Add monotonic clock calibration handling if clock speed is not found (#2776)

Currently, monotonic clock initialization relies on the model name field
from /proc/cpuinfo to retrieve the clock speed. However, this is not
always present. In case it is not present, measure the clock tick and
use it instead.

Before fix:
```
monotonic: x86 linux, unable to determine clock rate
```

After fix:
```
21695:M 25 Oct 2025 20:16:23.168 * monotonic clock: X86 TSC @ 2649 ticks/us
```

Fixes #2774

---------

Signed-off-by: Ken Nam <[email protected]>
Signed-off-by: Ran Shidlansik <[email protected]>
Co-authored-by: Ran Shidlansik <[email protected]>

Author: otherscase

src/monotonic.c

@@ -27,6 +27,8 @@ static char monotonic_info_string[32];
 #include <regex.h>
 #include <x86intrin.h>
 
+#define TSC_CALIBRATION_ITERATIONS 3
+
 static long mono_ticksPerMicrosecond = 0;
 
 static monotime getMonotonicUs_x86(void) {
@@ -64,6 +66,31 @@ static void monotonicInit_x86linux(void) {
                 break;
             }
         }
+        /* Some CPUs may not contain clock speed in the model name */
+        if (mono_ticksPerMicrosecond == 0) {
+            for (int i = 0; i < TSC_CALIBRATION_ITERATIONS; ++i) {
+                /* Calibrate TSC against CLOCK_MONOTONIC */
+                struct timespec start, end;
+                uint64_t tsc_start, tsc_end;
+
+                clock_gettime(CLOCK_MONOTONIC, &start);
+                tsc_start = __rdtsc();
+                usleep(10000); /* Sleep for 10ms */
+                tsc_end = __rdtsc();
+                clock_gettime(CLOCK_MONOTONIC, &end);
+
+                uint64_t elapsed_us = (end.tv_sec - start.tv_sec) * 1000000ULL + (end.tv_nsec - start.tv_nsec) / 1000;
+                uint64_t tsc_elapsed = tsc_end - tsc_start;
+                long sample_ticksPerMicrosecond = tsc_elapsed / elapsed_us;
+
+                /* Use the maximum out of TSC_CALIBRATION_ITERATIONS iterations for accuracy */
+                if (sample_ticksPerMicrosecond > mono_ticksPerMicrosecond) {
+                    mono_ticksPerMicrosecond = sample_ticksPerMicrosecond;
+                }
+            }
+        }
+        /* Rewind file to search for constant_tsc flag */
+        rewind(cpuinfo);
         while (fgets(buf, bufflen, cpuinfo) != NULL) {
             if (regexec(&constTscRegex, buf, nmatch, pmatch, 0) == 0) {
                 constantTsc = 1;