On Tue 28-02-17 16:40:03, Johannes Weiner wrote:
> 246e87a93934 ("memcg: fix get_scan_count() for small targets") sought
> to avoid high reclaim priorities for kswapd by forcing it to scan a
> minimum amount of pages when lru_pages >> priority yielded nothing.
>
> b95a2f2d486d ("mm: vmscan: convert global reclaim to per-memcg LRU
> lists"), due to switching global reclaim to a round-robin scheme over
> all cgroups, had to restrict this forceful behavior to unreclaimable
> zones in order to prevent massive overreclaim with many cgroups.
>
> The latter patch effectively neutered the behavior completely for all
> but extreme memory pressure. But in those situations we might as well
> drop the reclaimers to lower priority levels. Remove the check.
>
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
> mm/vmscan.c | 19 +++++--------------
> 1 file changed, 5 insertions(+), 14 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 911957b66622..46b6223fe7f3 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2129,22 +2129,13 @@ static void get_scan_count(struct lruvec *lruvec, struct mem_cgroup *memcg,
> int pass;
>
> /*
> - * If the zone or memcg is small, nr[l] can be 0. This
> - * results in no scanning on this priority and a potential
> - * priority drop. Global direct reclaim can go to the next
> - * zone and tends to have no problems. Global kswapd is for
> - * zone balancing and it needs to scan a minimum amount. When
> + * If the zone or memcg is small, nr[l] can be 0. When
> * reclaiming for a memcg, a priority drop can cause high
> - * latencies, so it's better to scan a minimum amount there as
> - * well.
> + * latencies, so it's better to scan a minimum amount. When a
> + * cgroup has already been deleted, scrape out the remaining
> + * cache forcefully to get rid of the lingering state.
> */
> - if (current_is_kswapd()) {
> - if (!pgdat_reclaimable(pgdat))
> - force_scan = true;
> - if (!mem_cgroup_online(memcg))
> - force_scan = true;
> - }
> - if (!global_reclaim(sc))
> + if (!global_reclaim(sc) || !mem_cgroup_online(memcg))
> force_scan = true;
>
> /* If we have no swap space, do not bother scanning anon pages. */
> --
> 2.11.1
--
Michal Hocko
SUSE Labs
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