Re: [PATCH V2 4/5] memcg: detect no memcgs above softlimit under zone reclaim.

[Hillf Danton <dhillf@gmail.com>]

On Thu, Apr 12, 2012 at 6:00 AM, Ying Han <yinghan@google.com> wrote:
> The function zone_reclaimable() marks zone->all_unreclaimable based on
> per-zone pages_scanned and reclaimable_pages. If all_unreclaimable is true,
> alloc_pages could go to OOM instead of getting stuck in page reclaim.
>
> In memcg kernel, cgroup under its softlimit is not targeted under global
> reclaim. It could be possible that all memcgs are under their softlimit for
> a particular zone. So the direct reclaim do_try_to_free_pages() will always
> return 1 which causes the caller __alloc_pages_direct_reclaim() enter tight
> loop.
>
> The reclaim priority check we put in should_reclaim_mem_cgroup() should help
> this case, but we still don't want to burn cpu cycles for first few priorities
> to get to that point. The idea is from LSF discussion where we detect it after
> the first round of scanning and restart the reclaim by not looking at softlimit
> at all. This allows us to make forward progress on shrink_zone() and free some
> pages on the zone.
>
> In order to do the detection for scanning all the memcgs under shrink_zone(),
> i have to change the mem_cgroup_iter() from shared walk to full walk. Otherwise,
> it would be very easy to skip lots of memcgs above softlimit and it causes the
> flag "ignore_softlimit" being mistakenly set.
>
Perhaps that detection could be covered by

	return target_mem_cgroup ||
		mem_cgroup_soft_limit_exceeded(memcg) ||
		(priority <= DEF_PRIORITY - 3);

then consider replacing shared walk with full walk.


> Signed-off-by: Ying Han <yinghan@google.com>
> ---
>  mm/vmscan.c |   23 ++++++++++++++++-------
>  1 files changed, 16 insertions(+), 7 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 2dbc300..d65eae4 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2158,21 +2158,25 @@ static void shrink_zone(int priority, struct zone *zone,
>                        struct scan_control *sc)
>  {
>        struct mem_cgroup *root = sc->target_mem_cgroup;
> -       struct mem_cgroup_reclaim_cookie reclaim = {
> -               .zone = zone,
> -               .priority = priority,
> -       };
>        struct mem_cgroup *memcg;
> +       int above_softlimit, ignore_softlimit = 0;
> +
>
> -       memcg = mem_cgroup_iter(root, NULL, &reclaim);
> +restart:
> +       above_softlimit = 0;
> +       memcg = mem_cgroup_iter(root, NULL, NULL);
>        do {
>                struct mem_cgroup_zone mz = {
>                        .mem_cgroup = memcg,
>                        .zone = zone,
>                };
>
> -               if (should_reclaim_mem_cgroup(root, memcg, priority))
> +               if (ignore_softlimit ||
> +                  should_reclaim_mem_cgroup(root, memcg, priority)) {
> +
>                        shrink_mem_cgroup_zone(priority, &mz, sc);
> +                       above_softlimit = 1;
> +               }
>
>                /*
>                 * Limit reclaim has historically picked one memcg and
> @@ -2188,8 +2192,13 @@ static void shrink_zone(int priority, struct zone *zone,
>                        mem_cgroup_iter_break(root, memcg);
>                        break;
>                }
> -               memcg = mem_cgroup_iter(root, memcg, &reclaim);
> +               memcg = mem_cgroup_iter(root, memcg, NULL);
>        } while (memcg);
> +
> +       if (!above_softlimit) {
> +               ignore_softlimit = 1;
> +               goto restart;
> +       }
>  }
>
>  /* Returns true if compaction should go ahead for a high-order request */
> --
> 1.7.7.3
>

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