Re: [RFC] mm/vmscan.c: avoid possible long latency caused by too_many_isolated()

[Shakeel Butt <shakeelb@google.com>]

On Thu, Apr 22, 2021 at 10:13 AM Yu Zhao <yuzhao@google.com> wrote:
>
[...]
>         spin_lock_irq(&lruvec->lru_lock);
> @@ -3302,6 +3252,7 @@ static bool throttle_direct_reclaim(gfp_t gfp_mask, struct zonelist *zonelist,
>  unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
>                                 gfp_t gfp_mask, nodemask_t *nodemask)
>  {
> +       int nr_cpus;
>         unsigned long nr_reclaimed;
>         struct scan_control sc = {
>                 .nr_to_reclaim = SWAP_CLUSTER_MAX,
> @@ -3334,8 +3285,17 @@ unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
>         set_task_reclaim_state(current, &sc.reclaim_state);
>         trace_mm_vmscan_direct_reclaim_begin(order, sc.gfp_mask);
>
> +       nr_cpus = current_is_kswapd() ? 0 : num_online_cpus();

kswapd does not call this function (directly or indirectly).

> +       while (nr_cpus && !atomic_add_unless(&pgdat->nr_reclaimers, 1, nr_cpus)) {

At most nr_nodes * nr_cpus direct reclaimers are allowed?

> +               if (schedule_timeout_killable(HZ / 10))

trace_mm_vmscan_direct_reclaim_end() and set_task_reclaim_state(NULL)?

> +                       return SWAP_CLUSTER_MAX;
> +       }
> +
>         nr_reclaimed = do_try_to_free_pages(zonelist, &sc);
>
> +       if (nr_cpus)
> +               atomic_dec(&pgdat->nr_reclaimers);
> +
>         trace_mm_vmscan_direct_reclaim_end(nr_reclaimed);
>         set_task_reclaim_state(current, NULL);

BTW I think this approach needs to be more sophisticated. What if a
direct reclaimer within the reclaim is scheduled away and is out of
CPU quota?