linux-kernel - Re: [PATCH v9 06/14] mm: multi-gen LRU: minimal implementation

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Date:   Sun, 20 Mar 2022 00:15:00 +1300
From:   Barry Song <21cnbao@...il.com>
To:     Yu Zhao <yuzhao@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Aneesh Kumar <aneesh.kumar@...ux.ibm.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Hillf Danton <hdanton@...a.com>, Jens Axboe <axboe@...nel.dk>,
        Jesse Barnes <jsbarnes@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Michael Larabel <Michael@...haellarabel.com>,
        Michal Hocko <mhocko@...nel.org>,
        Mike Rapoport <rppt@...nel.org>,
        Rik van Riel <riel@...riel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Will Deacon <will@...nel.org>,
        Ying Huang <ying.huang@...el.com>,
        LAK <linux-arm-kernel@...ts.infradead.org>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Kernel Page Reclaim v2 <page-reclaim@...gle.com>,
        x86 <x86@...nel.org>, Brian Geffon <bgeffon@...gle.com>,
        Jan Alexander Steffens <heftig@...hlinux.org>,
        Oleksandr Natalenko <oleksandr@...alenko.name>,
        Steven Barrett <steven@...uorix.net>,
        Suleiman Souhlal <suleiman@...gle.com>,
        Daniel Byrne <djbyrne@....edu>,
        Donald Carr <d@...os-reins.com>,
        Holger Hoffstätte <holger@...lied-asynchrony.com>,
        Konstantin Kharlamov <Hi-Angel@...dex.ru>,
        Shuang Zhai <szhai2@...rochester.edu>,
        Sofia Trinh <sofia.trinh@....works>,
        Vaibhav Jain <vaibhav@...ux.ibm.com>
Subject: Re: [PATCH v9 06/14] mm: multi-gen LRU: minimal implementation

> +                            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