LinuxQuestions.org - Using Netlink to get Taskstats data (libnl-3)

Hey, I want to read out linux kernel statistics of a single thread using netlink socket and taskstats.

I could get taskstats to work using a python wrapper (https://github.com/facebook/gnlpy) but I want to do a C implementation.

After setting up the socket, the message parameters and do the sending, the receiving

Code:

nl_recvmsgs_default(sock)

always returns an error code -7 ("Invalid input data or parameter") or -12 ("Object not found") depending on how I create the message to send.

I checked all method invocations before nl_recvmsgs_default(sock) but dont get any error back. I guess I am missing a part in setting up the message or socket, but dont know what part it is.

Quote:

#include <stdlib.h>
#include <unistd.h>
#include <linux/taskstats.h>
#include <netlink/netlink.h>
#include <netlink/genl/genl.h>
#include <netlink/genl/ctrl.h>

int callback_message(struct nl_msg *, void *);

int main(int argc, char ** argv) {
struct nl_sock * sock;
struct nl_msg * msg;
int family;

sock = nl_socket_alloc();

// Connect to generic netlink socket on kernel side
genl_connect(sock);

// get the id for the TASKSTATS generic family
family = genl_ctrl_resolve(sock, "TASKSTATS");

// Allocate a new netlink message and inherit netlink message header.
msg = nlmsg_alloc();

genlmsg_put(msg, NL_AUTO_PID, NL_AUTO_SEQ, family, 0, 0, TASKSTATS_CMD_GET, TASKSTATS_VERSION))

//error code: -7 NLE_INVAL "Invalid input data or parameter",
nla_put_string(msg, TASKSTATS_CMD_ATTR_REGISTER_CPUMASK, "0");

//error code: -12 NLE_OBJ_NOTFOUND "Obj not found"
//nla_put_string(msg, TASKSTATS_CMD_ATTR_PID, "583");

nl_send_auto(sock, msg);

nlmsg_free(msg);

// specify a callback for inbound messages
nl_socket_modify_cb(sock, NL_CB_MSG_IN, NL_CB_CUSTOM, callback_message, NULL);

// gives error code -7 or -12 depending on the two nla_put_string alternatives above
printf("recv code (0 = success): d", nl_recvmsgs_default(sock));
}

Quote:

int callback_message(struct nl_msg * nlmsg, void * arg) {

struct nlmsghdr * nlhdr;
struct nlattr * nlattrs[TASKSTATS_TYPE_MAX + 1];
struct nlattr * nlattr;
struct taskstats * stats;
int rem;

nlhdr = nlmsg_hdr(nlmsg);
int answer;

if ((answer = genlmsg_parse(nlhdr, 0, nlattrs, TASKSTATS_TYPE_MAX, NULL))
< 0) {
printf("error parsing msg\n");
}

if ((nlattr = nlattrs[TASKSTATS_TYPE_AGGR_TGID]) || (nlattr =
nlattrs[TASKSTATS_TYPE_AGGR_PID]) || (nlattr =
nlattrs[TASKSTATS_TYPE_NULL])) {
stats = nla_data(nla_next(nla_data(nlattr), &rem));

printf("---\n");
printf("pid: %u\n", stats->ac_pid);
printf("command: %s\n", stats->ac_comm);
printf("status: %u\n", stats->ac_exitcode);
printf("time:\n");
printf(" start: %u\n", stats->ac_btime);
printf(" elapsed: %llu\n", stats->ac_etime);
printf(" user: %llu\n", stats->ac_utime);
printf(" system: %llu\n", stats->ac_stime);
printf("memory:\n");
printf(" bytetime:\n");
printf(" rss: %llu\n", stats->coremem);
printf(" vsz: %llu\n", stats->virtmem);
printf(" peak:\n");
printf(" rss: %llu\n", stats->hiwater_rss);
printf(" vsz: %llu\n", stats->hiwater_vm);
printf("io:\n");
printf(" bytes:\n");
printf(" read: %llu\n", stats->read_char);
printf(" write: %llu\n", stats->write_char);
printf(" syscalls:\n");
printf(" read: %llu\n", stats->read_syscalls);
printf(" write: %llu\n", stats->write_syscalls);
} else {
printf("unknown attribute format received\n");
}
return 0;
}

Does someone of you have a hint what I am doing wrong or what is missing to set up a correct netlink/taskstats message?

Thanks in advance, bkk