Re: [PATCH v7 05/12] mm: multigenerational LRU: minimal implementation

[Hillf Danton <hdanton@sina.com>]

Hello Yu

On Tue,  8 Feb 2022 01:18:55 -0700 Yu Zhao wrote:
> +
> +/******************************************************************************
> + *                          the aging
> + ******************************************************************************/
> +
> +static int folio_inc_gen(struct lruvec *lruvec, struct folio *folio, bool reclaiming)
> +{
> +	unsigned long old_flags, new_flags;
> +	int type = folio_is_file_lru(folio);
> +	struct lru_gen_struct *lrugen = &lruvec->lrugen;
> +	int new_gen, old_gen = lru_gen_from_seq(lrugen->min_seq[type]);
> +
> +	do {
> +		new_flags = old_flags = READ_ONCE(folio->flags);
> +		VM_BUG_ON_FOLIO(!(new_flags & LRU_GEN_MASK), folio);
> +
> +		new_gen = ((new_flags & LRU_GEN_MASK) >> LRU_GEN_PGOFF) - 1;

Is the chance zero for deadloop if new_gen != old_gen?

> +		new_gen = (old_gen + 1) % MAX_NR_GENS;
> +
> +		new_flags &= ~LRU_GEN_MASK;
> +		new_flags |= (new_gen + 1UL) << LRU_GEN_PGOFF;
> +		new_flags &= ~(LRU_REFS_MASK | LRU_REFS_FLAGS);
> +		/* for folio_end_writeback() */

		/* for folio_end_writeback() and sort_folio() */ in terms of
reclaiming?

> +		if (reclaiming)
> +			new_flags |= BIT(PG_reclaim);
> +	} while (cmpxchg(&folio->flags, old_flags, new_flags) != old_flags);
> +
> +	lru_gen_balance_size(lruvec, folio, old_gen, new_gen);
> +
> +	return new_gen;
> +}

...

> +/******************************************************************************
> + *                          the eviction
> + ******************************************************************************/
> +
> +static bool sort_folio(struct lruvec *lruvec, struct folio *folio, int tier_idx)
> +{

Nit, the 80-column-char format is prefered.

> +	bool success;
> +	int gen = folio_lru_gen(folio);
> +	int type = folio_is_file_lru(folio);
> +	int zone = folio_zonenum(folio);
> +	int tier = folio_lru_tier(folio);
> +	int delta = folio_nr_pages(folio);
> +	struct lru_gen_struct *lrugen = &lruvec->lrugen;
> +
> +	VM_BUG_ON_FOLIO(gen >= MAX_NR_GENS, folio);
> +
> +	if (!folio_evictable(folio)) {
> +		success = lru_gen_del_folio(lruvec, folio, true);
> +		VM_BUG_ON_FOLIO(!success, folio);
> +		folio_set_unevictable(folio);
> +		lruvec_add_folio(lruvec, folio);
> +		__count_vm_events(UNEVICTABLE_PGCULLED, delta);
> +		return true;
> +	}
> +
> +	if (type && folio_test_anon(folio) && folio_test_dirty(folio)) {
> +		success = lru_gen_del_folio(lruvec, folio, true);
> +		VM_BUG_ON_FOLIO(!success, folio);
> +		folio_set_swapbacked(folio);
> +		lruvec_add_folio_tail(lruvec, folio);
> +		return true;
> +	}
> +
> +	if (tier > tier_idx) {
> +		int hist = lru_hist_from_seq(lrugen->min_seq[type]);
> +
> +		gen = folio_inc_gen(lruvec, folio, false);
> +		list_move_tail(&folio->lru, &lrugen->lists[gen][type][zone]);
> +
> +		WRITE_ONCE(lrugen->promoted[hist][type][tier - 1],
> +			   lrugen->promoted[hist][type][tier - 1] + delta);
> +		__mod_lruvec_state(lruvec, WORKINGSET_ACTIVATE_BASE + type, delta);
> +		return true;
> +	}
> +
> +	if (folio_test_locked(folio) || folio_test_writeback(folio) ||
> +	    (type && folio_test_dirty(folio))) {
> +		gen = folio_inc_gen(lruvec, folio, true);
> +		list_move(&folio->lru, &lrugen->lists[gen][type][zone]);
> +		return true;

Make the cold dirty page cache younger instead of writeout in the backgroungd
reclaimer context, and the question rising is if laundry is defered until the
flusher threads are waken up in the following patches.

> +	}
> +
> +	return false;
> +}

Hillf