The struct taskstats

struct taskstats field groups와 확장 규칙

이 문서는 `struct taskstats` field를 설명합니다. Common/basic accounting field는 `CONFIG_TASKSTATS`가 설정되면 taskstats interface가 활성화되고 task의 `do_exit()` 때 전달할 값이 수집됩니다.

Delay accounting field는 `/* Delay accounting fields start */`와 `/* Delay accounting fields end */` 사이에 있으며 `CONFIG_TASK_DELAY_ACCT`가 설정될 때 수집합니다. Extended accounting field는 대응하는 start/end marker 사이에 있고 `CONFIG_TASK_XACCT`가 설정될 때 수집합니다.

그 밖에 per-task/per-thread context switch count, SMT machine time accounting, memory reclaim용 extended delay accounting field가 있습니다. 향후 확장은 `taskstats` structure 끝에 field를 추가해야 하며 기존 field의 상대적 위치를 바꾸면 안 됩니다.

Common/basic accounting fields

`version`은 `<linux/taskstats.h>`의 `TASKSTATS_VERSION`으로 항상 설정하며 structure가 바뀔 때마다 증가해야 합니다. `ac_exitcode`는 task exit status, `ac_flag`는 `<linux/acct.h>`의 `AFORK`, `ASU`, `ACOMPAT`, `ACORE`, `AXSIG`, `ac_nice`는 `task_nice()` 값, `ac_comm`은 task를 시작한 command name, `ac_sched`는 `task->policy` scheduling discipline입니다.

`ac_uid`, `ac_gid`, `ac_pid`, `ac_ppid`는 user, group, process, parent process ID입니다. `ac_btime`은 1970년 이후 task begin seconds, `ac_etime`은 elapsed microseconds, `ac_utime`과 `ac_stime`은 user/system CPU microseconds입니다. `ac_minflt`와 `ac_majflt`는 각각 `task->min_flt`, `task->maj_flt`의 minor/major page fault count입니다.

  struct taskstats {

1) Common and basic accounting fields::

	/* The version number of this struct. This field is always set to
	 * TASKSTATS_VERSION, which is defined in <linux/taskstats.h>.
	 * Each time the struct is changed, the value should be incremented.
	 */
	__u16	version;

	/* The exit code of a task. */
	__u32	ac_exitcode;		/* Exit status */

	/* The accounting flags of a task as defined in <linux/acct.h>
	 * Defined values are AFORK, ASU, ACOMPAT, ACORE, and AXSIG.
	 */
	__u8	ac_flag;		/* Record flags */

	/* The value of task_nice() of a task. */
	__u8	ac_nice;		/* task_nice */

	/* The name of the command that started this task. */
	char	ac_comm[TS_COMM_LEN];	/* Command name */

	/* The scheduling discipline as set in task->policy field. */
	__u8	ac_sched;		/* Scheduling discipline */

	__u8	ac_pad[3];
	__u32	ac_uid;			/* User ID */
	__u32	ac_gid;			/* Group ID */
	__u32	ac_pid;			/* Process ID */
	__u32	ac_ppid;		/* Parent process ID */

	/* The time when a task begins, in [secs] since 1970. */
	__u32	ac_btime;		/* Begin time [sec since 1970] */

	/* The elapsed time of a task, in [usec]. */
	__u64	ac_etime;		/* Elapsed time [usec] */

	/* The user CPU time of a task, in [usec]. */
	__u64	ac_utime;		/* User CPU time [usec] */

	/* The system CPU time of a task, in [usec]. */
	__u64	ac_stime;		/* System CPU time [usec] */

	/* The minor page fault count of a task, as set in task->min_flt. */
	__u64	ac_minflt;		/* Minor Page Fault Count */

	/* The major page fault count of a task, as set in task->maj_flt. */
	__u64	ac_majflt;		/* Major Page Fault Count */

Delay accounting fields

Delay accounting이 enable된 경우 start/end marker 사이의 값을 사용할 수 있습니다. `xxx_count`는 기록한 delay value 수, `xxx_delay_total`은 nanoseconds 단위 cumulative delay입니다. `xxx_delay_total`은 overflow 때 0으로 wrap하지만 `xxx_count`는 overflow와 무관하게 증가합니다.

`cpu_count`, `cpu_delay_total`은 runnable 상태에서 CPU를 기다린 횟수와 delay이며 atomically update되지 않습니다. 이후 네 field는 `task->delays->lock`으로 atomic update됩니다. `blkio_count`, `blkio_delay_total`은 synchronous block I/O completion 대기이며 I/O submission delay는 포함하지 않습니다. `swapin_count`, `swapin_delay_total`은 page-fault I/O 중 swap-in 대기입니다.

`cpu_run_real_total`은 CPU wall-clock running time입니다. 일부 architecture에서는 virtualization 때문에 kernel이 원치 않게 wait하며 빼앗긴 CPU time을 보정합니다. `cpu_run_virtual_total`은 kernel이 본 interval을 사용해 그 보정을 하지 않습니다. 둘 다 nanoseconds cumulative value이고 count가 없으며 overflow 때 조용히 0으로 wrap합니다. 여기까지가 version 1의 끝입니다.

	/* Delay accounting fields start
	 *
	 * All values, until the comment "Delay accounting fields end" are
	 * available only if delay accounting is enabled, even though the last
	 * few fields are not delays
	 *
	 * xxx_count is the number of delay values recorded
	 * xxx_delay_total is the corresponding cumulative delay in nanoseconds
	 *
	 * xxx_delay_total wraps around to zero on overflow
	 * xxx_count incremented regardless of overflow
	 */

	/* Delay waiting for cpu, while runnable
	 * count, delay_total NOT updated atomically
	 */
	__u64	cpu_count;
	__u64	cpu_delay_total;

	/* Following four fields atomically updated using task->delays->lock */

	/* Delay waiting for synchronous block I/O to complete
	 * does not account for delays in I/O submission
	 */
	__u64	blkio_count;
	__u64	blkio_delay_total;

	/* Delay waiting for page fault I/O (swap in only) */
	__u64	swapin_count;
	__u64	swapin_delay_total;

	/* cpu "wall-clock" running time
	 * On some architectures, value will adjust for cpu time stolen
	 * from the kernel in involuntary waits due to virtualization.
	 * Value is cumulative, in nanoseconds, without a corresponding count
	 * and wraps around to zero silently on overflow
	 */
	__u64	cpu_run_real_total;

	/* cpu "virtual" running time
	 * Uses time intervals seen by the kernel i.e. no adjustment
	 * for kernel's involuntary waits due to virtualization.
	 * Value is cumulative, in nanoseconds, without a corresponding count
	 * and wraps around to zero silently on overflow
	 */
	__u64	cpu_run_virtual_total;
	/* Delay accounting fields end */
	/* version 1 ends here */

Extended accounting fields

`coremem`은 task lifetime의 accumulated RSS usage를 MB-usec 단위로 나타냅니다. Task system time에 tick이 charge될 때마다 current RSS를 더하므로 memory usage와 system time의 곱이 되고 system time unit당 average usage를 계산할 수 있습니다. `virtmem`은 같은 방식으로 virtual memory usage를 추적합니다.

`hiwater_rss`와 `hiwater_vm`은 task lifetime의 RSS와 VM high-watermark를 KBytes로 나타냅니다. `read_char`, `write_char`, `read_syscalls`, `write_syscalls`는 task의 bytes read/written과 read/write syscall 수입니다.

	/* Extended accounting fields start */

	/* Accumulated RSS usage in duration of a task, in MBytes-usecs.
	 * The current rss usage is added to this counter every time
	 * a tick is charged to a task's system time. So, at the end we
	 * will have memory usage multiplied by system time. Thus an
	 * average usage per system time unit can be calculated.
	 */
	__u64	coremem;		/* accumulated RSS usage in MB-usec */

	/* Accumulated virtual memory usage in duration of a task.
	 * Same as acct_rss_mem1 above except that we keep track of VM usage.
	 */
	__u64	virtmem;		/* accumulated VM usage in MB-usec */

	/* High watermark of RSS usage in duration of a task, in KBytes. */
	__u64	hiwater_rss;		/* High-watermark of RSS usage */

	/* High watermark of VM  usage in duration of a task, in KBytes. */
	__u64	hiwater_vm;		/* High-water virtual memory usage */

	/* The following four fields are I/O statistics of a task. */
	__u64	read_char;		/* bytes read */
	__u64	write_char;		/* bytes written */
	__u64	read_syscalls;		/* read syscalls */
	__u64	write_syscalls;		/* write syscalls */

	/* Extended accounting fields end */

Context switch, SMT와 reclaim fields

`nvcsw`와 `nivcsw`는 voluntary/involuntary context switch counter입니다. SMT machine용 `ac_utimescaled`, `ac_stimescaled`는 frequency 등을 반영해 scale한 user/system time이고 `cpu_scaled_run_real_total`은 scale한 `cpu_run_real_total`입니다.

Memory reclaim용 extended delay field `freepages_count`, `freepages_delay_total`은 memory reclaim을 기다린 횟수와 cumulative delay를 저장합니다.

	__u64	nvcsw;			/* Context voluntary switch counter */
	__u64	nivcsw;			/* Context involuntary switch counter */

5) Time accounting for SMT machines::

	__u64	ac_utimescaled;		/* utime scaled on frequency etc */
	__u64	ac_stimescaled;		/* stime scaled on frequency etc */
	__u64	cpu_scaled_run_real_total; /* scaled cpu_run_real_total */

6) Extended delay accounting fields for memory reclaim::

	/* Delay waiting for memory reclaim */
	__u64	freepages_count;
	__u64	freepages_delay_total;

::

  }