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Message-ID: <f6525b03-0864-4bd3-b083-3532f0042745@linux.alibaba.com>
Date: Wed, 31 Jan 2024 09:19:35 +0800
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Subject: Re: [PATCHv2 1/2] watchdog/softlockup: low-overhead detection of
 interrupt storm
To: Bitao Hu <yaoma@linux.alibaba.com>, dianders@chromium.org,
 akpm@linux-foundation.org, pmladek@suse.com, lecopzer.chen@mediatek.com,
 kernelfans@gmail.com
Cc: linux-kernel@vger.kernel.org
References: <20240130074744.45759-1-yaoma@linux.alibaba.com>
 <20240130074744.45759-2-yaoma@linux.alibaba.com>
From: Liu Song <liusong@linux.alibaba.com>
In-Reply-To: <20240130074744.45759-2-yaoma@linux.alibaba.com>
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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@linux.alibaba.com>
> ---
>   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?
> +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.

> +
> +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.


> +
> +	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)