linux-kernel - Re: [PATCH 1/1] mm: add swapiness= arg to memory.reclaim

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Date:   Thu, 30 Nov 2023 18:05:02 -0800
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Huan Yang <11133793@...o.com>
Cc:     Dan Schatzberg <schatzberg.dan@...il.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Huan Yang <link@...o.com>, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, linux-mm@...ck.org,
        Michal Hocko <mhocko@...nel.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Huang Ying <ying.huang@...el.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Peter Xu <peterx@...hat.com>,
        "Vishal Moola (Oracle)" <vishal.moola@...il.com>,
        Yue Zhao <findns94@...il.com>, Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH 1/1] mm: add swapiness= arg to memory.reclaim

> @@ -2327,7 +2330,8 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
>         struct pglist_data *pgdat = lruvec_pgdat(lruvec);
>         struct mem_cgroup *memcg = lruvec_memcg(lruvec);
>         unsigned long anon_cost, file_cost, total_cost;
> -       int swappiness = mem_cgroup_swappiness(memcg);
> +       int swappiness = sc->swappiness ?
> +               *sc->swappiness : mem_cgroup_swappiness(memcg);
>
> Should we use "unlikely" here to indicate that sc->swappiness is an unexpected behavior?
> Due to current use case only apply in proactive reclaim.

On a system that is not under memory pressure, the rate of proactive
reclaim could be higher than reactive reclaim. We should only use
likely/unlikely when it's obvious a scenario will happen most of the
time. I don't believe that's the case here.

>
>         u64 fraction[ANON_AND_FILE];
>         u64 denominator = 0;    /* gcc */
>         enum scan_balance scan_balance;
> @@ -2608,6 +2612,9 @@ static int get_swappiness(struct lruvec *lruvec, struct scan_control *sc)
>             mem_cgroup_get_nr_swap_pages(memcg) < MIN_LRU_BATCH)
>                 return 0;
>
> +       if (sc->swappiness)
> +               return *sc->swappiness;
>
> Also there.
>
> +
>         return mem_cgroup_swappiness(memcg);
>  }
>
> @@ -6433,7 +6440,8 @@ unsigned long mem_cgroup_shrink_node(struct mem_cgroup *memcg,
>  unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
>                                            unsigned long nr_pages,
>                                            gfp_t gfp_mask,
> -                                          unsigned int reclaim_options)
> +                                          unsigned int reclaim_options,
> +                                          int *swappiness)
>  {
>         unsigned long nr_reclaimed;
>         unsigned int noreclaim_flag;
> @@ -6448,6 +6456,7 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
>                 .may_unmap = 1,
>                 .may_swap = !!(reclaim_options & MEMCG_RECLAIM_MAY_SWAP),
>                 .proactive = !!(reclaim_options & MEMCG_RECLAIM_PROACTIVE),
> +               .swappiness = swappiness,
>         };
>         /*
>          * Traverse the ZONELIST_FALLBACK zonelist of the current node to put
> --
> 2.34.1
>
> My previous patch attempted to ensure fully deterministic semantics under extreme swappiness.
> For example, when swappiness is set to 200, only anonymous pages will be reclaimed.
> Due to code in MGLRU isolate_folios will try scan anon if no scanned, will try other type.(We do not want
> it to attempt this behavior.)
> How do you think about extreme swappiness scenarios?

I think having different semantics between swappiness passed to
proactive reclaim and global swappiness can be confusing. If it's
needed to have a swappiness value that says "anon only no matter
what", perhaps we should introduce such a new value and make it
supported by both global and proactive reclaim swappiness? We could
support writing "max" or something similar instead of a special value
to mean that.

Writing such value to global swappiness may cause problems and
premature OOMs IIUC, but that would be misconfiguration. If we think
that's dangerous, we can introduce this new value but make it valid
only for proactive reclaim for now.