[PATCH 6.19 762/844] watchdog/softlockup: fix sample ring index wrap in need_counting_irqs()

[Sasha Levin]

From: Shengming Hu <hu.shengming@xxxxxxxxxx>

[ 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@xxxxxx
Fixes: e9a9292e2368 ("watchdog/softlockup: Report the most frequent interrupts")
Signed-off-by: Shengming Hu <hu.shengming@xxxxxxxxxx>
Reviewed-by: Petr Mladek <pmladek@xxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Mark Brown <broonie@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Zhang Run <zhang.run@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 kernel/watchdog.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 366122f4a0f87..70eaa77242814 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;
 }
-- 
2.51.0