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

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Date:   Mon, 21 Mar 2022 22:02:57 -0600
From:   Yu Zhao <yuzhao@...gle.com>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andi Kleen <ak@...ux.intel.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>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Kernel Page Reclaim v2 <page-reclaim@...gle.com>,
        "the arch/x86 maintainers" <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

On Mon, Mar 21, 2022 at 6:52 AM Aneesh Kumar K.V
<aneesh.kumar@...ux.ibm.com> wrote:
>
>  +
> > +static long get_nr_evictable(struct lruvec *lruvec, unsigned long max_seq,
> > +                          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;
>
> Can you explain/document the reason for the considering the below
> conditions for ageing?
>
> > +     else if (young * MIN_NR_GENS > total)
> > +             *need_aging = true;
>
> Are we trying to consdier the case of more than half the total pages
> young as needing ageing? If so should MIN_NR_GENS be 2 instead of using
> that #define? Or
>
> > +     else if (old * (MIN_NR_GENS + 2) < total)
> > +             *need_aging = true;
>
> What is the significance of '+ 2' ?

Will improve the comment according to my previous reply here [1].

[1] https://lore.kernel.org/linux-mm/CAOUHufYmUPZY0gCC+wYk6Vr1L8KEx+tJeEAhjpBfUnLJsAHq5A@mail.gmail.com/