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From:   Barry Song <21cnbao@gmail.com>
Date:   Sun, 20 Mar 2022 00:15:00 +1300
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Subject: Re: [PATCH v9 06/14] mm: multi-gen LRU: minimal implementation
To:     Yu Zhao <yuzhao@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andi Kleen <ak@linux.intel.com>,
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> +                            unsigned long *min_seq, bool can_swap, bool *need_aging)
> +{
> +       int gen, type, zone;
> +       long old = 0;
> +       long young = 0;
> +       long total = 0;
> +       struct lru_gen_struct *lrugen = &lruvec->lrugen;
> +
> +       for (type = !can_swap; type < ANON_AND_FILE; type++) {
> +               unsigned long seq;
> +
> +               for (seq = min_seq[type]; seq <= max_seq; seq++) {
> +                       long size = 0;
> +
> +                       gen = lru_gen_from_seq(seq);
> +
> +                       for (zone = 0; zone < MAX_NR_ZONES; zone++)
> +                               size += READ_ONCE(lrugen->nr_pages[gen][type][zone]);
> +
> +                       total += size;
> +                       if (seq == max_seq)
> +                               young += size;
> +                       if (seq + MIN_NR_GENS == max_seq)
> +                               old += size;
> +               }
> +       }
> +
> +       /* try to spread pages out across MIN_NR_GENS+1 generations */
> +       if (min_seq[LRU_GEN_FILE] + MIN_NR_GENS > max_seq)
> +               *need_aging = true;
> +       else if (min_seq[LRU_GEN_FILE] + MIN_NR_GENS < max_seq)
> +               *need_aging = false;
> +       else if (young * MIN_NR_GENS > total)
> +               *need_aging = true;

Could we have some doc here? Given MIN_NR_GENS=2 and MAX_NR_GENS=4,
it seems you mean if we have three generations and the youngest pages are more
than 1/2 of the total pages, we need aging?


> +       else if (old * (MIN_NR_GENS + 2) < total)
> +               *need_aging = true;

it seems you mean if the oldest pages are less than 1/4 of the total pages,
we need aging? Can we have comments to explain why here?

your commit message only says " The aging produces young generations.
Given an lruvec, it increments max_seq when max_seq-min_seq+1
approaches MIN_NR_GENS." it can't explain what the code is doing
here.


> +       else
> +               *need_aging = false;
> +
> +       return total > 0 ? total : 0;
> +}

Thanks
Barry