Start_Kernel (Week 14)

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init_numa_balancing | Notion

함수 선언

https://linux-study.notion.site/init_numa_balancing-0abb1f860ebf44e3be72538b6adb360f

함수 선언

함수 정의

void init_numa_balancing(unsigned long clone_flags, struct task_struct *p)
{
	int mm_users = 0;
	struct mm_struct *mm = p->mm;

	if (mm) {
		mm_users = atomic_read(&mm->mm_users);
		if (mm_users == 1) {
			mm->numa_next_scan = jiffies + msecs_to_jiffies(sysctl_numa_balancing_scan_delay);
			mm->numa_scan_seq = 0;
		}
	}
	p->node_stamp			= 0;
	p->numa_scan_seq		= mm ? mm->numa_scan_seq : 0;
	p->numa_scan_period		= sysctl_numa_balancing_scan_delay;
	p->numa_migrate_retry		= 0;
	/* Protect against double add, see task_tick_numa and task_numa_work */
	p->numa_work.next		= &p->numa_work;
	p->numa_faults			= NULL;
	p->numa_pages_migrated		= 0;
	p->total_numa_faults		= 0;
	RCU_INIT_POINTER(p->numa_group, NULL);
	p->last_task_numa_placement	= 0;
	p->last_sum_exec_runtime	= 0;

	init_task_work(&p->numa_work, task_numa_work);

	/* New address space, reset the preferred nid */
	if (!(clone_flags & CLONE_VM)) {
		p->numa_preferred_nid = NUMA_NO_NODE;
		return;
	}

	/*
	 * New thread, keep existing numa_preferred_nid which should be copied
	 * already by arch_dup_task_struct but stagger when scans start.
	 */
	if (mm) {
		unsigned int delay;

		delay = min_t(unsigned int, task_scan_max(current),
			current->numa_scan_period * mm_users * NSEC_PER_MSEC);
		delay += 2 * TICK_NSEC;
		p->node_stamp = delay;
	}
}

함수 역할

• numa scan과 관련 파라미터를 초기화한다.
• NUMA scan은 새 태스크가 생성될 때 마다 실행됨

• kernel/sched/fair.c에 위치
• NUMA Balancing과 관련된 코드들이 모여있는 곳

언제 호출되는가?

• copy_process() -> sched_fork() ->  __sched_fork() -> init_numa_balancing()

파라미터

• 복습, task_struct란?

• 복습, mm_struct란?

함수 코드 살펴보기

int mm_users = 0;
struct mm_struct *mm = p->mm;

if (mm) {
	mm_users = atomic_read(&mm->mm_users);
	if (mm_users == 1) {
		mm->numa_next_scan = jiffies + msecs_to_jiffies(sysctl_numa_balancing_scan_delay);
		mm->numa_scan_seq = 0;
	}
}

	p->node_stamp			= 0;
	p->numa_scan_seq		= mm ? mm->numa_scan_seq : 0;
	p->numa_scan_period		= sysctl_numa_balancing_scan_delay;
	p->numa_migrate_retry		= 0;

/* Protect against double add, see task_tick_numa and task_numa_work */
p->numa_work.next		= &p->numa_work;

p->numa_faults			= NULL;
p->numa_pages_migrated		= 0;
p->total_numa_faults		= 0;
RCU_INIT_POINTER(p->numa_group, NULL);
p->last_task_numa_placement	= 0;
p->last_sum_exec_runtime	= 0;

• numa_faults

https://elixir.bootlin.com/linux/v6.10.3/source/include/linux/sched.h#L748
/*
	 * numa_faults is an array split into four regions:
	 * faults_memory, faults_cpu, faults_memory_buffer, faults_cpu_buffer
	 * in this precise order.
	 *
	 * faults_memory: Exponential decaying average of faults on a per-node
	 * basis. Scheduling placement decisions are made based on these
	 * counts. The values remain static for the duration of a PTE scan.
	 * faults_cpu: Track the nodes the process was running on when a NUMA
	 * hinting fault was incurred.
	 * faults_memory_buffer and faults_cpu_buffer: Record faults per node
	 * during the current scan window. When the scan completes, the counts
	 * in faults_memory and faults_cpu decay and these values are copied.
	 */
	unsigned long			*numa_faults;
	unsigned long			total_numa_faults;

• numa_group

static inline void
init_task_work(struct callback_head *twork, task_work_func_t func)
{
	twork->func = func;
}

/*
 * The expensive part of numa migration is done from task_work context.
 * Triggered from task_tick_numa().
 */
 static void task_numa_work(struct callback_head *work)

1. numa migration에서 부담이 많이 가는 작업은 task_work 컨텍스트에서 끝난다

1. task_tick_numa()에서 동작한다.

/* New address space, reset the preferred nid */
	if (!(clone_flags & CLONE_VM)) {
		p->numa_preferred_nid = NUMA_NO_NODE;
		return;
	}

if (mm) {
		unsigned int delay;

		delay = min_t(unsigned int, task_scan_max(current),
			current->numa_scan_period * mm_users * NSEC_PER_MSEC);
		delay += 2 * TICK_NSEC;
		p->node_stamp = delay;
	}