Linuxのvm_swapinessについてコードリーディングしてみた

$ ag vm_swappiness
mm/vmscan.c
166:int vm_swappiness = 60;

mm/memcontrol.c
3827:           vm_swappiness = val;

include/uapi/linux/sysctl.h
182:    VM_SWAPPINESS=19,       /* Tendency to steal mapped memory */

include/linux/swap.h
364:extern int vm_swappiness;
633:            return vm_swappiness;
637:            return vm_swappiness;
644:    return vm_swappiness;

kernel/sysctl.c
1410:           .data           = &vm_swappiness,
1411:           .maxlen         = sizeof(vm_swappiness),

  VM_SWAPPINESS=19, /* Tendency to steal mapped memory */

  {
    .procname = "swappiness",
    .data   = &vm_swappiness,
    .maxlen   = sizeof(vm_swappiness),
    .mode   = 0644,
    .proc_handler = proc_dointvec_minmax,
    .extra1   = SYSCTL_ZERO,
    .extra2   = &one_hundred,
  },

static int mem_cgroup_swappiness_write(struct cgroup_subsys_state *css,
				       struct cftype *cft, u64 val)
{
	struct mem_cgroup *memcg = mem_cgroup_from_css(css);

	if (val > 100)
		return -EINVAL;

	if (css->parent)
		memcg->swappiness = val;
	else
		vm_swappiness = val;

	return 0;
}

#ifdef CONFIG_MEMCG
static inline int mem_cgroup_swappiness(struct mem_cgroup *memcg)
{
	/* Cgroup2 doesn't have per-cgroup swappiness */
	if (cgroup_subsys_on_dfl(memory_cgrp_subsys))
		return vm_swappiness;

	/* root ? */
	if (mem_cgroup_disabled() || mem_cgroup_is_root(memcg))
		return vm_swappiness;

	return memcg->swappiness;
}
#else
static inline int mem_cgroup_swappiness(struct mem_cgroup *mem)
{
	return vm_swappiness;
}
#endif

/*
 * From 0 .. 100.  Higher means more swappy.
 */
int vm_swappiness = 60;

/*
 * Determine how aggressively the anon and file LRU lists should be
 * scanned.  The relative value of each set of LRU lists is determined
 * by looking at the fraction of the pages scanned we did rotate back
 * onto the active list instead of evict.
 *
 * nr[0] = anon inactive pages to scan; nr[1] = anon active pages to scan
 * nr[2] = file inactive pages to scan; nr[3] = file active pages to scan
 */
static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
			   unsigned long *nr)
{

	struct mem_cgroup *memcg = lruvec_memcg(lruvec);
	int swappiness = mem_cgroup_swappiness(memcg);
	struct zone_reclaim_stat *reclaim_stat = &lruvec->reclaim_stat;
	u64 fraction[2];
	u64 denominator = 0;	/* gcc */
	struct pglist_data *pgdat = lruvec_pgdat(lruvec);
	unsigned long anon_prio, file_prio;
	enum scan_balance scan_balance;
	unsigned long anon, file;
	unsigned long ap, fp;
	enum lru_list lru;

enum scan_balance {
	SCAN_EQUAL,
	SCAN_FRACT,
	SCAN_ANON,
	SCAN_FILE,
};

	/* If we have no swap space, do not bother scanning anon pages. */
	if (!sc->may_swap || mem_cgroup_get_nr_swap_pages(memcg) <= 0) {
		scan_balance = SCAN_FILE;
		goto out;
	}

	/*
	 * Global reclaim will swap to prevent OOM even with no
	 * swappiness, but memcg users want to use this knob to
	 * disable swapping for individual groups completely when
	 * using the memory controller's swap limit feature would be
	 * too expensive.
	 */
	if (cgroup_reclaim(sc) && !swappiness) {
		scan_balance = SCAN_FILE;
		goto out;
	}

	/*
	 * Do not apply any pressure balancing cleverness when the
	 * system is close to OOM, scan both anon and file equally
	 * (unless the swappiness setting disagrees with swapping).
	 */
	if (!sc->priority && swappiness) {
		scan_balance = SCAN_EQUAL;
		goto out;
	}

	/*
	 * If the system is almost out of file pages, force-scan anon.
	 */
	if (sc->file_is_tiny) {
		scan_balance = SCAN_ANON;
		goto out;
	}

	/*
	 * If there is enough inactive page cache, we do not reclaim
	 * anything from the anonymous working right now.
	 */
	if (sc->cache_trim_mode) {
		scan_balance = SCAN_FILE;
		goto out;
	}

	scan_balance = SCAN_FRACT;

	/*
	 * With swappiness at 100, anonymous and file have the same priority.
	 * This scanning priority is essentially the inverse of IO cost.
	 */
	anon_prio = swappiness;
	file_prio = 200 - anon_prio;

	/*
	 * OK, so we have swap space and a fair amount of page cache
	 * pages.  We use the recently rotated / recently scanned
	 * ratios to determine how valuable each cache is.
	 *
	 * Because workloads change over time (and to avoid overflow)
	 * we keep these statistics as a floating average, which ends
	 * up weighing recent references more than old ones.
	 *
	 * anon in [0], file in [1]
	 */

	anon  = lruvec_lru_size(lruvec, LRU_ACTIVE_ANON, MAX_NR_ZONES) +
		lruvec_lru_size(lruvec, LRU_INACTIVE_ANON, MAX_NR_ZONES);
	file  = lruvec_lru_size(lruvec, LRU_ACTIVE_FILE, MAX_NR_ZONES) +
		lruvec_lru_size(lruvec, LRU_INACTIVE_FILE, MAX_NR_ZONES);

	spin_lock_irq(&pgdat->lru_lock);
	if (unlikely(reclaim_stat->recent_scanned[0] > anon / 4)) {
		reclaim_stat->recent_scanned[0] /= 2;
		reclaim_stat->recent_rotated[0] /= 2;
	}

	if (unlikely(reclaim_stat->recent_scanned[1] > file / 4)) {
		reclaim_stat->recent_scanned[1] /= 2;
		reclaim_stat->recent_rotated[1] /= 2;
	}

	/*
	 * The amount of pressure on anon vs file pages is inversely
	 * proportional to the fraction of recently scanned pages on
	 * each list that were recently referenced and in active use.
	 */
	ap = anon_prio * (reclaim_stat->recent_scanned[0] + 1);
	ap /= reclaim_stat->recent_rotated[0] + 1;

	fp = file_prio * (reclaim_stat->recent_scanned[1] + 1);
	fp /= reclaim_stat->recent_rotated[1] + 1;
	spin_unlock_irq(&pgdat->lru_lock);

	fraction[0] = ap;
	fraction[1] = fp;
	denominator = ap + fp + 1;

	enum lru_list lru;

/*
 * We do arithmetic on the LRU lists in various places in the code,
 * so it is important to keep the active lists LRU_ACTIVE higher in
 * the array than the corresponding inactive lists, and to keep
 * the *_FILE lists LRU_FILE higher than the corresponding _ANON lists.
 *
 * This has to be kept in sync with the statistics in zone_stat_item
 * above and the descriptions in vmstat_text in mm/vmstat.c
 */
#define LRU_BASE 0
#define LRU_ACTIVE 1
#define LRU_FILE 2

enum lru_list {
	LRU_INACTIVE_ANON = LRU_BASE,
	LRU_ACTIVE_ANON = LRU_BASE + LRU_ACTIVE,
	LRU_INACTIVE_FILE = LRU_BASE + LRU_FILE,
	LRU_ACTIVE_FILE = LRU_BASE + LRU_FILE + LRU_ACTIVE,
	LRU_UNEVICTABLE,
	NR_LRU_LISTS
};

#define for_each_lru(lru) for (lru = 0; lru < NR_LRU_LISTS; lru++)

#define for_each_evictable_lru(lru) for (lru = 0; lru <= LRU_ACTIVE_FILE; lru++)

out:
	for_each_evictable_lru(lru) {
		int file = is_file_lru(lru);
		unsigned long lruvec_size;
		unsigned long scan;
		unsigned long protection;

		lruvec_size = lruvec_lru_size(lruvec, lru, sc->reclaim_idx);
		protection = mem_cgroup_protection(memcg,
						   sc->memcg_low_reclaim);

#ifdef CONFIG_MEMCG

static inline unsigned long mem_cgroup_protection(struct mem_cgroup *memcg,
						  bool in_low_reclaim)
{
	if (mem_cgroup_disabled())
		return 0;

	if (in_low_reclaim)
		return READ_ONCE(memcg->memory.emin);

	return max(READ_ONCE(memcg->memory.emin),
		   READ_ONCE(memcg->memory.elow));
}

static inline unsigned long mem_cgroup_protection(struct mem_cgroup *memcg,
						  bool in_low_reclaim)
{
	return 0;
}

		if (protection) {
			/*
			 * Scale a cgroup's reclaim pressure by proportioning
			 * its current usage to its memory.low or memory.min
			 * setting.
			 *
			 * This is important, as otherwise scanning aggression
			 * becomes extremely binary -- from nothing as we
			 * approach the memory protection threshold, to totally
			 * nominal as we exceed it.  This results in requiring
			 * setting extremely liberal protection thresholds. It
			 * also means we simply get no protection at all if we
			 * set it too low, which is not ideal.
			 *
			 * If there is any protection in place, we reduce scan
			 * pressure by how much of the total memory used is
			 * within protection thresholds.
			 *
			 * There is one special case: in the first reclaim pass,
			 * we skip over all groups that are within their low
			 * protection. If that fails to reclaim enough pages to
			 * satisfy the reclaim goal, we come back and override
			 * the best-effort low protection. However, we still
			 * ideally want to honor how well-behaved groups are in
			 * that case instead of simply punishing them all
			 * equally. As such, we reclaim them based on how much
			 * memory they are using, reducing the scan pressure
			 * again by how much of the total memory used is under
			 * hard protection.
			 */
			unsigned long cgroup_size = mem_cgroup_size(memcg);

			/* Avoid TOCTOU with earlier protection check */
			cgroup_size = max(cgroup_size, protection);

			scan = lruvec_size - lruvec_size * protection /
				cgroup_size;

			/*
			 * Minimally target SWAP_CLUSTER_MAX pages to keep
			 * reclaim moving forwards, avoiding decremeting
			 * sc->priority further than desirable.
			 */
			scan = max(scan, SWAP_CLUSTER_MAX);
		} else {
			scan = lruvec_size;
		}

		scan >>= sc->priority;

		/*
		 * If the cgroup's already been deleted, make sure to
		 * scrape out the remaining cache.
		 */
		if (!scan && !mem_cgroup_online(memcg))
			scan = min(lruvec_size, SWAP_CLUSTER_MAX);

static inline bool mem_cgroup_online(struct mem_cgroup *memcg)
{
	if (mem_cgroup_disabled())
		return true;
	return !!(memcg->css.flags & CSS_ONLINE);
}

static inline bool mem_cgroup_online(struct mem_cgroup *memcg)
{
	return true;
}

		switch (scan_balance) {
		case SCAN_EQUAL:
			/* Scan lists relative to size */
			break;
		case SCAN_FRACT:
			/*
			 * Scan types proportional to swappiness and
			 * their relative recent reclaim efficiency.
			 * Make sure we don't miss the last page
			 * because of a round-off error.
			 */
			scan = DIV64_U64_ROUND_UP(scan * fraction[file],
						  denominator);
			break;
		case SCAN_FILE:
		case SCAN_ANON:
			/* Scan one type exclusively */
			if ((scan_balance == SCAN_FILE) != file) {
				lruvec_size = 0;
				scan = 0;
			}
			break;
		default:
			/* Look ma, no brain */
			BUG();
		}

		nr[lru] = scan;
	}
}

 * nr[0] = anon inactive pages to scan; nr[1] = anon active pages to scan
 * nr[2] = file inactive pages to scan; nr[3] = file active pages to scan