Re: [PATCH] watchdog/softlockup: Fix sample ring index wrap in need_counting_irqs()

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 28 Jan 2026 18:52:11 +0100 Petr Mladek <pmladek@suse.com> wrote:

> On Mon 2026-01-19 21:59:05, shengminghu512 wrote:
> > From: Shengming Hu <hu.shengming@zte.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@suse.com>

Fixes: e9a9292e2368 ("watchdog/softlockup: Report the most frequent
interrupts"), yes?

What are the runtime effects of this?  "most frequent interrupts" data
is messed up?

I'm assuming we want to fix earlier kernels, so cc:stable?

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

Sure, I can queue it.  e9a9292e2368 was merged by tglx so he might want
to take it - if so I'll drop the mm.git copy if/when this appears in
linux-next.