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Message-ID: <b51ace6a-100b-4d79-9a48-c6528f25f6d2@linux.alibaba.com>
Date: Mon, 5 Feb 2024 09:27:39 +0800
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Subject: Re: [PATCHv4 1/3] watchdog/softlockup: low-overhead detection of
 interrupt
To: Bitao Hu <yaoma@linux.alibaba.com>, dianders@chromium.org,
 akpm@linux-foundation.org, pmladek@suse.com, kernelfans@gmail.com
Cc: linux-kernel@vger.kernel.org
References: <20240204145154.11069-1-yaoma@linux.alibaba.com>
 <20240204145154.11069-2-yaoma@linux.alibaba.com>
From: Liu Song <liusong@linux.alibaba.com>
In-Reply-To: <20240204145154.11069-2-yaoma@linux.alibaba.com>
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在 2024/2/4 22:51, 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@linux.alibaba.com>
> ---
>   kernel/watchdog.c | 91 +++++++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 91 insertions(+)
>
> diff --git a/kernel/watchdog.c b/kernel/watchdog.c
> index 81a8862295d6..7b121e166b81 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>
> @@ -333,6 +335,92 @@ __setup("watchdog_thresh=", watchdog_thresh_setup);
>   
>   static void __lockup_detector_cleanup(void);
>   
> +#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 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_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_ns)
> +{
> +	return data_ns >> 24LL; /* 2^24ns ~= 16.8ms */
> +}
> +
> +static void update_cpustat(void)
> +{
> +	int i;
> +	u8 util;
> +	u16 old_stat;
> +	u16 new_stat;
> +	u8 tail = __this_cpu_read(cpustat_tail);
> +	struct kernel_cpustat kcpustat;
> +	u64 *cpustat = kcpustat.cpustat;
> +	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_STATS_GROUPS);
> +}
> +
> +static void print_cpustat(void)
> +{
> +	int i;
> +	int 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_STATS_GROUPS; i++) {
> +		group = (tail + i) % NUM_STATS_GROUPS;
> +		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
> @@ -504,6 +592,8 @@ 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 +629,7 @@ 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)

Looks good, and advisable to combine declarations of variables of the 
same type into one line to save on the number of lines.

Reviewed-by: Liu Song <liusong@linux.alibaba.com>