[PATCH 6.18 679/752] watchdog/softlockup: fix sample ring index wrap in need_counting_irqs()

Sasha Levin
2026-02-28T17:58:15Z

From: Shengming Hu <hu.shengming@zte.com.cn>

[ Upstream commit cafe4074a7221dca2fa954dd1ab0cf99b6318e23 ]

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.

Link: https://lkml.kernel.org/r/tencent_7068189CB6D6689EB353F3D17BF5A5311A07@qq.com
Fixes: e9a9292e2368 ("watchdog/softlockup: Report the most frequent interrupts")
Signed-off-by: Shengming Hu <hu.shengming@zte.com.cn>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Zhang Run <zhang.run@zte.com.cn>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/watchdog.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 5b62d10027836..25d134306e146 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -521,7 +521,7 @@ 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;
}
--
2.51.0