fix(benchmarks): use CLOCK_PROCESS_CPUTIME_ID for actual CPU time

nsi_host_clock_gettime maps CLOCK_MONOTONIC to native_sim's simulated
time (which starts at 0 and doesn't advance during CPU-bound loops).

CLOCK_PROCESS_CPUTIME_ID measures actual CPU time consumed by the process.
It is maintained by the OS kernel per-process clock and should not be
redirected by native_sim's user-space time simulation.

Co-Authored-By: Oz <oz-agent@warp.dev>

Author: tbitcs

tests/benchmarks/kalman_benchmark/src/main.c

@@ -25,11 +25,16 @@
  * host OS clock, bypassing all simulated-time interception.
  */
 #include <time.h>
+/* nsi_host_clock_gettime maps CLOCK_MONOTONIC to simulated time.
+ * CLOCK_PROCESS_CPUTIME_ID measures actual CPU time consumed by this
+ * process — it is OS-maintained, not interceptable by user-space time
+ * simulation, and gives the true benchmark execution time.
+ */
 extern int nsi_host_clock_gettime(int clk_id, struct timespec *tp);
 static inline uint64_t bench_ns(void)
 {
 	struct timespec ts;
-	nsi_host_clock_gettime(CLOCK_MONOTONIC, &ts);
+	nsi_host_clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &ts);
 	return (uint64_t)ts.tv_sec * 1000000000ULL + (uint64_t)ts.tv_nsec;
 }
 #else

tests/benchmarks/pid_benchmark/src/main.c

@@ -27,7 +27,7 @@ extern int nsi_host_clock_gettime(int clk_id, struct timespec *tp);
 static inline uint64_t bench_ns(void)
 {
 	struct timespec ts;
-	nsi_host_clock_gettime(CLOCK_MONOTONIC, &ts);
+	nsi_host_clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &ts);
 	return (uint64_t)ts.tv_sec * 1000000000ULL + (uint64_t)ts.tv_nsec;
 }
 #else