Obtaining counter data through an API

Red Hat Enterprise Linux 9.2 LPAR mode

Applications can access the CPU-measurement counters through ioctls.

For example, the lshwc tool uses these ioctls.

Application programmers: This information is intended for programmers who want to write applications that extract data from CPU-measurement counters.

Table 1. CPU-measurement counter facility ioctls
Name	Structure passed	Description
S390_HWCTR_START	s390_ctrset_start	Start counter sets on CPUs.
S390_HWCTR_READ	s390_ctrset_read	Read counter sets on CPUs.
S390_HWCTR_STOP	n/a	Stop counter sets on CPUs.

For details about the calls and structures, see the arch/s390/include/uapi/asm/hwctrset.h header file in the Linux® kernel source.

Issue the ioctls on the misc character device /dev/hwctr. You must first open this device node in read/write mode.

Sample program

unsigned long cpumask[1];
struct s390_hwctr_start start;

struct s390_hwctr_read read;
int fd, rc;

/* Open device */
fd = open(S390_HWCTR_DEVICE, O_RDWR);

/* Start counter sets */
cpumask[0] = 0xffff; /* Use CPUs 0-15 */
start.version = 1;
start.counter_sets = S390_HWCTR_ALL;
start.cpumask_len = sizeof(cpumask);
start.cpumask = cpumask;
rc = ioctl(fd, S390_HWCTR_START, &start);

/* Read counter sets */
read.data = malloc(start.data_bytes + sizeof(*read));
rc = ioctl(fd, S390_HWCTR_READ, &read);

/* Stop counter sets */
rc = ioctl(fd, S390_HWCTR_STOP, 0);

/* Close device */
close(fd);