[PATCHv7 0/2] *** Detect interrupt storm in softlockup ***

[PATCHv7 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 v6 to v7:

- Remove "READ_ONCE" in "start_counting_irqs"

- Replace the hard-coded 5 with "NUM_SAMPLE_PERIODS" macro in
"set_sample_period".

- Add empty lines to help with reading the code.

- Remove the branch that processes IRQs where "counts_diff = 0".

- Add the Reviewed-by of Liu Song and Douglas.

Changes from v5 to v6:

- Use "./scripts/checkpatch.pl --strict" to get a few extra
style nits and fix them.

- Squash patch #3 into patch #1, and wrapp the help text to
80 columns.

- Sort existing headers alphabetically in watchdog.c

- Drop "softlockup_hardirq_cpus", just read "hardirq_counts"
and see if it's non-NULL.

- Store "nr_irqs" in a local variable.

- Simplify the calculation of "cpu_diff".

Changes from v4 to v5:

- Rearranging variable placement to make code look neater.

Changes from v3 to v4:

- Renaming some variable and function names to make the code logic
more readable.

- Change the code location to avoid predeclaring.

- Just swap rather than a double loop in tabulate_irq_count.

- Since nr_irqs has the potential to grow at runtime, bounds-check
logic has been implemented.

- Add SOFTLOCKUP_DETECTOR_INTR_STORM Kconfig knob.

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
  watchdog/softlockup: report the most frequent interrupts

 kernel/watchdog.c | 255 +++++++++++++++++++++++++++++++++++++++++++++-
 lib/Kconfig.debug |  13 +++
 2 files changed, 263 insertions(+), 5 deletions(-)

-- 
2.37.1 (Apple Git-137.1)

[PATCHv7 1/2] watchdog/softlockup: low-overhead detection of interrupt

[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.

The mechanism requires a considerable amount of global storage space
when configured for the maximum number of CPUs. Therefore, adding a
SOFTLOCKUP_DETECTOR_INTR_STORM Kconfig knob that defaults to "yes"
if the max number of CPUs is <= 128.

Signed-off-by: Bitao Hu <yaoma@linux.alibaba.com>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Liu Song <liusong@linux.alibaba.com>
---
 kernel/watchdog.c | 98 ++++++++++++++++++++++++++++++++++++++++++++++-
 lib/Kconfig.debug | 13 +++++++
 2 files changed, 110 insertions(+), 1 deletion(-)

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 81a8862295d6..69e72d7e461d 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -16,6 +16,8 @@
 #include <linux/cpu.h>
 #include <linux/nmi.h>
 #include <linux/init.h>
+#include <linux/kernel_stat.h>
+#include <linux/math64.h>
 #include <linux/module.h>
 #include <linux/sysctl.h>
 #include <linux/tick.h>
@@ -35,6 +37,8 @@ static DEFINE_MUTEX(watchdog_mutex);
 # define WATCHDOG_HARDLOCKUP_DEFAULT	0
 #endif
 
+#define NUM_SAMPLE_PERIODS	5
+
 unsigned long __read_mostly watchdog_enabled;
 int __read_mostly watchdog_user_enabled = 1;
 static int __read_mostly watchdog_hardlockup_user_enabled = WATCHDOG_HARDLOCKUP_DEFAULT;
@@ -333,6 +337,95 @@ __setup("watchdog_thresh=", watchdog_thresh_setup);
 
 static void __lockup_detector_cleanup(void);
 
+#ifdef CONFIG_SOFTLOCKUP_DETECTOR_INTR_STORM
+enum stats_per_group {
+	STATS_SYSTEM,
+	STATS_SOFTIRQ,
+	STATS_HARDIRQ,
+	STATS_IDLE,
+	NUM_STATS_PER_GROUP,
+};
+
+static const enum cpu_usage_stat tracked_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_util[NUM_SAMPLE_PERIODS][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_ns)
+{
+	return data_ns >> 24LL; /* 2^24ns ~= 16.8ms */
+}
+
+static void update_cpustat(void)
+{
+	int i;
+	u8 util;
+	u16 old_stat, new_stat;
+	struct kernel_cpustat kcpustat;
+	u64 *cpustat = kcpustat.cpustat;
+	u8 tail = __this_cpu_read(cpustat_tail);
+	u16 sample_period_16 = get_16bit_precision(sample_period);
+
+	kcpustat_cpu_fetch(&kcpustat, smp_processor_id());
+
+	for (i = 0; i < NUM_STATS_PER_GROUP; i++) {
+		old_stat = __this_cpu_read(cpustat_old[i]);
+		new_stat = get_16bit_precision(cpustat[tracked_stats[i]]);
+		util = DIV_ROUND_UP(100 * (new_stat - old_stat), sample_period_16);
+		__this_cpu_write(cpustat_util[tail][i], util);
+		__this_cpu_write(cpustat_old[i], new_stat);
+	}
+
+	__this_cpu_write(cpustat_tail, (tail + 1) % NUM_SAMPLE_PERIODS);
+}
+
+static void print_cpustat(void)
+{
+	int i, group;
+	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 (i = 0; i < NUM_SAMPLE_PERIODS; i++) {
+		group = (tail + i) % NUM_SAMPLE_PERIODS;
+		printk(KERN_CRIT "\t#%d: %3u%% system,\t%3u%% softirq,\t"
+			"%3u%% hardirq,\t%3u%% idle\n", i + 1,
+			__this_cpu_read(cpustat_util[group][STATS_SYSTEM]),
+			__this_cpu_read(cpustat_util[group][STATS_SOFTIRQ]),
+			__this_cpu_read(cpustat_util[group][STATS_HARDIRQ]),
+			__this_cpu_read(cpustat_util[group][STATS_IDLE]));
+	}
+}
+
+static void report_cpu_status(void)
+{
+	print_cpustat();
+}
+#else
+static inline void update_cpustat(void) { }
+static inline void report_cpu_status(void) { }
+#endif
+
 /*
  * Hard-lockup warnings should be triggered after just a few seconds. Soft-
  * lockups can have false positives under extreme conditions. So we generally
@@ -364,7 +457,7 @@ static void set_sample_period(void)
 	 * and hard thresholds) to increment before the
 	 * hardlockup detector generates a warning
 	 */
-	sample_period = get_softlockup_thresh() * ((u64)NSEC_PER_SEC / 5);
+	sample_period = get_softlockup_thresh() * ((u64)NSEC_PER_SEC / NUM_SAMPLE_PERIODS);
 	watchdog_update_hrtimer_threshold(sample_period);
 }
 
@@ -504,6 +597,8 @@ 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 +634,7 @@ 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));
+		report_cpu_status();
 		print_modules();
 		print_irqtrace_events(current);
 		if (regs)
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 975a07f9f1cc..49f652674bd8 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1029,6 +1029,19 @@ config SOFTLOCKUP_DETECTOR
 	  chance to run.  The current stack trace is displayed upon
 	  detection and the system will stay locked up.
 
+config SOFTLOCKUP_DETECTOR_INTR_STORM
+	bool "Detect Interrupt Storm in Soft Lockups"
+	depends on SOFTLOCKUP_DETECTOR && IRQ_TIME_ACCOUNTING
+	default y if NR_CPUS <= 128
+	help
+	  Say Y here to enable the kernel to detect interrupt storm
+	  during "soft lockups".
+
+	  "soft lockups" can be caused by a variety of reasons. If one is
+	  caused by an interrupt storm, then the storming interrupts will not
+	  be on the callstack. To detect this case, it is necessary to report
+	  the CPU stats and the interrupt counts during the "soft lockups".
+
 config BOOTPARAM_SOFTLOCKUP_PANIC
 	bool "Panic (Reboot) On Soft Lockups"
 	depends on SOFTLOCKUP_DETECTOR
-- 
2.37.1 (Apple Git-137.1)

[PATCHv7 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>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Liu Song <liusong@linux.alibaba.com>
---
 kernel/watchdog.c | 157 ++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 153 insertions(+), 4 deletions(-)

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 69e72d7e461d..0c521b8ae7aa 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -12,22 +12,25 @@
 
 #define pr_fmt(fmt) "watchdog: " fmt
 
-#include <linux/mm.h>
 #include <linux/cpu.h>
-#include <linux/nmi.h>
 #include <linux/init.h>
+#include <linux/irq.h>
+#include <linux/irqdesc.h>
 #include <linux/kernel_stat.h>
+#include <linux/kvm_para.h>
 #include <linux/math64.h>
+#include <linux/mm.h>
 #include <linux/module.h>
+#include <linux/nmi.h>
+#include <linux/stop_machine.h>
 #include <linux/sysctl.h>
 #include <linux/tick.h>
+
 #include <linux/sched/clock.h>
 #include <linux/sched/debug.h>
 #include <linux/sched/isolation.h>
-#include <linux/stop_machine.h>
 
 #include <asm/irq_regs.h>
-#include <linux/kvm_para.h>
 
 static DEFINE_MUTEX(watchdog_mutex);
 
@@ -417,13 +420,146 @@ static void print_cpustat(void)
 	}
 }
 
+#define HARDIRQ_PERCENT_THRESH		50
+#define NUM_HARDIRQ_REPORT		5
+static DEFINE_PER_CPU(u32 *, hardirq_counts);
+static DEFINE_PER_CPU(int, actual_nr_irqs);
+struct irq_counts {
+	int irq;
+	u32 counts;
+};
+
+/* Tabulate the most frequent interrupts. */
+static void tabulate_irq_count(struct irq_counts *irq_counts, int irq, u32 counts, int rank)
+{
+	int i;
+	struct irq_counts new_count = {irq, counts};
+
+	for (i = 0; i < rank; i++) {
+		if (counts > irq_counts[i].counts)
+			swap(new_count, irq_counts[i]);
+	}
+}
+
+/*
+ * If the hardirq time exceeds HARDIRQ_PERCENT_THRESH% of the sample_period,
+ * then the cause of softlockup might be interrupt storm. In this case, it
+ * would be useful to start interrupt counting.
+ */
+static bool need_counting_irqs(void)
+{
+	u8 util;
+	int tail = __this_cpu_read(cpustat_tail);
+
+	tail = (tail + NUM_HARDIRQ_REPORT - 1) % NUM_HARDIRQ_REPORT;
+	util = __this_cpu_read(cpustat_util[tail][STATS_HARDIRQ]);
+	return util > HARDIRQ_PERCENT_THRESH;
+}
+
+static void start_counting_irqs(void)
+{
+	int i;
+	int local_nr_irqs;
+	struct irq_desc *desc;
+	u32 *counts = __this_cpu_read(hardirq_counts);
+
+	if (!counts) {
+		/*
+		 * nr_irqs has the potential to grow at runtime. We should read
+		 * it and store locally to avoid array out-of-bounds access.
+		 */
+		local_nr_irqs = nr_irqs;
+		counts = kcalloc(local_nr_irqs, sizeof(u32), GFP_ATOMIC);
+		if (!counts)
+			return;
+
+		for (i = 0; i < local_nr_irqs; i++) {
+			desc = irq_to_desc(i);
+			if (!desc)
+				continue;
+			counts[i] = desc->kstat_irqs ?
+				*this_cpu_ptr(desc->kstat_irqs) : 0;
+		}
+
+		__this_cpu_write(actual_nr_irqs, local_nr_irqs);
+		__this_cpu_write(hardirq_counts, counts);
+	}
+}
+
+static void stop_counting_irqs(void)
+{
+	kfree(__this_cpu_read(hardirq_counts));
+	__this_cpu_write(hardirq_counts, NULL);
+}
+
+static void print_irq_counts(void)
+{
+	int i;
+	struct irq_desc *desc;
+	u32 counts_diff;
+	int local_nr_irqs = __this_cpu_read(actual_nr_irqs);
+	u32 *counts = __this_cpu_read(hardirq_counts);
+	struct irq_counts irq_counts_sorted[NUM_HARDIRQ_REPORT] = {
+		{-1, 0}, {-1, 0}, {-1, 0}, {-1, 0},
+	};
+
+	if (counts) {
+		for_each_irq_desc(i, desc) {
+			/*
+			 * We need to bounds-check in case someone on a different CPU
+			 * expanded nr_irqs.
+			 */
+			if (desc->kstat_irqs) {
+				counts_diff = *this_cpu_ptr(desc->kstat_irqs);
+				if (i < local_nr_irqs)
+					counts_diff -= counts[i];
+				tabulate_irq_count(irq_counts_sorted, 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 (irq_counts_sorted[i].irq == -1)
+				break;
+
+			desc = irq_to_desc(irq_counts_sorted[i].irq);
+			if (desc && desc->action)
+				printk(KERN_CRIT "\t#%u: %-10u\tirq#%d(%s)\n",
+				       i + 1, irq_counts_sorted[i].counts,
+				       irq_counts_sorted[i].irq, desc->action->name);
+			else
+				printk(KERN_CRIT "\t#%u: %-10u\tirq#%d\n",
+				       i + 1, irq_counts_sorted[i].counts,
+				       irq_counts_sorted[i].irq);
+		}
+
+		/*
+		 * If the hardirq time is less than HARDIRQ_PERCENT_THRESH% in the last
+		 * sample_period, then we suspect the interrupt storm might be subsiding.
+		 */
+		if (!need_counting_irqs())
+			stop_counting_irqs();
+	}
+}
+
 static void report_cpu_status(void)
 {
 	print_cpustat();
+	print_irq_counts();
 }
 #else
 static inline void update_cpustat(void) { }
 static inline void report_cpu_status(void) { }
+static inline bool need_counting_irqs(void) { return false; }
+static inline void start_counting_irqs(void) { }
+static inline void stop_counting_irqs(void) { }
 #endif
 
 /*
@@ -527,6 +663,18 @@ static int is_softlockup(unsigned long touch_ts,
 			 unsigned long now)
 {
 	if ((watchdog_enabled & WATCHDOG_SOFTOCKUP_ENABLED) && watchdog_thresh) {
+		/*
+		 * If period_ts has not been updated during a sample_period, then
+		 * in the subsequent few sample_periods, period_ts might also not
+		 * be updated, which could indicate a potential softlockup. In
+		 * this case, if we suspect the cause of the potential softlockup
+		 * might be interrupt storm, then we need to count the interrupts
+		 * to find which interrupt is storming.
+		 */
+		if (time_after_eq(now, period_ts + get_softlockup_thresh() / 5) &&
+		    need_counting_irqs())
+			start_counting_irqs();
+
 		/* Warn about unreasonable delays. */
 		if (time_after(now, period_ts + get_softlockup_thresh()))
 			return now - touch_ts;
@@ -549,6 +697,7 @@ static DEFINE_PER_CPU(struct cpu_stop_work, softlockup_stop_work);
 static int softlockup_fn(void *data)
 {
 	update_touch_ts();
+	stop_counting_irqs();
 	complete(this_cpu_ptr(&softlockup_completion));
 
 	return 0;
-- 
2.37.1 (Apple Git-137.1)

Re: [PATCHv7 2/2] watchdog/softlockup: report the most frequent interrupts

[Thomas Gleixner]

On Wed, Feb 14 2024 at 10:14, Bitao Hu wrote:
> +static void start_counting_irqs(void)
> +{
> +	int i;
> +	int local_nr_irqs;
> +	struct irq_desc *desc;
> +	u32 *counts = __this_cpu_read(hardirq_counts);
> +
> +	if (!counts) {
> +		/*
> +		 * nr_irqs has the potential to grow at runtime. We should read
> +		 * it and store locally to avoid array out-of-bounds access.
> +		 */
> +		local_nr_irqs = nr_irqs;
> +		counts = kcalloc(local_nr_irqs, sizeof(u32), GFP_ATOMIC);

Seriously? The system has a problem and you allocate memory from the
detection code in hard interrupt context?

> +		if (!counts)
> +			return;
> +
> +		for (i = 0; i < local_nr_irqs; i++) {
> +			desc = irq_to_desc(i);
> +			if (!desc)
> +				continue;
> +			counts[i] = desc->kstat_irqs ?
> +				*this_cpu_ptr(desc->kstat_irqs) : 0;
> +		}

This code has absolutely no business to access an interrupt
descriptor. There is an existing interface to retrieve the stats.

Also iterating one by one over the total number of interrupts is a
complete waste as the interrupt number space is sparse.

> +		for (i = 0; i < NUM_HARDIRQ_REPORT; i++) {
> +			if (irq_counts_sorted[i].irq == -1)
> +				break;
> +
> +			desc = irq_to_desc(irq_counts_sorted[i].irq);
> +			if (desc && desc->action)
> +				printk(KERN_CRIT "\t#%u: %-10u\tirq#%d(%s)\n",
> +				       i + 1, irq_counts_sorted[i].counts,
> +				       irq_counts_sorted[i].irq, desc->action->name);

You cannot dereference desc->action here:

    1) It can be NULL'ed between check and dereference.

    2) Both 'action' and 'action->name' can be freed in parallel

And no, you cannot take desc->lock here to prevent this. Stop fiddling
in the internals of interrupt descriptors.


See my reply on V1 how the stats can be done. That neither needs a
memory allocation nor the local_nr_irqs heuristics and just can use
proper interfaces.

Your initialization code then becomes:

	if (!this_cpu_read(snapshot_taken)) {
        	kstat_snapshot_irqs();
        	this_cpu_write(snapshot_taken, true);
        }

and the analysis boils down to:

        u64 cnt, sorted[3] = {};
        unsigned int irq, i;

    	for_each_active_irq(irq) {
        	cnt = kstat_get_irq_since_snapshot(irq);

		if (cnt) {
                     	for (cnt = (cnt << 32) + irq, i = 0; i < 3; i++) {
                		if (cnt > sorted[i])
                        		swap(cnt, sorted[i]);
                	}
		}
	}

Resetting the thing just becomes:

	this_cpu_write(snapshot_taken, false);

No allocation/free, no bound checks, proper abstractions. See?

Thanks,

        tglx

Re: [PATCHv7 2/2] watchdog/softlockup: report the most frequent interrupts

[Bitao Hu]

Hi,

On 2024/2/15 19:30, Thomas Gleixner wrote:
> On Wed, Feb 14 2024 at 10:14, Bitao Hu wrote:
>> +static void start_counting_irqs(void)
>> +{
>> +	int i;
>> +	int local_nr_irqs;
>> +	struct irq_desc *desc;
>> +	u32 *counts = __this_cpu_read(hardirq_counts);
>> +
>> +	if (!counts) {
>> +		/*
>> +		 * nr_irqs has the potential to grow at runtime. We should read
>> +		 * it and store locally to avoid array out-of-bounds access.
>> +		 */
>> +		local_nr_irqs = nr_irqs;
>> +		counts = kcalloc(local_nr_irqs, sizeof(u32), GFP_ATOMIC);
> 
> Seriously? The system has a problem and you allocate memory from the
> detection code in hard interrupt context?
I want all the changes for this feature to be concentrated within the
watchdog module, and I am also unsure whether modifying the irq code
for this feature would be justified. Hence, I opted for this approach.
However, your reply on V1 demonstrated the proper way to do it, so I
will refactor accordingly.

>> +		for (i = 0; i < NUM_HARDIRQ_REPORT; i++) {
>> +			if (irq_counts_sorted[i].irq == -1)
>> +				break;
>> +
>> +			desc = irq_to_desc(irq_counts_sorted[i].irq);
>> +			if (desc && desc->action)
>> +				printk(KERN_CRIT "\t#%u: %-10u\tirq#%d(%s)\n",
>> +				       i + 1, irq_counts_sorted[i].counts,
>> +				       irq_counts_sorted[i].irq, desc->action->name);
> 
> You cannot dereference desc->action here:
> 
>      1) It can be NULL'ed between check and dereference.
> 
>      2) Both 'action' and 'action->name' can be freed in parallel
> 
> And no, you cannot take desc->lock here to prevent this. Stop fiddling
> in the internals of interrupt descriptors.
Thanks for your analysis. However, I have a question. 'action->name'
cannot be accessed here, and it seems that merely outputting the
irq number provides insufficient information?

> 
> 
> and the analysis boils down to:
> 
>          u64 cnt, sorted[3] = {};
>          unsigned int irq, i;
> 
>      	for_each_active_irq(irq) {
>          	cnt = kstat_get_irq_since_snapshot(irq);
> 
> 		if (cnt) {
>                       	for (cnt = (cnt << 32) + irq, i = 0; i < 3; i++) {
>                  		if (cnt > sorted[i])
>                          		swap(cnt, sorted[i]);
Hmm, I think the approach here isn't optimal. If some interrupts
have the same count, then it effectively results in sorting by the
irq number. Is my understanding correct?

Best Regards,
	Bitao

Re: [PATCHv7 2/2] watchdog/softlockup: report the most frequent interrupts

[Thomas Gleixner]

On Mon, Feb 19 2024 at 17:12, Bitao Hu wrote:
> On 2024/2/15 19:30, Thomas Gleixner wrote:
>> On Wed, Feb 14 2024 at 10:14, Bitao Hu wrote:
>>> +		for (i = 0; i < NUM_HARDIRQ_REPORT; i++) {
>>> +			if (irq_counts_sorted[i].irq == -1)
>>> +				break;
>>> +
>>> +			desc = irq_to_desc(irq_counts_sorted[i].irq);
>>> +			if (desc && desc->action)
>>> +				printk(KERN_CRIT "\t#%u: %-10u\tirq#%d(%s)\n",
>>> +				       i + 1, irq_counts_sorted[i].counts,
>>> +				       irq_counts_sorted[i].irq, desc->action->name);
>> 
>> You cannot dereference desc->action here:
>> 
>>      1) It can be NULL'ed between check and dereference.
>> 
>>      2) Both 'action' and 'action->name' can be freed in parallel
>> 
>> And no, you cannot take desc->lock here to prevent this. Stop fiddling
>> in the internals of interrupt descriptors.
>
> Thanks for your analysis. However, I have a question. 'action->name'
> cannot be accessed here, and it seems that merely outputting the
> irq number provides insufficient information?

That's what you can access without risk. It's better than nothing, no?

>> and the analysis boils down to:
>> 
>>          u64 cnt, sorted[3] = {};
>>          unsigned int irq, i;
>> 
>>      	for_each_active_irq(irq) {
>>          	cnt = kstat_get_irq_since_snapshot(irq);
>> 
>> 		if (cnt) {
>>                       	for (cnt = (cnt << 32) + irq, i = 0; i < 3; i++) {
>>                  		if (cnt > sorted[i])
>>                          		swap(cnt, sorted[i]);
> Hmm, I think the approach here isn't optimal. If some interrupts
> have the same count, then it effectively results in sorting by the
> irq number. Is my understanding correct?

Sure, but what's the problem? If two interrupts have the same count then
the ordering is pretty much irrelevant, no?

Thanks,

        tglx