Re: [PATCH] mm: throttle and inc min_seq when both page types reach MIN_NR_GENS

[Yu Zhao <yuzhao@google.com>]

On Wed, Oct 9, 2024 at 1:50 AM zhaoyang.huang <zhaoyang.huang@unisoc.com> wrote:
>
> From: Zhaoyang Huang <zhaoyang.huang@unisoc.com>
>
> The test case of [1] leads to system hang which caused by a local
> watchdog thread starved over 20s on a 5.5GB RAM ANDROID15(v6.6)
> system. This commit solve the issue by have the reclaimer be throttled
> and increase min_seq if both page types reach MIN_NR_GENS, which may
> introduce a livelock of switching type with holding lruvec->lru_lock.
>
> [1]
> launch below script 8 times simutanously which allocates 1GB virtual
> memory and access it from user space by each thread.
> $ costmem -c1024000 -b12800 -o0 &
>
> Signed-off-by: Zhaoyang Huang <zhaoyang.huang@unisoc.com>
> ---
>  mm/vmscan.c | 16 ++++++++++++++--
>  1 file changed, 14 insertions(+), 2 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index cfa839284b92..83e450d0ce3c 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -4384,11 +4384,23 @@ static int scan_folios(struct lruvec *lruvec, struct scan_control *sc,
>         int remaining = MAX_LRU_BATCH;
>         struct lru_gen_folio *lrugen = &lruvec->lrugen;
>         struct mem_cgroup *memcg = lruvec_memcg(lruvec);
> +       struct pglist_data *pgdat = lruvec_pgdat(lruvec);
>
>         VM_WARN_ON_ONCE(!list_empty(list));
>
> -       if (get_nr_gens(lruvec, type) == MIN_NR_GENS)
> -               return 0;
> +       if (get_nr_gens(lruvec, type) == MIN_NR_GENS) {
> +               /*
> +                * throttle for a while and then increase the min_seq since
> +                * both page types reach the limit.
> +                */

Sorry but this isn't going to work because in try_to_inc_min_seq(), there is
   `while (min_seq[type] + MIN_NR_GENS <= lrugen->max_seq) {`
to prevent reclaimers from evicting hot memory -- they need to do aging first.

>
> +               if (get_nr_gens(lruvec, !type) == MIN_NR_GENS) {
> +                       spin_unlock_irq(&lruvec->lru_lock);
> +                       reclaim_throttle(pgdat, VMSCAN_THROTTLE_ISOLATED);
> +                       spin_lock_irq(&lruvec->lru_lock);
> +                       try_to_inc_min_seq(lruvec, get_swappiness(lruvec, sc));
> +               } else
> +                       return 0;
> +       }
>
>         gen = lru_gen_from_seq(lrugen->min_seq[type]);
>
> --
> 2.25.1
>