linux-kernel - Re: [PATCH v9 10/14] mm: multi-gen LRU: kill switch

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Date:   Tue, 22 Mar 2022 21:45:23 +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 10/14] mm: multi-gen LRU: kill switch

On Tue, Mar 22, 2022 at 9:20 PM Yu Zhao <yuzhao@...gle.com> wrote:
>
> On Tue, Mar 22, 2022 at 1:47 AM Barry Song <21cnbao@...il.com> wrote:
> >
> ...
> > > +static bool drain_evictable(struct lruvec *lruvec)
> > > +{
> > > +       int gen, type, zone;
> > > +       int remaining = MAX_LRU_BATCH;
> > > +
> > > +       for_each_gen_type_zone(gen, type, zone) {
> > > +               struct list_head *head = &lruvec->lrugen.lists[gen][type][zone];
> > > +
> > > +               while (!list_empty(head)) {
> > > +                       bool success;
> > > +                       struct folio *folio = lru_to_folio(head);
> > > +
> > > +                       VM_BUG_ON_FOLIO(folio_test_unevictable(folio), folio);
> > > +                       VM_BUG_ON_FOLIO(folio_test_active(folio), folio);
> > > +                       VM_BUG_ON_FOLIO(folio_is_file_lru(folio) != type, folio);
> > > +                       VM_BUG_ON_FOLIO(folio_zonenum(folio) != zone, folio);
> > > +
> > > +                       success = lru_gen_del_folio(lruvec, folio, false);
> > > +                       VM_BUG_ON(!success);
> > > +                       lruvec_add_folio(lruvec, folio);
> >
> > for example, max_seq=4(GEN=0) and max_seq-1=3, then we are supposed to put
> > max_seq in the head of active list. but your code seems to be putting max_seq-1
> > after putting max_seq, then max_seq is more likely to be evicted
> > afterwards as it
> > is in the tail of the active list.
>
> This is correct.

maybe something like below can fix it:
 #define for_each_gen_type_zone(gen, type, zone)
         \
-       for ((gen) = 0; (gen) < MAX_NR_GENS; (gen)++)                   \
+       for (int seq = min_seq[type], (gen)=(seq_to_gen(seq)); seq <=
max_seq ; seq++)                       \
                for ((type) = 0; (type) < ANON_AND_FILE; (type)++)      \
                        for ((zone) = 0; (zone) < MAX_NR_ZONES; (zone)++)

but i am not quite sure it is worth it if we don't switch mglru/lru that
often. so it is all up to you, either fix it or put a comment to describe
we are not trying to make an active list with completely the same
temperature (hot/cold) as pages were in mglru lists.

>
> > anyway, it might not be so important. I can't imagine we will
> > frequently switch mglru
> > with lru dynamically. will we?
>
> I certainly hope not :)

me too.

Thanks
Barry