linux-kernel - Re: [PATCH] watchdog/softlockup: Fix sample ring index wrap in need_counting

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Message-ID: <aXpMy67Ra5ejKVnm@pathway.suse.cz>
Date: Wed, 28 Jan 2026 18:52:11 +0100
From: Petr Mladek <pmladek@...e.com>
To: shengminghu512 <shengminghu512@...com>
Cc: akpm <akpm@...ux-foundation.org>, tglx <tglx@...utronix.de>,
	mingo <mingo@...nel.org>, broonie <broonie@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"hu.shengming" <hu.shengming@....com.cn>,
	"zhang.run" <zhang.run@....com.cn>
Subject: Re: [PATCH] watchdog/softlockup: Fix sample ring index wrap in
 need_counting_irqs()

On Mon 2026-01-19 21:59:05, shengminghu512 wrote:
> From: Shengming Hu <hu.shengming@....com.cn>
> 
> cpustat_tail indexes cpustat_util[], which is a NUM_SAMPLE_PERIODS-sized
> ring buffer. need_counting_irqs() currently wraps the index using
> NUM_HARDIRQ_REPORT, which only happens to match NUM_SAMPLE_PERIODS.
> 
> Use NUM_SAMPLE_PERIODS for the wrap to keep the ring math correct even if
> the NUM_HARDIRQ_REPORT or  NUM_SAMPLE_PERIODS changes.
> 
> ---
>  kernel/watchdog.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/watchdog.c b/kernel/watchdog.c
> index b4d5fbdb9..7d675781b 100644
> --- a/kernel/watchdog.c
> +++ b/kernel/watchdog.c
> @@ -550,7 +550,7 @@ static bool need_counting_irqs(void)
>  	u8 util;
>  	int tail = __this_cpu_read(cpustat_tail);
>  
> -	tail = (tail + NUM_HARDIRQ_REPORT - 1) % NUM_HARDIRQ_REPORT;
> +	tail = (tail + NUM_SAMPLE_PERIODS - 1) % NUM_SAMPLE_PERIODS;
>  	util = __this_cpu_read(cpustat_util[tail][STATS_HARDIRQ]);
>  	return util > HARDIRQ_PERCENT_THRESH;

Great catch! It makes perfect sense.

The NUM_HARDIRQ_REPORT is used for another array (irq_counts_sorted[])
with the most frequent IRQs. This code was added with the same commit
which added the other array. It would explain the mistake.

Reviewed-by: Petr Mladek <pmladek@...e.com>

Andrew, I assume that you would take it...

Best Regards,
Petr









>  }
> -- 
> 2.25.1
>