sched_getcpu example

README.adoc

@@ -17294,6 +17294,17 @@ The best article to understand spinlocks is: https://eli.thegreenplace.net/2018/
 
 The example in `man futex` is also a must.
 
+[[getcpu]]
+==== `getcpu` system call and the `sched_getaffinity` glibc wrapper
+
+Example: link:userland/linux/sched_getcpu.c[]
+
+Returns the CPU that the process/thread is currently running on: https://stackoverflow.com/questions/491520/how-can-i-get-the-cpu-core-number-from-within-a-user-space-app-linux-c
+
+So when running a multicore program, we may see that each thread can be running on a different core.
+
+The cores in which the process runs can be fixed with `sched_setaffinity` as shown at: link:userland/linux/sched_getaffinity.c[].
+
 === Linux calling conventions
 
 A summary of results is shown at: xref:table-linux-calling-conventions[xrefstyle=full].

path_properties.py

@@ -750,6 +750,7 @@ path_properties_tuples = (
                             'more_than_1s': True,
                             'requires_syscall_getcpu': True,
                         },
+                        'sched_getcpu.c': {'requires_syscall_getcpu': True},
                         'time_boot.c': {'requires_sudo': True},
                         'virt_to_phys_user.c': {'requires_argument': True},
                     }

userland/linux/sched_getcpu.c

@@ -0,0 +1,41 @@
+/* https://cirosantilli.com/linux-kernel-module-cheat#getcpu */
+
+#define _GNU_SOURCE
+#include <assert.h>
+#include <pthread.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+void* main_thread(void *arg) {
+    (void)arg;
+    printf("sched_getcpu = %d\n", sched_getcpu());
+    return NULL;
+}
+
+int main(int argc, char **argv) {
+    pthread_t *threads;
+    unsigned int nthreads, i;
+    if (argc > 1) {
+        nthreads = strtoll(argv[1], NULL, 0);
+    } else {
+        nthreads = 1;
+    }
+    threads = malloc(nthreads * sizeof(*threads));
+    for (i = 0; i < nthreads; ++i) {
+        assert(pthread_create(
+            &threads[i],
+            NULL,
+            main_thread,
+            NULL
+        ) == 0);
+    }
+    for (i = 0; i < nthreads; ++i) {
+        pthread_join(threads[i], NULL);
+    }
+    free(threads);
+    return EXIT_SUCCESS;
+}