERCENT_THRESH);
> +		for (i = 0; i < NUM_HARDIRQ_REPORT; i++) {
> +			if (hardirq_counts_top[i].irq == -1)
> +				break;
> +			desc = irq_to_desc(hardirq_counts_top[i].irq);
> +			if (desc && desc->action)
> +				printk(KERN_CRIT "\t#%u: %-10u\tirq#%d(%s)\n",
> +					i+1, hardirq_counts_top[i].counts,
> +					hardirq_counts_top[i].irq, desc->action->name);
> +			else
> +				printk(KERN_CRIT "\t#%u: %-10u\tirq#%d\n",
> +					i+1, hardirq_counts_top[i].counts,
> +					hardirq_counts_top[i].irq);
> +		}
> +		if (!need_record_irq_counts(STATS_HARDIRQ))
> +			clear_potential_softlockup_hardirq();
> +	}
> +}
> +
>   #else
>   static inline void update_cpustat(void) { }
>   static inline void print_cpustat(void) { }
> @@ -537,6 +690,7 @@ static DEFINE_PER_CPU(struct cpu_stop_work, softlockup_stop_work);
>   static int softlockup_fn(void *data)
>   {
>   	update_touch_ts();
> +	clear_potential_softlockup();
>   	complete(this_cpu_ptr(&softlockup_completion));
>   
>   	return 0;

[PATCHv3 0/2] *** Detect interrupt storm in softlockup ***

[Bitao Hu]

Hi, guys.
I have implemented a low-overhead method for detecting interrupt storm
in softlockup. Please review it, all comments are welcome.

Changes from v2 to v3:

- From Liu Song, using enum instead of macro for cpu_stats, shortening
the name 'idx_to_stat' to 'stats', adding 'get_16bit_precesion' instead
of using right shift operations, and using 'struct irq_counts'.

- From kernel robot test, using '__this_cpu_read' and '__this_cpu_write'
instead of accessing to an per-cpu array directly, in order to avoid
this warning.
'sparse: incorrect type in initializer (different modifiers)'

Changes from v1 to v2:

- From Douglas, optimize the memory of cpustats. With the maximum number
of CPUs, that's now this.
2 * 8192 * 4 + 1 * 8192 * 5 * 4 + 1 * 8192 = 237,568 bytes.

- From Liu Song, refactor the code format and add necessary comments.

- From Douglas, use interrupt counts instead of interrupt time to
determine the cause of softlockup.

- Remove the cmdline parameter added in PATCHv1.

Bitao Hu (2):
  watchdog/softlockup: low-overhead detection of interrupt storm
  watchdog/softlockup: report the most frequent interrupts

 kernel/watchdog.c | 240 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 240 insertions(+)

-- 
2.37.1 (Apple Git-137.1)

[PATCHv3 2/2] watchdog/softlockup: report the most frequent interrupts

[Bitao Hu]

When the watchdog determines that the current soft lockup is due
to an interrupt storm based on CPU utilization, reporting the
most frequent interrupts could be good enough for further
troubleshooting.

Below is an example of interrupt storm. The call tree does not
provide useful information, but we can analyze which interrupt
caused the soft lockup by comparing the counts of interrupts.

[ 2987.488075] watchdog: BUG: soft lockup - CPU#9 stuck for 23s! [kworker/9:1:214]
[ 2987.488607] CPU#9 Utilization every 4s during lockup:
[ 2987.488941]  #1:   0% system,          0% softirq,   100% hardirq,     0% idle
[ 2987.489357]  #2:   0% system,          0% softirq,   100% hardirq,     0% idle
[ 2987.489771]  #3:   0% system,          0% softirq,   100% hardirq,     0% idle
[ 2987.490186]  #4:   0% system,          0% softirq,   100% hardirq,     0% idle
[ 2987.490601]  #5:   0% system,          0% softirq,   100% hardirq,     0% idle
[ 2987.491034] CPU#9 Detect HardIRQ Time exceeds 50%. Most frequent HardIRQs:
[ 2987.491493]  #1: 330985      irq#7(IPI)
[ 2987.491743]  #2: 5000        irq#10(arch_timer)
[ 2987.492039]  #3: 9           irq#91(nvme0q2)
[ 2987.492318]  #4: 3           irq#118(virtio1-output.12)
...
[ 2987.492728] Call trace:
[ 2987.492729]  __do_softirq+0xa8/0x364

Signed-off-by: Bitao Hu <yaoma@linux.alibaba.com>
---
 kernel/watchdog.c | 156 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 156 insertions(+)

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 046507be4eb5..c4c25f25eae7 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -25,6 +25,9 @@
 #include <linux/stop_machine.h>
 #include <linux/kernel_stat.h>
 #include <linux/math64.h>
+#include <linux/irq.h>
+#include <linux/bitops.h>
+#include <linux/irqdesc.h>
 
 #include <asm/irq_regs.h>
 #include <linux/kvm_para.h>
@@ -431,11 +434,15 @@ void touch_softlockup_watchdog_sync(void)
 	__this_cpu_write(watchdog_report_ts, SOFTLOCKUP_DELAY_REPORT);
 }
 
+static void set_potential_softlockup(unsigned long now, unsigned long touch_ts);
+
 static int is_softlockup(unsigned long touch_ts,
 			 unsigned long period_ts,
 			 unsigned long now)
 {
 	if ((watchdog_enabled & WATCHDOG_SOFTOCKUP_ENABLED) && watchdog_thresh) {
+		/* Softlockup may occur in the current period */
+		set_potential_softlockup(now, period_ts);
 		/* Warn about unreasonable delays. */
 		if (time_after(now, period_ts + get_softlockup_thresh()))
 			return now - touch_ts;
@@ -462,6 +469,8 @@ static DEFINE_PER_CPU(u16, cpustat_old[NUM_STATS_PER_GROUP]);
 static DEFINE_PER_CPU(u8, cpustat_utilization[NUM_STATS_GROUPS][NUM_STATS_PER_GROUP]);
 static DEFINE_PER_CPU(u8, cpustat_tail);
 
+static void print_hardirq_counts(void);
+
 /*
  * We don't need nanosecond resolution. A granularity of 16ms is
  * sufficient for our precision, allowing us to use u16 to store
@@ -516,10 +525,156 @@ static void print_cpustat(void)
 			__this_cpu_read(cpustat_utilization[i][STATS_HARDIRQ]),
 			__this_cpu_read(cpustat_utilization[i][STATS_IDLE]));
 	}
+	print_hardirq_counts();
+}
+
+#define HARDIRQ_PERCENT_THRESH		50
+#define NUM_HARDIRQ_REPORT		5
+static DECLARE_BITMAP(softlockup_hardirq_cpus, CONFIG_NR_CPUS);
+static DEFINE_PER_CPU(u32 *, hardirq_counts);
+
+struct irq_counts {
+	int irq;
+	u32 counts;
+};
+
+static void find_counts_top(struct irq_counts *irq_counts, int irq, u32 counts, int range)
+{
+	unsigned int i, j;
+
+	for (i = 0; i < range; i++) {
+		if (counts > irq_counts[i].counts) {
+			for (j = range - 1; j > i; j--) {
+				irq_counts[j].counts = irq_counts[j - 1].counts;
+				irq_counts[j].irq = irq_counts[j - 1].irq;
+			}
+			irq_counts[j].counts = counts;
+			irq_counts[j].irq = irq;
+			break;
+		}
+	}
+}
+
+/*
+ * If the proportion of time spent handling irq exceeds HARDIRQ_PERCENT_THRESH%
+ * during sample_period, then it is necessary to record the counts of each irq.
+ */
+static inline bool need_record_irq_counts(int type)
+{
+	int tail = __this_cpu_read(cpustat_tail);
+	u8 utilization;
+
+	if (--tail == -1)
+		tail = 4;
+	utilization = __this_cpu_read(cpustat_utilization[tail][type]);
+	return utilization > HARDIRQ_PERCENT_THRESH;
+}
+
+/*
+ * Mark softlockup as potentially caused by hardirq
+ */
+static void set_potential_softlockup_hardirq(void)
+{
+	u32 i;
+	u32 *counts = __this_cpu_read(hardirq_counts);
+	int cpu = smp_processor_id();
+	struct irq_desc *desc;
+
+	if (!need_record_irq_counts(STATS_HARDIRQ))
+		return;
+
+	if (!test_bit(cpu, softlockup_hardirq_cpus)) {
+		counts = kmalloc_array(nr_irqs, sizeof(u32), GFP_ATOMIC);
+		if (!counts)
+			return;
+		for_each_irq_desc(i, desc) {
+			if (!desc)
+				continue;
+			counts[i] = desc->kstat_irqs ?
+				*this_cpu_ptr(desc->kstat_irqs) : 0;
+		}
+		__this_cpu_write(hardirq_counts, counts);
+		set_bit(cpu, softlockup_hardirq_cpus);
+	}
+}
+
+static void clear_potential_softlockup_hardirq(void)
+{
+	u32 *counts = __this_cpu_read(hardirq_counts);
+	int cpu = smp_processor_id();
+
+	if (test_bit(cpu, softlockup_hardirq_cpus)) {
+		kfree(counts);
+		counts = NULL;
+		__this_cpu_write(hardirq_counts, counts);
+		clear_bit(cpu, softlockup_hardirq_cpus);
+	}
 }
+
+/*
+ * Mark that softlockup may occur
+ */
+static void set_potential_softlockup(unsigned long now, unsigned long period_ts)
+{
+	if (time_after_eq(now, period_ts + get_softlockup_thresh() / 5))
+		set_potential_softlockup_hardirq();
+}
+
+static void clear_potential_softlockup(void)
+{
+	clear_potential_softlockup_hardirq();
+}
+
+static void print_hardirq_counts(void)
+{
+	u32 i;
+	struct irq_desc *desc;
+	u32 counts_diff;
+	u32 *counts = __this_cpu_read(hardirq_counts);
+	int cpu = smp_processor_id();
+	struct irq_counts hardirq_counts_top[NUM_HARDIRQ_REPORT] = {
+		{-1, 0}, {-1, 0}, {-1, 0}, {-1, 0},
+	};
+
+	if (test_bit(cpu, softlockup_hardirq_cpus)) {
+		/* Find the top NUM_HARDIRQ_REPORT most frequent interrupts */
+		for_each_irq_desc(i, desc) {
+			if (!desc)
+				continue;
+			counts_diff = desc->kstat_irqs ?
+				*this_cpu_ptr(desc->kstat_irqs) - counts[i] : 0;
+			find_counts_top(hardirq_counts_top, i, counts_diff,
+					NUM_HARDIRQ_REPORT);
+		}
+		/*
+		 * We do not want the "watchdog: " prefix on every line,
+		 * hence we use "printk" instead of "pr_crit".
+		 */
+		printk(KERN_CRIT "CPU#%d Detect HardIRQ Time exceeds %d%%. Most frequent HardIRQs:\n",
+			smp_processor_id(), HARDIRQ_PERCENT_THRESH);
+		for (i = 0; i < NUM_HARDIRQ_REPORT; i++) {
+			if (hardirq_counts_top[i].irq == -1)
+				break;
+			desc = irq_to_desc(hardirq_counts_top[i].irq);
+			if (desc && desc->action)
+				printk(KERN_CRIT "\t#%u: %-10u\tirq#%d(%s)\n",
+					i+1, hardirq_counts_top[i].counts,
+					hardirq_counts_top[i].irq, desc->action->name);
+			else
+				printk(KERN_CRIT "\t#%u: %-10u\tirq#%d\n",
+					i+1, hardirq_counts_top[i].counts,
+					hardirq_counts_top[i].irq);
+		}
+		if (!need_record_irq_counts(STATS_HARDIRQ))
+			clear_potential_softlockup_hardirq();
+	}
+}
+
 #else
 static inline void update_cpustat(void) { }
 static inline void print_cpustat(void) { }
+static inline void set_potential_softlockup(unsigned long now, unsigned long period_ts) { }
+static inline void clear_potential_softlockup(void) { }
 #endif
 
 /* watchdog detector functions */
@@ -537,6 +692,7 @@ static DEFINE_PER_CPU(struct cpu_stop_work, softlockup_stop_work);
 static int softlockup_fn(void *data)
 {
 	update_touch_ts();
+	clear_potential_softlockup();
 	complete(this_cpu_ptr(&softlockup_completion));
 
 	return 0;
-- 
2.37.1 (Apple Git-137.1)

Re: [PATCHv3 2/2] watchdog/softlockup: report the most frequent interrupts

[Liu Song]

在 2024/2/1 01:17, Bitao Hu 写道:
> When the watchdog determines that the current soft lockup is due
> to an interrupt storm based on CPU utilization, reporting the
> most frequent interrupts could be good enough for further
> troubleshooting.
>
> Below is an example of interrupt storm. The call tree does not
> provide useful information, but we can analyze which interrupt
> caused the soft lockup by comparing the counts of interrupts.
>
> [ 2987.488075] watchdog: BUG: soft lockup - CPU#9 stuck for 23s! [kworker/9:1:214]
> [ 2987.488607] CPU#9 Utilization every 4s during lockup:
> [ 2987.488941]  #1:   0% system,          0% softirq,   100% hardirq,     0% idle
> [ 2987.489357]  #2:   0% system,          0% softirq,   100% hardirq,     0% idle
> [ 2987.489771]  #3:   0% system,          0% softirq,   100% hardirq,     0% idle
> [ 2987.490186]  #4:   0% system,          0% softirq,   100% hardirq,     0% idle
> [ 2987.490601]  #5:   0% system,          0% softirq,   100% hardirq,     0% idle
> [ 2987.491034] CPU#9 Detect HardIRQ Time exceeds 50%. Most frequent HardIRQs:
> [ 2987.491493]  #1: 330985      irq#7(IPI)
> [ 2987.491743]  #2: 5000        irq#10(arch_timer)
> [ 2987.492039]  #3: 9           irq#91(nvme0q2)
> [ 2987.492318]  #4: 3           irq#118(virtio1-output.12)
> ...
> [ 2987.492728] Call trace:
> [ 2987.492729]  __do_softirq+0xa8/0x364
>
> Signed-off-by: Bitao Hu <yaoma@linux.alibaba.com>
> ---
>   kernel/watchdog.c | 156 ++++++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 156 insertions(+)
>
> diff --git a/kernel/watchdog.c b/kernel/watchdog.c
> index 046507be4eb5..c4c25f25eae7 100644
> --- a/kernel/watchdog.c
> +++ b/kernel/watchdog.c
> @@ -25,6 +25,9 @@
>   #include <linux/stop_machine.h>
>   #include <linux/kernel_stat.h>
>   #include <linux/math64.h>
> +#include <linux/irq.h>
> +#include <linux/bitops.h>
> +#include <linux/irqdesc.h>
>   
>   #include <asm/irq_regs.h>
>   #include <linux/kvm_para.h>
> @@ -431,11 +434,15 @@ void touch_softlockup_watchdog_sync(void)
>   	__this_cpu_write(watchdog_report_ts, SOFTLOCKUP_DELAY_REPORT);
>   }
>   
> +static void set_potential_softlockup(unsigned long now, unsigned long touch_ts);
> +
>   static int is_softlockup(unsigned long touch_ts,
>   			 unsigned long period_ts,
>   			 unsigned long now)
>   {
>   	if ((watchdog_enabled & WATCHDOG_SOFTOCKUP_ENABLED) && watchdog_thresh) {
> +		/* Softlockup may occur in the current period */
> +		set_potential_softlockup(now, period_ts);
>   		/* Warn about unreasonable delays. */
>   		if (time_after(now, period_ts + get_softlockup_thresh()))
>   			return now - touch_ts;
> @@ -462,6 +469,8 @@ static DEFINE_PER_CPU(u16, cpustat_old[NUM_STATS_PER_GROUP]);
>   static DEFINE_PER_CPU(u8, cpustat_utilization[NUM_STATS_GROUPS][NUM_STATS_PER_GROUP]);
>   static DEFINE_PER_CPU(u8, cpustat_tail);
>   
> +static void print_hardirq_counts(void);
> +
>   /*
>    * We don't need nanosecond resolution. A granularity of 16ms is
>    * sufficient for our precision, allowing us to use u16 to store
> @@ -516,10 +525,156 @@ static void print_cpustat(void)
>   			__this_cpu_read(cpustat_utilization[i][STATS_HARDIRQ]),
>   			__this_cpu_read(cpustat_utilization[i][STATS_IDLE]));
>   	}
> +	print_hardirq_counts();
> +}
> +
> +#define HARDIRQ_PERCENT_THRESH		50
> +#define NUM_HARDIRQ_REPORT		5
> +static DECLARE_BITMAP(softlockup_hardirq_cpus, CONFIG_NR_CPUS);
> +static DEFINE_PER_CPU(u32 *, hardirq_counts);
> +
> +struct irq_counts {
> +	int irq;
> +	u32 counts;
> +};
> +
> +static void find_counts_top(struct irq_counts *irq_counts, int irq, u32 counts, int range)
> +{
> +	unsigned int i, j;
> +
> +	for (i = 0; i < range; i++) {
> +		if (counts > irq_counts[i].counts) {
> +			for (j = range - 1; j > i; j--) {
> +				irq_counts[j].counts = irq_counts[j - 1].counts;
> +				irq_counts[j].irq = irq_counts[j - 1].irq;
> +			}
> +			irq_counts[j].counts = counts;
> +			irq_counts[j].irq = irq;
> +			break;
> +		}
The current implementation can lead to a reordering with each iteration.
It is recommended to update in place if the value is larger and perform
the reordering just before printing at the end.
> +	}
> +}
> +
> +/*
> + * If the proportion of time spent handling irq exceeds HARDIRQ_PERCENT_THRESH%
> + * during sample_period, then it is necessary to record the counts of each irq.
> + */
> +static inline bool need_record_irq_counts(int type)
> +{
> +	int tail = __this_cpu_read(cpustat_tail);
> +	u8 utilization;
> +
> +	if (--tail == -1)
> +		tail = 4;
> +	utilization = __this_cpu_read(cpustat_utilization[tail][type]);
> +	return utilization > HARDIRQ_PERCENT_THRESH;
> +}
> +
> +/*
> + * Mark softlockup as potentially caused by hardirq
> + */
> +static void set_potential_softlockup_hardirq(void)
> +{
> +	u32 i;
> +	u32 *counts = __this_cpu_read(hardirq_counts);
> +	int cpu = smp_processor_id();
> +	struct irq_desc *desc;
> +
> +	if (!need_record_irq_counts(STATS_HARDIRQ))
> +		return;
> +
> +	if (!test_bit(cpu, softlockup_hardirq_cpus)) {
> +		counts = kmalloc_array(nr_irqs, sizeof(u32), GFP_ATOMIC);
> +		if (!counts)
> +			return;
> +		for_each_irq_desc(i, desc) {
> +			if (!desc)
> +				continue;
> +			counts[i] = desc->kstat_irqs ?
> +				*this_cpu_ptr(desc->kstat_irqs) : 0;
> +		}
> +		__this_cpu_write(hardirq_counts, counts);
> +		set_bit(cpu, softlockup_hardirq_cpus);
> +	}
> +}
> +
> +static void clear_potential_softlockup_hardirq(void)
> +{
> +	u32 *counts = __this_cpu_read(hardirq_counts);
> +	int cpu = smp_processor_id();
> +
> +	if (test_bit(cpu, softlockup_hardirq_cpus)) {
> +		kfree(counts);
> +		counts = NULL;
> +		__this_cpu_write(hardirq_counts, counts);
> +		clear_bit(cpu, softlockup_hardirq_cpus);
> +	}
>   }
> +
> +/*
> + * Mark that softlockup may occur
> + */
> +static void set_potential_softlockup(unsigned long now, unsigned long period_ts)
> +{
> +	if (time_after_eq(now, period_ts + get_softlockup_thresh() / 5))
> +		set_potential_softlockup_hardirq();
> +}
> +
> +static void clear_potential_softlockup(void)
> +{
> +	clear_potential_softlockup_hardirq();
> +}
Given that clear_potential_softlockup will only call
clear_potential_softlockup_hardirq, then why is there a need to declare
clear_potential_softlockup separately?
> +
> +static void print_hardirq_counts(void)
> +{
> +	u32 i;
> +	struct irq_desc *desc;
> +	u32 counts_diff;
> +	u32 *counts = __this_cpu_read(hardirq_counts);
> +	int cpu = smp_processor_id();
> +	struct irq_counts hardirq_counts_top[NUM_HARDIRQ_REPORT] = {
> +		{-1, 0}, {-1, 0}, {-1, 0}, {-1, 0},
> +	};
> +
> +	if (test_bit(cpu, softlockup_hardirq_cpus)) {
> +		/* Find the top NUM_HARDIRQ_REPORT most frequent interrupts */
> +		for_each_irq_desc(i, desc) {
> +			if (!desc)
> +				continue;
> +			counts_diff = desc->kstat_irqs ?
> +				*this_cpu_ptr(desc->kstat_irqs) - counts[i] : 0;
> +			find_counts_top(hardirq_counts_top, i, counts_diff,
> +					NUM_HARDIRQ_REPORT);
> +		}
> +		/*
> +		 * We do not want the "watchdog: " prefix on every line,
> +		 * hence we use "printk" instead of "pr_crit".
> +		 */
> +		printk(KERN_CRIT "CPU#%d Detect HardIRQ Time exceeds %d%%. Most frequent HardIRQs:\n",
> +			smp_processor_id(), HARDIRQ_PERCENT_THRESH);
> +		for (i = 0; i < NUM_HARDIRQ_REPORT; i++) {
> +			if (hardirq_counts_top[i].irq == -1)
> +				break;
> +			desc = irq_to_desc(hardirq_counts_top[i].irq);
> +			if (desc && desc->action)
> +				printk(KERN_CRIT "\t#%u: %-10u\tirq#%d(%s)\n",
> +					i+1, hardirq_counts_top[i].counts,
> +					hardirq_counts_top[i].irq, desc->action->name);
> +			else
> +				printk(KERN_CRIT "\t#%u: %-10u\tirq#%d\n",
> +					i+1, hardirq_counts_top[i].counts,
> +					hardirq_counts_top[i].irq);
> +		}
> +		if (!need_record_irq_counts(STATS_HARDIRQ))
> +			clear_potential_softlockup_hardirq();
> +	}
> +}
> +
>   #else
>   static inline void update_cpustat(void) { }
>   static inline void print_cpustat(void) { }
> +static inline void set_potential_softlockup(unsigned long now, unsigned long period_ts) { }
> +static inline void clear_potential_softlockup(void) { }
>   #endif
>   
>   /* watchdog detector functions */
> @@ -537,6 +692,7 @@ static DEFINE_PER_CPU(struct cpu_stop_work, softlockup_stop_work);
>   static int softlockup_fn(void *data)
>   {
>   	update_touch_ts();
> +	clear_potential_softlockup();
>   	complete(this_cpu_ptr(&softlockup_completion));
>   
>   	return 0;

Re: [PATCHv3 2/2] watchdog/softlockup: report the most frequent interrupts

[Bitao Hu]

On 2024/2/1 09:46, Liu Song wrote:
> 
> 在 2024/2/1 01:17, Bitao Hu 写道:
>> When the watchdog determines that the current soft lockup is due
>> to an interrupt storm based on CPU utilization, reporting the
>> most frequent interrupts could be good enough for further
>> troubleshooting.
>>
>> Below is an example of interrupt storm. The call tree does not
>> provide useful information, but we can analyze which interrupt
>> caused the soft lockup by comparing the counts of interrupts.
>>
>> [ 2987.488075] watchdog: BUG: soft lockup - CPU#9 stuck for 23s! 
>> [kworker/9:1:214]
>> [ 2987.488607] CPU#9 Utilization every 4s during lockup:
>> [ 2987.488941]  #1:   0% system,          0% softirq,   100% 
>> hardirq,     0% idle
>> [ 2987.489357]  #2:   0% system,          0% softirq,   100% 
>> hardirq,     0% idle
>> [ 2987.489771]  #3:   0% system,          0% softirq,   100% 
>> hardirq,     0% idle
>> [ 2987.490186]  #4:   0% system,          0% softirq,   100% 
>> hardirq,     0% idle
>> [ 2987.490601]  #5:   0% system,          0% softirq,   100% 
>> hardirq,     0% idle
>> [ 2987.491034] CPU#9 Detect HardIRQ Time exceeds 50%. Most frequent 
>> HardIRQs:
>> [ 2987.491493]  #1: 330985      irq#7(IPI)
>> [ 2987.491743]  #2: 5000        irq#10(arch_timer)
>> [ 2987.492039]  #3: 9           irq#91(nvme0q2)
>> [ 2987.492318]  #4: 3           irq#118(virtio1-output.12)
>> ...
>> [ 2987.492728] Call trace:
>> [ 2987.492729]  __do_softirq+0xa8/0x364
>>
>> Signed-off-by: Bitao Hu <yaoma@linux.alibaba.com>
>> ---
>>   kernel/watchdog.c | 156 ++++++++++++++++++++++++++++++++++++++++++++++
>>   1 file changed, 156 insertions(+)
>>
>> diff --git a/kernel/watchdog.c b/kernel/watchdog.c
>> index 046507be4eb5..c4c25f25eae7 100644
>> --- a/kernel/watchdog.c
>> +++ b/kernel/watchdog.c
>> @@ -25,6 +25,9 @@
>>   #include <linux/stop_machine.h>
>>   #include <linux/kernel_stat.h>
>>   #include <linux/math64.h>
>> +#include <linux/irq.h>
>> +#include <linux/bitops.h>
>> +#include <linux/irqdesc.h>
>>   #include <asm/irq_regs.h>
>>   #include <linux/kvm_para.h>
>> @@ -431,11 +434,15 @@ void touch_softlockup_watchdog_sync(void)
>>       __this_cpu_write(watchdog_report_ts, SOFTLOCKUP_DELAY_REPORT);
>>   }
>> +static void set_potential_softlockup(unsigned long now, unsigned long 
>> touch_ts);
>> +
>>   static int is_softlockup(unsigned long touch_ts,
>>                unsigned long period_ts,
>>                unsigned long now)
>>   {
>>       if ((watchdog_enabled & WATCHDOG_SOFTOCKUP_ENABLED) && 
>> watchdog_thresh) {
>> +        /* Softlockup may occur in the current period */
>> +        set_potential_softlockup(now, period_ts);
>>           /* Warn about unreasonable delays. */
>>           if (time_after(now, period_ts + get_softlockup_thresh()))
>>               return now - touch_ts;
>> @@ -462,6 +469,8 @@ static DEFINE_PER_CPU(u16, 
>> cpustat_old[NUM_STATS_PER_GROUP]);
>>   static DEFINE_PER_CPU(u8, 
>> cpustat_utilization[NUM_STATS_GROUPS][NUM_STATS_PER_GROUP]);
>>   static DEFINE_PER_CPU(u8, cpustat_tail);
>> +static void print_hardirq_counts(void);
>> +
>>   /*
>>    * We don't need nanosecond resolution. A granularity of 16ms is
>>    * sufficient for our precision, allowing us to use u16 to store
>> @@ -516,10 +525,156 @@ static void print_cpustat(void)
>>               __this_cpu_read(cpustat_utilization[i][STATS_HARDIRQ]),
>>               __this_cpu_read(cpustat_utilization[i][STATS_IDLE]));
>>       }
>> +    print_hardirq_counts();
>> +}
>> +
>> +#define HARDIRQ_PERCENT_THRESH        50
>> +#define NUM_HARDIRQ_REPORT        5
>> +static DECLARE_BITMAP(softlockup_hardirq_cpus, CONFIG_NR_CPUS);
>> +static DEFINE_PER_CPU(u32 *, hardirq_counts);
>> +
>> +struct irq_counts {
>> +    int irq;
>> +    u32 counts;
>> +};
>> +
>> +static void find_counts_top(struct irq_counts *irq_counts, int irq, 
>> u32 counts, int range)
>> +{
>> +    unsigned int i, j;
>> +
>> +    for (i = 0; i < range; i++) {
>> +        if (counts > irq_counts[i].counts) {
>> +            for (j = range - 1; j > i; j--) {
>> +                irq_counts[j].counts = irq_counts[j - 1].counts;
>> +                irq_counts[j].irq = irq_counts[j - 1].irq;
>> +            }
>> +            irq_counts[j].counts = counts;
>> +            irq_counts[j].irq = irq;
>> +            break;
>> +        }
> The current implementation can lead to a reordering with each iteration.
> It is recommended to update in place if the value is larger and perform
> the reordering just before printing at the end.
Sure, I will consider optimization.
>> +    }
>> +}
>> +
>> +/*
>> + * If the proportion of time spent handling irq exceeds 
>> HARDIRQ_PERCENT_THRESH%
>> + * during sample_period, then it is necessary to record the counts of 
>> each irq.
>> + */
>> +static inline bool need_record_irq_counts(int type)
>> +{
>> +    int tail = __this_cpu_read(cpustat_tail);
>> +    u8 utilization;
>> +
>> +    if (--tail == -1)
>> +        tail = 4;
>> +    utilization = __this_cpu_read(cpustat_utilization[tail][type]);
>> +    return utilization > HARDIRQ_PERCENT_THRESH;
>> +}
>> +
>> +/*
>> + * Mark softlockup as potentially caused by hardirq
>> + */
>> +static void set_potential_softlockup_hardirq(void)
>> +{
>> +    u32 i;
>> +    u32 *counts = __this_cpu_read(hardirq_counts);
>> +    int cpu = smp_processor_id();
>> +    struct irq_desc *desc;
>> +
>> +    if (!need_record_irq_counts(STATS_HARDIRQ))
>> +        return;
>> +
>> +    if (!test_bit(cpu, softlockup_hardirq_cpus)) {
>> +        counts = kmalloc_array(nr_irqs, sizeof(u32), GFP_ATOMIC);
>> +        if (!counts)
>> +            return;
>> +        for_each_irq_desc(i, desc) {
>> +            if (!desc)
>> +                continue;
>> +            counts[i] = desc->kstat_irqs ?
>> +                *this_cpu_ptr(desc->kstat_irqs) : 0;
>> +        }
>> +        __this_cpu_write(hardirq_counts, counts);
>> +        set_bit(cpu, softlockup_hardirq_cpus);
>> +    }
>> +}
>> +
>> +static void clear_potential_softlockup_hardirq(void)
>> +{
>> +    u32 *counts = __this_cpu_read(hardirq_counts);
>> +    int cpu = smp_processor_id();
>> +
>> +    if (test_bit(cpu, softlockup_hardirq_cpus)) {
>> +        kfree(counts);
>> +        counts = NULL;
>> +        __this_cpu_write(hardirq_counts, counts);
>> +        clear_bit(cpu, softlockup_hardirq_cpus);
>> +    }
>>   }
>> +
>> +/*
>> + * Mark that softlockup may occur
>> + */
>> +static void set_potential_softlockup(unsigned long now, unsigned long 
>> period_ts)
>> +{
>> +    if (time_after_eq(now, period_ts + get_softlockup_thresh() / 5))
>> +        set_potential_softlockup_hardirq();
>> +}
>> +
>> +static void clear_potential_softlockup(void)
>> +{
>> +    clear_potential_softlockup_hardirq();
>> +}
> Given that clear_potential_softlockup will only call
> clear_potential_softlockup_hardirq, then why is there a need to declare
> clear_potential_softlockup separately?
I had considered adding "clear_potential_softlockup_softirq", but
ultimately I removed it. Indeed, it is confusing; I will remove the
"clear_potential_softlockup".
>> +
>> +static void print_hardirq_counts(void)
>> +{
>> +    u32 i;
>> +    struct irq_desc *desc;
>> +    u32 counts_diff;
>> +    u32 *counts = __this_cpu_read(hardirq_counts);
>> +    int cpu = smp_processor_id();
>> +    struct irq_counts hardirq_counts_top[NUM_HARDIRQ_REPORT] = {
>> +        {-1, 0}, {-1, 0}, {-1, 0}, {-1, 0},
>> +    };
>> +
>> +    if (test_bit(cpu, softlockup_hardirq_cpus)) {
>> +        /* Find the top NUM_HARDIRQ_REPORT most frequent interrupts */
>> +        for_each_irq_desc(i, desc) {
>> +            if (!desc)
>> +                continue;
>> +            counts_diff = desc->kstat_irqs ?
>> +                *this_cpu_ptr(desc->kstat_irqs) - counts[i] : 0;
>> +            find_counts_top(hardirq_counts_top, i, counts_diff,
>> +                    NUM_HARDIRQ_REPORT);
>> +        }
>> +        /*
>> +         * We do not want the "watchdog: " prefix on every line,
>> +         * hence we use "printk" instead of "pr_crit".
>> +         */
>> +        printk(KERN_CRIT "CPU#%d Detect HardIRQ Time exceeds %d%%. 
>> Most frequent HardIRQs:\n",
>> +            smp_processor_id(), HARDIRQ_PERCENT_THRESH);
>> +        for (i = 0; i < NUM_HARDIRQ_REPORT; i++) {
>> +            if (hardirq_counts_top[i].irq == -1)
>> +                break;
>> +            desc = irq_to_desc(hardirq_counts_top[i].irq);
>> +            if (desc && desc->action)
>> +                printk(KERN_CRIT "\t#%u: %-10u\tirq#%d(%s)\n",
>> +                    i+1, hardirq_counts_top[i].counts,
>> +                    hardirq_counts_top[i].irq, desc->action->name);
>> +            else
>> +                printk(KERN_CRIT "\t#%u: %-10u\tirq#%d\n",
>> +                    i+1, hardirq_counts_top[i].counts,
>> +                    hardirq_counts_top[i].irq);
>> +        }
>> +        if (!need_record_irq_counts(STATS_HARDIRQ))
>> +            clear_potential_softlockup_hardirq();
>> +    }
>> +}
>> +
>>   #else
>>   static inline void update_cpustat(void) { }
>>   static inline void print_cpustat(void) { }
>> +static inline void set_potential_softlockup(unsigned long now, 
>> unsigned long period_ts) { }
>> +static inline void clear_potential_softlockup(void) { }
>>   #endif
>>   /* watchdog detector functions */
>> @@ -537,6 +692,7 @@ static DEFINE_PER_CPU(struct cpu_stop_work, 
>> softlockup_stop_work);
>>   static int softlockup_fn(void *data)
>>   {
>>       update_touch_ts();
>> +    clear_potential_softlockup();
>>       complete(this_cpu_ptr(&softlockup_completion));
>>       return 0;

Re: [PATCHv3 2/2] watchdog/softlockup: report the most frequent interrupts

[Doug Anderson]

Hi,

On Wed, Jan 31, 2024 at 9:17 AM Bitao Hu <yaoma@linux.alibaba.com> wrote:
>
> When the watchdog determines that the current soft lockup is due
> to an interrupt storm based on CPU utilization, reporting the
> most frequent interrupts could be good enough for further
> troubleshooting.
>
> Below is an example of interrupt storm. The call tree does not
> provide useful information, but we can analyze which interrupt
> caused the soft lockup by comparing the counts of interrupts.
>
> [ 2987.488075] watchdog: BUG: soft lockup - CPU#9 stuck for 23s! [kworker/9:1:214]
> [ 2987.488607] CPU#9 Utilization every 4s during lockup:
> [ 2987.488941]  #1:   0% system,          0% softirq,   100% hardirq,     0% idle
> [ 2987.489357]  #2:   0% system,          0% softirq,   100% hardirq,     0% idle
> [ 2987.489771]  #3:   0% system,          0% softirq,   100% hardirq,     0% idle
> [ 2987.490186]  #4:   0% system,          0% softirq,   100% hardirq,     0% idle
> [ 2987.490601]  #5:   0% system,          0% softirq,   100% hardirq,     0% idle
> [ 2987.491034] CPU#9 Detect HardIRQ Time exceeds 50%. Most frequent HardIRQs:
> [ 2987.491493]  #1: 330985      irq#7(IPI)
> [ 2987.491743]  #2: 5000        irq#10(arch_timer)
> [ 2987.492039]  #3: 9           irq#91(nvme0q2)
> [ 2987.492318]  #4: 3           irq#118(virtio1-output.12)
> ...
> [ 2987.492728] Call trace:
> [ 2987.492729]  __do_softirq+0xa8/0x364
>
> Signed-off-by: Bitao Hu <yaoma@linux.alibaba.com>
> ---
>  kernel/watchdog.c | 156 ++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 156 insertions(+)
>
> diff --git a/kernel/watchdog.c b/kernel/watchdog.c
> index 046507be4eb5..c4c25f25eae7 100644
> --- a/kernel/watchdog.c
> +++ b/kernel/watchdog.c
> @@ -25,6 +25,9 @@
>  #include <linux/stop_machine.h>
>  #include <linux/kernel_stat.h>
>  #include <linux/math64.h>
> +#include <linux/irq.h>
> +#include <linux/bitops.h>
> +#include <linux/irqdesc.h>

Like in patch #1, don't just jam headers at the end. Put them in a
sensible order.



>  #include <asm/irq_regs.h>
>  #include <linux/kvm_para.h>
> @@ -431,11 +434,15 @@ void touch_softlockup_watchdog_sync(void)
>         __this_cpu_write(watchdog_report_ts, SOFTLOCKUP_DELAY_REPORT);
>  }
>
> +static void set_potential_softlockup(unsigned long now, unsigned long touch_ts);
> +
>  static int is_softlockup(unsigned long touch_ts,
>                          unsigned long period_ts,
>                          unsigned long now)
>  {
>         if ((watchdog_enabled & WATCHDOG_SOFTOCKUP_ENABLED) && watchdog_thresh) {
> +               /* Softlockup may occur in the current period */
> +               set_potential_softlockup(now, period_ts);

Something is really confusing to me about the
set_potential_softlockup() and set_potential_softlockup_hardirq()
functions and the comment above this line doesn't help me. From the
comment and the name of the function it sounds like at this point in
the code you've already determined that a softlockup is likely. ...but
I don't think that's the case. At this point in the code all we know
is that the softlockup detector is running, right?

I guess the first thing that would help would be to just get rid of
the set_potential_softlockup() wrapper and just inline here:

if (time_after_eq(now, period_ts + get_softlockup_thresh() / 5))
  set_potential_softlockup_hardirq();

...but then I'd want a comment explaining what that "if" test means.
Maybe something like this (assuming it's correct):

The "sample_period" is set so that we should get called ~5 times
between the start of the softlockup and when it is detected /
reported. If we've already been called twice and it looks like a
softlockup might be occurring, start counting interrupts.

Also: assuming I understand correctly, won't your "time_after_eq()"
always be true as you've written it? Shouldn't it be something like:

if (time_after_eq(now, period_ts + 2 * get_softlockup_thresh() / 5))

...or maybe you don't need this "if" test at all since you're using
"need_record_irq_counts(STATS_HARDIRQ)" here. IMO that should be
pulled out here as well since it makes it more obvious...



>                 /* Warn about unreasonable delays. */
>                 if (time_after(now, period_ts + get_softlockup_thresh()))
>                         return now - touch_ts;
> @@ -462,6 +469,8 @@ static DEFINE_PER_CPU(u16, cpustat_old[NUM_STATS_PER_GROUP]);
>  static DEFINE_PER_CPU(u8, cpustat_utilization[NUM_STATS_GROUPS][NUM_STATS_PER_GROUP]);
>  static DEFINE_PER_CPU(u8, cpustat_tail);
>
> +static void print_hardirq_counts(void);
> +

Rather than predeclaring, can't you just put the functions here?


>  /*
>   * We don't need nanosecond resolution. A granularity of 16ms is
>   * sufficient for our precision, allowing us to use u16 to store
> @@ -516,10 +525,156 @@ static void print_cpustat(void)
>                         __this_cpu_read(cpustat_utilization[i][STATS_HARDIRQ]),
>                         __this_cpu_read(cpustat_utilization[i][STATS_IDLE]));
>         }
> +       print_hardirq_counts();
> +}
> +
> +#define HARDIRQ_PERCENT_THRESH         50
> +#define NUM_HARDIRQ_REPORT             5
> +static DECLARE_BITMAP(softlockup_hardirq_cpus, CONFIG_NR_CPUS);
> +static DEFINE_PER_CPU(u32 *, hardirq_counts);
> +
> +struct irq_counts {
> +       int irq;
> +       u32 counts;
> +};
> +
> +static void find_counts_top(struct irq_counts *irq_counts, int irq, u32 counts, int range)

nit: it's not really "finding" anything. Maybe "tabulate_irq_count" or
something?


> +{
> +       unsigned int i, j;
> +
> +       for (i = 0; i < range; i++) {
> +               if (counts > irq_counts[i].counts) {
> +                       for (j = range - 1; j > i; j--) {
> +                               irq_counts[j].counts = irq_counts[j - 1].counts;
> +                               irq_counts[j].irq = irq_counts[j - 1].irq;
> +                       }
> +                       irq_counts[j].counts = counts;
> +                       irq_counts[j].irq = irq;
> +                       break;
> +               }
> +       }

Rather than a double loop, can't you just swap? Untested:

  unsigned int i;
  struct irq_counts new_count = { irq, counts };

  for (i = 0; i < range; i++) {
    if (count > irq_counts[i].counts)
      swap(new_count, irq_counts[i])
  }


> +}
> +
> +/*
> + * If the proportion of time spent handling irq exceeds HARDIRQ_PERCENT_THRESH%
> + * during sample_period, then it is necessary to record the counts of each irq.
> + */
> +static inline bool need_record_irq_counts(int type)

Let the compiler decide if this should be inline. No need for the
forced "inline" keyword.

Also: why do you need to pass in the "type". This function only makes
sense for "STATS_HARDIRQ"


> +{
> +       int tail = __this_cpu_read(cpustat_tail);
> +       u8 utilization;
> +
> +       if (--tail == -1)
> +               tail = 4;

Instead of the above:

tail = (tail + NUM_HARDIRQ_REPORT - 1) % NUM_HARDIRQ_REPORT;


> +       utilization = __this_cpu_read(cpustat_utilization[tail][type]);
> +       return utilization > HARDIRQ_PERCENT_THRESH;
> +}
> +
> +/*
> + * Mark softlockup as potentially caused by hardirq
> + */
> +static void set_potential_softlockup_hardirq(void)
> +{
> +       u32 i;
> +       u32 *counts = __this_cpu_read(hardirq_counts);
> +       int cpu = smp_processor_id();
> +       struct irq_desc *desc;
> +
> +       if (!need_record_irq_counts(STATS_HARDIRQ))
> +               return;
> +
> +       if (!test_bit(cpu, softlockup_hardirq_cpus)) {
> +               counts = kmalloc_array(nr_irqs, sizeof(u32), GFP_ATOMIC);

I think "nr_irqs" has the potential to grow at runtime, right? That
means you should read it and store locally how big your array actually
is. Otherwise you could allocate enough space for 64 IRQs, someone
could grow nr_irqs, and you could try looping over 128. Presumably
when you loop over with "for_each_irq_desc" you'd also need to
bounds-check in case someone on a different CPU expanded the number
after you read it...


-Doug

Re: [PATCHv3 2/2] watchdog/softlockup: report the most frequent interrupts

[Bitao Hu]

On 2024/2/1 10:23, Doug Anderson wrote:
> Hi,
> 
> On Wed, Jan 31, 2024 at 9:17 AM Bitao Hu <yaoma@linux.alibaba.com> wrote:
>>
>> When the watchdog determines that the current soft lockup is due
>> to an interrupt storm based on CPU utilization, reporting the
>> most frequent interrupts could be good enough for further
>> troubleshooting.
>>
>> Below is an example of interrupt storm. The call tree does not
>> provide useful information, but we can analyze which interrupt
>> caused the soft lockup by comparing the counts of interrupts.
>>
>> [ 2987.488075] watchdog: BUG: soft lockup - CPU#9 stuck for 23s! [kworker/9:1:214]
>> [ 2987.488607] CPU#9 Utilization every 4s during lockup:
>> [ 2987.488941]  #1:   0% system,          0% softirq,   100% hardirq,     0% idle
>> [ 2987.489357]  #2:   0% system,          0% softirq,   100% hardirq,     0% idle
>> [ 2987.489771]  #3:   0% system,          0% softirq,   100% hardirq,     0% idle
>> [ 2987.490186]  #4:   0% system,          0% softirq,   100% hardirq,     0% idle
>> [ 2987.490601]  #5:   0% system,          0% softirq,   100% hardirq,     0% idle
>> [ 2987.491034] CPU#9 Detect HardIRQ Time exceeds 50%. Most frequent HardIRQs:
>> [ 2987.491493]  #1: 330985      irq#7(IPI)
>> [ 2987.491743]  #2: 5000        irq#10(arch_timer)
>> [ 2987.492039]  #3: 9           irq#91(nvme0q2)
>> [ 2987.492318]  #4: 3           irq#118(virtio1-output.12)
>> ...
>> [ 2987.492728] Call trace:
>> [ 2987.492729]  __do_softirq+0xa8/0x364
>>
>> Signed-off-by: Bitao Hu <yaoma@linux.alibaba.com>
>> ---
>>   kernel/watchdog.c | 156 ++++++++++++++++++++++++++++++++++++++++++++++
>>   1 file changed, 156 insertions(+)
>>
>> diff --git a/kernel/watchdog.c b/kernel/watchdog.c
>> index 046507be4eb5..c4c25f25eae7 100644
>> --- a/kernel/watchdog.c
>> +++ b/kernel/watchdog.c
>> @@ -25,6 +25,9 @@
>>   #include <linux/stop_machine.h>
>>   #include <linux/kernel_stat.h>
>>   #include <linux/math64.h>
>> +#include <linux/irq.h>
>> +#include <linux/bitops.h>
>> +#include <linux/irqdesc.h>
> 
> Like in patch #1, don't just jam headers at the end. Put them in a
> sensible order.
Sure, I will standardize the code.
> 
> 
> 
>>   #include <asm/irq_regs.h>
>>   #include <linux/kvm_para.h>
>> @@ -431,11 +434,15 @@ void touch_softlockup_watchdog_sync(void)
>>          __this_cpu_write(watchdog_report_ts, SOFTLOCKUP_DELAY_REPORT);
>>   }
>>
>> +static void set_potential_softlockup(unsigned long now, unsigned long touch_ts);
>> +
>>   static int is_softlockup(unsigned long touch_ts,
>>                           unsigned long period_ts,
>>                           unsigned long now)
>>   {
>>          if ((watchdog_enabled & WATCHDOG_SOFTOCKUP_ENABLED) && watchdog_thresh) {
>> +               /* Softlockup may occur in the current period */
>> +               set_potential_softlockup(now, period_ts);
> 
> Something is really confusing to me about the
> set_potential_softlockup() and set_potential_softlockup_hardirq()
> functions and the comment above this line doesn't help me. From the
> comment and the name of the function it sounds like at this point in
> the code you've already determined that a softlockup is likely. ...but
> I don't think that's the case. At this point in the code all we know
> is that the softlockup detector is running, right?
> 
> I guess the first thing that would help would be to just get rid of
> the set_potential_softlockup() wrapper and just inline here:
> 
> if (time_after_eq(now, period_ts + get_softlockup_thresh() / 5))
>    set_potential_softlockup_hardirq();
> 
> ...but then I'd want a comment explaining what that "if" test means.
> Maybe something like this (assuming it's correct):
> 
> The "sample_period" is set so that we should get called ~5 times
> between the start of the softlockup and when it is detected /
> reported. If we've already been called twice and it looks like a
> softlockup might be occurring, start counting interrupts.
> 
> Also: assuming I understand correctly, won't your "time_after_eq()"
> always be true as you've written it? Shouldn't it be something like:
> 
> if (time_after_eq(now, period_ts + 2 * get_softlockup_thresh() / 5))
> 
> ...or maybe you don't need this "if" test at all since you're using
> "need_record_irq_counts(STATS_HARDIRQ)" here. IMO that should be
> pulled out here as well since it makes it more obvious...
I agree with your this suggestion here. It is easier to understand:

if (time_after_eq(now, period_ts + get_softlockup_thresh() / 5))
   set_potential_softlockup_hardirq();

Please let me explain the criteria for the judgment here. Under normal
circumstances, "softlockup_fn" will be woken up every "sample_period" to
update "period_ts", and the "time_after_eq" I written will be false. If
"period_ts" has not been updated after a "sample_period" has passed,
then the "time_after_eq" will be true. And I suspect that in the 
subsequent few "sample_period", "period_ts" might also not be updated,
which could indicate a potential softlockup. At this point, I use
"need_record_irq_counts" to determine if this phenomenon is caused by an
interrupt storm.

To summarize, my condition to start counting interrupts is that
"period_ts" has not been updated during "sample_period" AND the 
proportion of hardirq time during "sample_period" exceeds 50%.

What do you think?

By the way, The reason for having both set_potential_softlockup() and
set_potential_softlockup_hardirq() is that I once wrote a
set_potential_softlockup_softirq() for starting counting softirqs.
However, I found it might not very meaningful and removed it. I will
follow your suggestion and make improvements to make this area more
understandable.
> 
> 
> 
>>                  /* Warn about unreasonable delays. */
>>                  if (time_after(now, period_ts + get_softlockup_thresh()))
>>                          return now - touch_ts;
>> @@ -462,6 +469,8 @@ static DEFINE_PER_CPU(u16, cpustat_old[NUM_STATS_PER_GROUP]);
>>   static DEFINE_PER_CPU(u8, cpustat_utilization[NUM_STATS_GROUPS][NUM_STATS_PER_GROUP]);
>>   static DEFINE_PER_CPU(u8, cpustat_tail);
>>
>> +static void print_hardirq_counts(void);
>> +
> 
> Rather than predeclaring, can't you just put the functions here?
> 
> 
>>   /*
>>    * We don't need nanosecond resolution. A granularity of 16ms is
>>    * sufficient for our precision, allowing us to use u16 to store
>> @@ -516,10 +525,156 @@ static void print_cpustat(void)
>>                          __this_cpu_read(cpustat_utilization[i][STATS_HARDIRQ]),
>>                          __this_cpu_read(cpustat_utilization[i][STATS_IDLE]));
>>          }
>> +       print_hardirq_counts();
>> +}
>> +
>> +#define HARDIRQ_PERCENT_THRESH         50
>> +#define NUM_HARDIRQ_REPORT             5
>> +static DECLARE_BITMAP(softlockup_hardirq_cpus, CONFIG_NR_CPUS);
>> +static DEFINE_PER_CPU(u32 *, hardirq_counts);
>> +
>> +struct irq_counts {
>> +       int irq;
>> +       u32 counts;
>> +};
>> +
>> +static void find_counts_top(struct irq_counts *irq_counts, int irq, u32 counts, int range)
> 
> nit: it's not really "finding" anything. Maybe "tabulate_irq_count" or
> something?
Agree, I will rename it.
> 
> 
>> +{
>> +       unsigned int i, j;
>> +
>> +       for (i = 0; i < range; i++) {
>> +               if (counts > irq_counts[i].counts) {
>> +                       for (j = range - 1; j > i; j--) {
>> +                               irq_counts[j].counts = irq_counts[j - 1].counts;
>> +                               irq_counts[j].irq = irq_counts[j - 1].irq;
>> +                       }
>> +                       irq_counts[j].counts = counts;
>> +                       irq_counts[j].irq = irq;
>> +                       break;
>> +               }
>> +       }
> 
> Rather than a double loop, can't you just swap? Untested:
> 
>    unsigned int i;
>    struct irq_counts new_count = { irq, counts };
> 
>    for (i = 0; i < range; i++) {
>      if (count > irq_counts[i].counts)
>        swap(new_count, irq_counts[i])
>    }
I will try it.
> 
> 
>> +}
>> +
>> +/*
>> + * If the proportion of time spent handling irq exceeds HARDIRQ_PERCENT_THRESH%
>> + * during sample_period, then it is necessary to record the counts of each irq.
>> + */
>> +static inline bool need_record_irq_counts(int type)
> 
> Let the compiler decide if this should be inline. No need for the
> forced "inline" keyword.
OK.
> 
> Also: why do you need to pass in the "type". This function only makes
> sense for "STATS_HARDIRQ
As previously mentioned, I had considered counting softirqs. I will 
refactor 'need_record_irq_counts'.

> 
> 
>> +{
>> +       int tail = __this_cpu_read(cpustat_tail);
>> +       u8 utilization;
>> +
>> +       if (--tail == -1)
>> +               tail = 4;
> 
> Instead of the above:
> 
> tail = (tail + NUM_HARDIRQ_REPORT - 1) % NUM_HARDIRQ_REPORT;
Agree, I will follow your suggestion.
> 
> 
>> +       utilization = __this_cpu_read(cpustat_utilization[tail][type]);
>> +       return utilization > HARDIRQ_PERCENT_THRESH;
>> +}
>> +
>> +/*
>> + * Mark softlockup as potentially caused by hardirq
>> + */
>> +static void set_potential_softlockup_hardirq(void)
>> +{
>> +       u32 i;
>> +       u32 *counts = __this_cpu_read(hardirq_counts);
>> +       int cpu = smp_processor_id();
>> +       struct irq_desc *desc;
>> +
>> +       if (!need_record_irq_counts(STATS_HARDIRQ))
>> +               return;
>> +
>> +       if (!test_bit(cpu, softlockup_hardirq_cpus)) {
>> +               counts = kmalloc_array(nr_irqs, sizeof(u32), GFP_ATOMIC);
> 
> I think "nr_irqs" has the potential to grow at runtime, right? That
> means you should read it and store locally how big your array actually
> is. Otherwise you could allocate enough space for 64 IRQs, someone
> could grow nr_irqs, and you could try looping over 128. Presumably
> when you loop over with "for_each_irq_desc" you'd also need to
> bounds-check in case someone on a different CPU expanded the number
> after you read it...
Oh, I assumed that "nr_irqs" would remain constant at runtime, but I 
will consider scenarios where it might grow.

> 
> 
> -Doug

Re: [PATCHv3 2/2] watchdog/softlockup: report the most frequent interrupts

[Doug Anderson]

Hi,

On Fri, Feb 2, 2024 at 6:22 AM Bitao Hu <yaoma@linux.alibaba.com> wrote:
>
> > ...or maybe you don't need this "if" test at all since you're using
> > "need_record_irq_counts(STATS_HARDIRQ)" here. IMO that should be
> > pulled out here as well since it makes it more obvious...
> I agree with your this suggestion here. It is easier to understand:
>
> if (time_after_eq(now, period_ts + get_softlockup_thresh() / 5))
>    set_potential_softlockup_hardirq();
>
> Please let me explain the criteria for the judgment here. Under normal
> circumstances, "softlockup_fn" will be woken up every "sample_period" to
> update "period_ts", and the "time_after_eq" I written will be false. If
> "period_ts" has not been updated after a "sample_period" has passed,
> then the "time_after_eq" will be true. And I suspect that in the
> subsequent few "sample_period", "period_ts" might also not be updated,
> which could indicate a potential softlockup. At this point, I use
> "need_record_irq_counts" to determine if this phenomenon is caused by an
> interrupt storm.
>
> To summarize, my condition to start counting interrupts is that
> "period_ts" has not been updated during "sample_period" AND the
> proportion of hardirq time during "sample_period" exceeds 50%.
>
> What do you think?

OK, sounds reasonable. Given that this is non-obvious, it would be
great if your patch included a comment explaining it. :-)