Re: [PATCHv2 1/2] watchdog/softlockup: low-overhead detection of interrupt storm
From: Bitao Hu
Date: Wed Jan 31 2024 - 06:08:13 EST
On 2024/1/31 09:19, Liu Song wrote:
在 2024/1/30 15:47, 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% 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@xxxxxxxxxxxxxxxxx>
---
kernel/watchdog.c | 72 +++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 72 insertions(+)
diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 81a8862295d6..0efe9604c3c2 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 +443,73 @@ static int is_softlockup(unsigned long touch_ts,
return 0;
}
+#ifdef CONFIG_IRQ_TIME_ACCOUNTING
+#define NUM_STATS_GROUPS 5
+#define STATS_SYSTEM 0
+#define STATS_SOFTIRQ 1
+#define STATS_HARDIRQ 2
+#define STATS_IDLE 3
+#define NUM_STATS_PER_GROUP 4
This is a set of related numbers; wouldn't it be better to use an enum?
Agree.
+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 enum cpu_usage_stat idx_to_stat[NUM_STATS_PER_GROUP] = {
+ CPUTIME_SYSTEM, CPUTIME_SOFTIRQ, CPUTIME_IRQ, CPUTIME_IDLE
+};
To be honest, I'm not particularly fond of the name 'idx_to_stat' as the
concept of an
index is already implied by the nature of an array, so adding 'idx' is
redundant.
I suggest shortening the name.
OK, the name 'stats' is clear enough here.
+
+static void update_cpustat(void)
+{
+ u8 i;
+ u16 *old = this_cpu_ptr(cpustat_old);
+ u8 (*utilization)[NUM_STATS_PER_GROUP] =
this_cpu_ptr(cpustat_utilization);
+ u8 tail = this_cpu_read(cpustat_tail);
+ struct kernel_cpustat kcpustat;
+ u64 *cpustat = kcpustat.cpustat;
+ u16 sample_period_ms = sample_period >> 24LL; /* 2^24ns ~= 16.8ms */
There are two instances where right shift operations are used; it is
suggested to employ a helper macro for a more comfortable look.
OK.
+
+ kcpustat_cpu_fetch(&kcpustat, smp_processor_id());
+ for (i = STATS_SYSTEM; i < NUM_STATS_PER_GROUP; i++) {
+ /*
+ * 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
+ */
+ cpustat[idx_to_stat[i]] =
lower_16_bits(cpustat[idx_to_stat[i]] >> 24LL);
+ utilization[tail][i] = 100 * (u16)(cpustat[idx_to_stat[i]] -
old[i])
+ / sample_period_ms;
+ old[i] = cpustat[idx_to_stat[i]];
+ }
+ this_cpu_write(cpustat_tail, (tail + 1) % NUM_STATS_GROUPS);
+}
+
+static void print_cpustat(void)
+{
+ u8 i, j;
+ u8 (*utilization)[NUM_STATS_PER_GROUP] =
this_cpu_ptr(cpustat_utilization);
+ 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,
+ utilization[i][STATS_SYSTEM], utilization[i][STATS_SOFTIRQ],
+ utilization[i][STATS_HARDIRQ], 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 +573,8 @@ 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 +610,7 @@ 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)