Re: [PATCH v10 06/14] mm: multi-gen LRU: minimal implementation

[Yu Zhao]

On Thu, Apr 14, 2022 at 07:47:54PM +0800, Chen Wandun wrote:
> 
> On 2022/4/7 11:15, Yu Zhao wrote:
> > +static void inc_min_seq(struct lruvec *lruvec)
> > +{
> > +	int type;
> > +	struct lru_gen_struct *lrugen = &lruvec->lrugen;
> > +
> > +	VM_BUG_ON(!seq_is_valid(lruvec));
> > +
> > +	for (type = 0; type < ANON_AND_FILE; type++) {
> > +		if (get_nr_gens(lruvec, type) != MAX_NR_GENS)
> > +			continue;
> I'm confused about relation between aging and LRU list operation.
> 
> In function inc_max_seq,  both min_seq and max_seq will increase，
> the lrugen->lists[] indexed by lru_gen_from_seq(max_seq + 1) may
> be non-empty?

Yes.

> for example,
> before inc_max_seq:
> min_seq == 0, lrugen->lists[0][type][zone]
> max_seq ==3, lrugen->lists[3][type][zone]
> 
> after inc_max_seq:
> min_seq ==1, lrugen->lists[1][type][zone]
> max_seq ==4, lrugen->lists[0][type][zone]
> 
> If lrugen->lists[0][type][zone] is not empty before inc_max_seq and it is
> the most inactive list，however lurgen->lists[0][type][zone] will become
> the most active list after inc_max_seq.

Correct.

> So,  in this place,
> 
> if (get_nr_gens(lruvec, type) != MAX_NR_GENS)
> 	continue;
> 
> should change to
> 
> if (get_nr_gens(lruvec, type) == MAX_NR_GENS)
> 	continue;

No, because max/min_seq will overlap if we do so.

lrugen->lists[max_seq+1] can only be non-empty for anon LRU, for a
couple of reasons:
1. We can't swap at all.
2. Swapping is constrained, e.g., swapfile is full.

Both cases are similar to a producer (the aging) overrunning a
consumer (the eviction). We used to handle them, but I simplified the
code because I don't feel they are worth handling [1].

[1] https://lore.kernel.org/r/CAOUHufbDfwgm8PgCGkhCjbhMbm=fekfjgRR56NL-j+5iUGfVuw@xxxxxxxxxxxxxx/