[PATCH] per-task-delay-accounting: documentation

Delay accounting

----------------

Tasks encounter delays in execution when they wait

for some kernel resource to become available e.g. a

runnable task may wait for a free CPU to run on.

The per-task delay accounting functionality measures

the delays experienced by a task while

a) waiting for a CPU (while being runnable)

b) completion of synchronous block I/O initiated by the task

c) swapping in pages

and makes these statistics available to userspace through

the taskstats interface.

Such delays provide feedback for setting a task's cpu priority,

io priority and rss limit values appropriately. Long delays for

important tasks could be a trigger for raising its corresponding priority.

The functionality, through its use of the taskstats interface, also provides

delay statistics aggregated for all tasks (or threads) belonging to a

thread group (corresponding to a traditional Unix process). This is a commonly

needed aggregation that is more efficiently done by the kernel.

Userspace utilities, particularly resource management applications, can also

aggregate delay statistics into arbitrary groups. To enable this, delay

statistics of a task are available both during its lifetime as well as on its

exit, ensuring continuous and complete monitoring can be done.

Interface

---------

Delay accounting uses the taskstats interface which is described

in detail in a separate document in this directory. Taskstats returns a

generic data structure to userspace corresponding to per-pid and per-tgid

statistics. The delay accounting functionality populates specific fields of

this structure. See

     include/linux/taskstats.h

for a description of the fields pertaining to delay accounting.

It will generally be in the form of counters returning the cumulative

delay seen for cpu, sync block I/O, swapin etc.

Taking the difference of two successive readings of a given

counter (say cpu_delay_total) for a task will give the delay

experienced by the task waiting for the corresponding resource

in that interval.

When a task exits, records containing the per-task and per-process statistics

are sent to userspace without requiring a command. More details are given in

the taskstats interface description.

The getdelays.c userspace utility in this directory allows simple commands to

be run and the corresponding delay statistics to be displayed. It also serves

as an example of using the taskstats interface.

Usage

-----

Compile the kernel with

	CONFIG_TASK_DELAY_ACCT=y

	CONFIG_TASKSTATS=y

Enable the accounting at boot time by adding

the following to the kernel boot options

	delayacct

and after the system has booted up, use a utility

similar to  getdelays.c to access the delays

seen by a given task or a task group (tgid).

The utility also allows a given command to be

executed and the corresponding delays to be

seen.

General format of the getdelays command

getdelays [-t tgid] [-p pid] [-c cmd...]

Get delays, since system boot, for pid 10

# ./getdelays -p 10

(output similar to next case)

Get sum of delays, since system boot, for all pids with tgid 5

# ./getdelays -t 5

CPU	count	real total	virtual total	delay total

	7876	92005750	100000000	24001500

IO	count	delay total

	0	0

MEM	count	delay total

	0	0

Get delays seen in executing a given simple command

# ./getdelays -c ls /

bin   data1  data3  data5  dev  home  media  opt   root  srv        sys  usr

boot  data2  data4  data6  etc  lib   mnt    proc  sbin  subdomain  tmp  var

CPU	count	real total	virtual total	delay total

	6	4000250		4000000		0

IO	count	delay total

	0	0

MEM	count	delay total

	0	0

/* getdelays.c

 *

 * Utility to get per-pid and per-tgid delay accounting statistics

 * Also illustrates usage of the taskstats interface

 *

 * Copyright (C) Shailabh Nagar, IBM Corp. 2005

 * Copyright (C) Balbir Singh, IBM Corp. 2006

 *

 */

#include <stdio.h>

#include <stdlib.h>

#include <errno.h>

#include <unistd.h>

#include <poll.h>

#include <string.h>

#include <fcntl.h>

#include <sys/types.h>

#include <sys/stat.h>

#include <sys/socket.h>

#include <sys/types.h>

#include <signal.h>

#include <linux/genetlink.h>

#include <linux/taskstats.h>

/*

 * Generic macros for dealing with netlink sockets. Might be duplicated

 * elsewhere. It is recommended that commercial grade applications use

 * libnl or libnetlink and use the interfaces provided by the library

 */

#define GENLMSG_DATA(glh)	((void *)(NLMSG_DATA(glh) + GENL_HDRLEN))

#define GENLMSG_PAYLOAD(glh)	(NLMSG_PAYLOAD(glh, 0) - GENL_HDRLEN)

#define NLA_DATA(na)		((void *)((char*)(na) + NLA_HDRLEN))

#define NLA_PAYLOAD(len)	(len - NLA_HDRLEN)

#define err(code, fmt, arg...) do { printf(fmt, ##arg); exit(code); } while (0)

int done = 0;

/*

 * Create a raw netlink socket and bind

 */

static int create_nl_socket(int protocol, int groups)

{

    socklen_t addr_len;

    int fd;

    struct sockaddr_nl local;

    fd = socket(AF_NETLINK, SOCK_RAW, protocol);

    if (fd < 0)

	return -1;

    memset(&local, 0, sizeof(local));

    local.nl_family = AF_NETLINK;

    local.nl_groups = groups;

    if (bind(fd, (struct sockaddr *) &local, sizeof(local)) < 0)

	goto error;

    return fd;

  error:

    close(fd);

    return -1;

}

int sendto_fd(int s, const char *buf, int bufLen)

{

    struct sockaddr_nl nladdr;

    int r;

    memset(&nladdr, 0, sizeof(nladdr));

    nladdr.nl_family = AF_NETLINK;

    while ((r = sendto(s, buf, bufLen, 0, (struct sockaddr *) &nladdr,

		       sizeof(nladdr))) < bufLen) {

	if (r > 0) {

	    buf += r;

	    bufLen -= r;

	} else if (errno != EAGAIN)

	    return -1;

    }

    return 0;

}

/*

 * Probe the controller in genetlink to find the family id

 * for the TASKSTATS family

 */

int get_family_id(int sd)

{

    struct {

	struct nlmsghdr n;

	struct genlmsghdr g;

	char buf[256];

    } family_req;

    struct {

	struct nlmsghdr n;

	struct genlmsghdr g;

	char buf[256];

    } ans;

    int id;

    struct nlattr *na;

    int rep_len;

    /* Get family name */

    family_req.n.nlmsg_type = GENL_ID_CTRL;

    family_req.n.nlmsg_flags = NLM_F_REQUEST;

    family_req.n.nlmsg_seq = 0;

    family_req.n.nlmsg_pid = getpid();

    family_req.n.nlmsg_len = NLMSG_LENGTH(GENL_HDRLEN);

    family_req.g.cmd = CTRL_CMD_GETFAMILY;

    family_req.g.version = 0x1;

    na = (struct nlattr *) GENLMSG_DATA(&family_req);

    na->nla_type = CTRL_ATTR_FAMILY_NAME;

    na->nla_len = strlen(TASKSTATS_GENL_NAME) + 1 + NLA_HDRLEN;

    strcpy(NLA_DATA(na), TASKSTATS_GENL_NAME);

    family_req.n.nlmsg_len += NLMSG_ALIGN(na->nla_len);

    if (sendto_fd(sd, (char *) &family_req, family_req.n.nlmsg_len) < 0)

	err(1, "error sending message via Netlink\n");

    rep_len = recv(sd, &ans, sizeof(ans), 0);

    if (rep_len < 0)

	err(1, "error receiving reply message via Netlink\n");

    /* Validate response message */

    if (!NLMSG_OK((&ans.n), rep_len))

	err(1, "invalid reply message received via Netlink\n");

    if (ans.n.nlmsg_type == NLMSG_ERROR) {	/* error */

	printf("error received NACK - leaving\n");

	exit(1);

    }

    na = (struct nlattr *) GENLMSG_DATA(&ans);

    na = (struct nlattr *) ((char *) na + NLA_ALIGN(na->nla_len));

    if (na->nla_type == CTRL_ATTR_FAMILY_ID) {

	id = *(__u16 *) NLA_DATA(na);

    }

    return id;

}

void print_taskstats(struct taskstats *t)

{

    printf("\n\nCPU   %15s%15s%15s%15s\n"

	   "      %15llu%15llu%15llu%15llu\n"

	   "IO    %15s%15s\n"

	   "      %15llu%15llu\n"

	   "MEM   %15s%15s\n"

	   "      %15llu%15llu\n\n",

	   "count", "real total", "virtual total", "delay total",

	   t->cpu_count, t->cpu_run_real_total, t->cpu_run_virtual_total,

	   t->cpu_delay_total,

	   "count", "delay total",

	   t->blkio_count, t->blkio_delay_total,

	   "count", "delay total", t->swapin_count, t->swapin_delay_total);

}

void sigchld(int sig)

{

    done = 1;

}

int main(int argc, char *argv[])

{

    int rc;

    int sk_nl;

    struct nlmsghdr *nlh;

    struct genlmsghdr *genlhdr;

    char *buf;

    struct taskstats_cmd_param *param;

    __u16 id;

    struct nlattr *na;

    /* For receiving */

    struct sockaddr_nl kern_nla, from_nla;

    socklen_t from_nla_len;

    int recv_len;

    struct taskstats_reply *reply;

    struct {

	struct nlmsghdr n;

	struct genlmsghdr g;

	char buf[256];

    } req;

    struct {

	struct nlmsghdr n;

	struct genlmsghdr g;

	char buf[256];

    } ans;

    int nl_sd = -1;

    int rep_len;

    int len = 0;

    int aggr_len, len2;

    struct sockaddr_nl nladdr;

    pid_t tid = 0;

    pid_t rtid = 0;

    int cmd_type = TASKSTATS_TYPE_TGID;

    int c, status;

    int forking = 0;

    struct sigaction act = {

	.sa_handler = SIG_IGN,

	.sa_mask = SA_NOMASK,

    };

    struct sigaction tact ;

    if (argc < 3) {

	printf("usage %s [-t tgid][-p pid][-c cmd]\n", argv[0]);

	exit(-1);

    }

    tact.sa_handler = sigchld;

    sigemptyset(&tact.sa_mask);

    if (sigaction(SIGCHLD, &tact, NULL) < 0)

	err(1, "sigaction failed for SIGCHLD\n");

    while (1) {

	c = getopt(argc, argv, "t:p:c:");

	if (c < 0)

	    break;

	switch (c) {

	case 't':

	    tid = atoi(optarg);

	    if (!tid)

		err(1, "Invalid tgid\n");

	    cmd_type = TASKSTATS_CMD_ATTR_TGID;

	    break;

	case 'p':

	    tid = atoi(optarg);

	    if (!tid)

		err(1, "Invalid pid\n");

	    cmd_type = TASKSTATS_CMD_ATTR_TGID;

	    break;

	case 'c':

	    opterr = 0;

	    tid = fork();

	    if (tid < 0)

		err(1, "fork failed\n");

	    if (tid == 0) {	/* child process */

		if (execvp(argv[optind - 1], &argv[optind - 1]) < 0) {

		    exit(-1);

		}

	    }

	    forking = 1;

	    break;

	default:

	    printf("usage %s [-t tgid][-p pid][-c cmd]\n", argv[0]);

	    exit(-1);

	    break;

	}

	if (c == 'c')

	    break;

    }

    /* Construct Netlink request message */

    /* Send Netlink request message & get reply */

    if ((nl_sd =

	 create_nl_socket(NETLINK_GENERIC, TASKSTATS_LISTEN_GROUP)) < 0)

	err(1, "error creating Netlink socket\n");

    id = get_family_id(nl_sd);

    /* Send command needed */

    req.n.nlmsg_len = NLMSG_LENGTH(GENL_HDRLEN);

    req.n.nlmsg_type = id;

    req.n.nlmsg_flags = NLM_F_REQUEST;

    req.n.nlmsg_seq = 0;

    req.n.nlmsg_pid = tid;

    req.g.cmd = TASKSTATS_CMD_GET;

    na = (struct nlattr *) GENLMSG_DATA(&req);

    na->nla_type = cmd_type;

    na->nla_len = sizeof(unsigned int) + NLA_HDRLEN;

    *(__u32 *) NLA_DATA(na) = tid;

    req.n.nlmsg_len += NLMSG_ALIGN(na->nla_len);

    if (!forking && sendto_fd(nl_sd, (char *) &req, req.n.nlmsg_len) < 0)

	err(1, "error sending message via Netlink\n");

    act.sa_handler = SIG_IGN;

    sigemptyset(&act.sa_mask);

    if (sigaction(SIGINT, &act, NULL) < 0)

	err(1, "sigaction failed for SIGINT\n");

    do {

	int i;

	struct pollfd pfd;

	int pollres;

	pfd.events = 0xffff & ~POLLOUT;

	pfd.fd = nl_sd;

	pollres = poll(&pfd, 1, 5000);

	if (pollres < 0 || done) {

	    break;

	}

	rep_len = recv(nl_sd, &ans, sizeof(ans), 0);

	nladdr.nl_family = AF_NETLINK;

	nladdr.nl_groups = TASKSTATS_LISTEN_GROUP;

	if (ans.n.nlmsg_type == NLMSG_ERROR) {	/* error */

	    printf("error received NACK - leaving\n");

	    exit(1);

	}

	if (rep_len < 0) {

	    err(1, "error receiving reply message via Netlink\n");

	    break;

	}

	/* Validate response message */

	if (!NLMSG_OK((&ans.n), rep_len))

	    err(1, "invalid reply message received via Netlink\n");

	rep_len = GENLMSG_PAYLOAD(&ans.n);

	na = (struct nlattr *) GENLMSG_DATA(&ans);

	len = 0;

	i = 0;

	while (len < rep_len) {

	    len += NLA_ALIGN(na->nla_len);

	    switch (na->nla_type) {

	    case TASKSTATS_TYPE_AGGR_PID:

		/* Fall through */

	    case TASKSTATS_TYPE_AGGR_TGID:

		aggr_len = NLA_PAYLOAD(na->nla_len);

		len2 = 0;

		/* For nested attributes, na follows */

		na = (struct nlattr *) NLA_DATA(na);

		done = 0;

		while (len2 < aggr_len) {

		    switch (na->nla_type) {

		    case TASKSTATS_TYPE_PID:

			rtid = *(int *) NLA_DATA(na);

			break;

		    case TASKSTATS_TYPE_TGID:

			rtid = *(int *) NLA_DATA(na);

			break;

		    case TASKSTATS_TYPE_STATS:

			if (rtid == tid) {

			    print_taskstats((struct taskstats *)

					    NLA_DATA(na));

			    done = 1;

			}

			break;

		    }

		    len2 += NLA_ALIGN(na->nla_len);

		    na = (struct nlattr *) ((char *) na + len2);

		    if (done)

			break;

		}

	    }

	    na = (struct nlattr *) (GENLMSG_DATA(&ans) + len);

	    if (done)

		break;

	}

	if (done)

	    break;

    }

    while (1);

    close(nl_sd);

    return 0;

}

per-tgid stats to be sent for each exiting task is explained in the per-tgid

stats section below.

getdelays.c is a simple utility demonstrating usage of the taskstats interface

for reporting delay accounting statistics.

Interface

---------