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.

[[email protected]: make get_16bit_precision() more accurate, fix comment layout]
  Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: ZhenguoYao <[email protected]>
Cc: Bitao Hu <[email protected]>
Cc: Li Huafei <[email protected]>
Cc: Max Kellermann <[email protected]>
Cc: Thomas Gleinxer <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: ZhenguoYao1

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);
 	}