Re: [PATCHv3 2/2] watchdog/softlockup: report the most frequent interrupts
From: Bitao Hu
Date: Fri Feb 02 2024 - 09:22:37 EST
On 2024/2/1 10:23, Doug Anderson wrote:
Hi,
On Wed, Jan 31, 2024 at 9:17 AM Bitao Hu <yaoma@xxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxx>
---
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