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From:   Barry Song <21cnbao@gmail.com>
Date:   Tue, 22 Mar 2022 21:45:23 +1300
Message-ID: <CAGsJ_4wPWRkmioxGYOwZ9KwdiJUUdgH9Xd38MJH6B2sKJNG+iA@mail.gmail.com>
Subject: Re: [PATCH v9 10/14] mm: multi-gen LRU: kill switch
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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On Tue, Mar 22, 2022 at 9:20 PM Yu Zhao <yuzhao@google.com> wrote:
>
> On Tue, Mar 22, 2022 at 1:47 AM Barry Song <21cnbao@gmail.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