Re: [PATCH 1/3] watchdog/softlockup: low-overhead detection of interrupt storm

From: Liu Song
Date: Tue Jan 23 2024 - 20:44:13 EST


在 2024/1/23 20:12, Bitao Hu 写道:
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: 0.0% system, 0.0% softirq, 100.0% hardirq, 0.0% idle
#2: 0.0% system, 0.0% softirq, 100.0% hardirq, 0.0% idle
#3: 0.0% system, 0.0% softirq, 100.0% hardirq, 0.0% idle
#4: 0.0% system, 0.0% softirq, 100.0% hardirq, 0.0% idle
#5: 0.0% system, 0.0% softirq, 100.0% hardirq, 0.0% 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@xxxxxxxxxxxxxxxxx>
---
kernel/watchdog.c | 58 +++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 58 insertions(+)

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 81a8862295d6..9fad10e0a147 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 +443,58 @@ static int is_softlockup(unsigned long touch_ts,
return 0;
}
+#ifdef CONFIG_IRQ_TIME_ACCOUNTING
+static DEFINE_PER_CPU(u64, cpustat_old[NR_STATS]);
+static DEFINE_PER_CPU(u64, cpustat_diff[5][NR_STATS]);
+static DEFINE_PER_CPU(int, cpustat_tail);
+
+static void update_cpustat(void)
+{
+ u64 *old = this_cpu_ptr(cpustat_old);
+ u64 (*diff)[NR_STATS] = this_cpu_ptr(cpustat_diff);
+ int tail = this_cpu_read(cpustat_tail), i;
+ struct kernel_cpustat kcpustat;
+ u64 *cpustat = kcpustat.cpustat;
+
+ kcpustat_cpu_fetch(&kcpustat, smp_processor_id());
+ for (i = 0; i < NR_STATS; i++) {
+ diff[tail][i] = cpustat[i] - old[i];
+ old[i] = cpustat[i];
+ }
+ this_cpu_write(cpustat_tail, (tail + 1) % 5);
The number 5 here is related to the 5 in cpustat_diff[5], and it is recommended to use a macro definition instead of using the number 5 directly.
+}
+
+static void print_cpustat(void)
+{
+ int i, j, k;
+ u64 a[5][NR_STATS], b[5][NR_STATS];
Use define instead of the literal number 5.
+ u64 (*diff)[NR_STATS] = this_cpu_ptr(cpustat_diff);
+ int tail = this_cpu_read(cpustat_tail);
+ u32 period_us = sample_period / 1000;
Use NSEC_PER_USEC
+
+ for (i = 0; i < 5; i++) {
+ for (j = 0; j < NR_STATS; j++) {
+ a[i][j] = 100 * (diff[i][j] / 1000);
+ b[i][j] = 10 * do_div(a[i][j], period_us);
+ do_div(b[i][j], period_us);
+ }
+ }
+ printk(KERN_CRIT "CPU#%d Utilization every %us during lockup:\n",
better use "pr_crit", and was the original intent here microseconds (us) or milliseconds (ms)?
+ smp_processor_id(), period_us/1000000);
better use "period_us /NSEC_PER_MSEC"?
+ for (k = 0, i = tail; k < 5; k++, i = (i + 1) % 5) {

It seems that only i and j are necessary, k is not essential.

+ printk(KERN_CRIT "\t#%d: %llu.%llu%% system,\t%llu.%llu%% softirq,\t"
+ "%llu.%llu%% hardirq,\t%llu.%llu%% idle\n", k+1,
+ a[i][CPUTIME_SYSTEM], b[i][CPUTIME_SYSTEM],
+ a[i][CPUTIME_SOFTIRQ], b[i][CPUTIME_SOFTIRQ],
+ a[i][CPUTIME_IRQ], b[i][CPUTIME_IRQ],
+ a[i][CPUTIME_IDLE], b[i][CPUTIME_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 +558,9 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
*/
period_ts = READ_ONCE(*this_cpu_ptr(&watchdog_report_ts));
+ /* update cpu usage stat */
The function name already indicates that it involves graphs, so the comment here appears superfluous.
If a comment is absolutely necessary, please provide more detailed information.
+ 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 +596,7 @@ 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)