watchdog/softlockup: fix incorrect CPU utilization output during softlockup

Since we use 16-bit precision, the raw data will undergo integer division,
which may sometimes result in data loss.  This can lead to slightly
inaccurate CPU utilization calculations.  Under normal circumstances, this
isn't an issue.  However, when CPU utilization reaches 100%, the
calculated result might exceed 100%.  For example, with raw data like the
following:

sample_period 400000134 new_stat 83648414036 old_stat 83247417494

sample_period=400000134/2^24=23
new_stat=83648414036/2^24=4985
old_stat=83247417494/2^24=4961
util=105%

Below log will output：

CPU#3 Utilization every 0s during lockup:
    #1:   0% system,          0% softirq,   105% hardirq,     0% idle
    #2:   0% system,          0% softirq,   105% hardirq,     0% idle
    #3:   0% system,          0% softirq,   100% hardirq,     0% idle
    #4:   0% system,          0% softirq,   105% hardirq,     0% idle
    #5:   0% system,          0% softirq,   105% hardirq,     0% idle

To avoid confusion, we enforce a 100% display cap when calculations exceed
this threshold.

We also round to the nearest multiple of 16.8 milliseconds to improve the
accuracy.

[yaozhenguo1@gmail.com: make get_16bit_precision() more accurate, fix comment layout]
Link: https://lkml.kernel.org/r/20250818081438.40540-1-yaozhenguo@jd.com
Link: https://lkml.kernel.org/r/20250812082510.32291-1-yaozhenguo@jd.com
Signed-off-by: ZhenguoYao <yaozhenguo1@gmail.com>
Cc: Bitao Hu <yaoma@linux.alibaba.com>
Cc: Li Huafei <lihuafei1@huawei.com>
Cc: Max Kellermann <max.kellermann@ionos.com>
Cc: Thomas Gleinxer <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 9c7134f7d2c48b4e9bba16c0a374f2df1da34448..5413aa85e8a4f93126654bf7f0b28923ee98e013 100644 (file)
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -425,7 +425,11 @@ static DEFINE_PER_CPU(u8, cpustat_tail);
  */
 static u16 get_16bit_precision(u64 data_ns)
 {
-       return data_ns >> 24LL; /* 2^24ns ~= 16.8ms */
+       /*
+        * 2^24ns ~= 16.8ms
+        * Round to the nearest multiple of 16.8 milliseconds.
+        */
+       return (data_ns + (1 << 23)) >> 24LL;
 }
 
 static void update_cpustat(void)
@@ -444,6 +448,14 @@ static void update_cpustat(void)
                old_stat = __this_cpu_read(cpustat_old[i]);
                new_stat = get_16bit_precision(cpustat[tracked_stats[i]]);
                util = DIV_ROUND_UP(100 * (new_stat - old_stat), sample_period_16);
+               /*
+                * Since we use 16-bit precision, the raw data will undergo
+                * integer division, which may sometimes result in data loss,
+                * and then result might exceed 100%. To avoid confusion,
+                * we enforce a 100% display cap when calculations exceed this threshold.
+                */
+               if (util > 100)
+                       util = 100;
                __this_cpu_write(cpustat_util[tail][i], util);
                __this_cpu_write(cpustat_old[i], new_stat);
        }