I've noticed, that dying memory cgroups are often pinned in memory by a single pagecache page. Even under moderate memory pressure they sometimes stayed in such state for a long time. That looked strange. My investigation showed that the problem is caused by applying the LRU pressure balancing math: scan = div64_u64(scan * fraction[lru], denominator), where denominator = fraction[anon] + fraction[file] + 1. Because fraction[lru] is always less than denominator, if the initial scan size is 1, the result is always 0. This means the last page is not scanned and has no chances to be reclaimed. Fix this by skipping the balancing logic if the initial scan count is 1. In practice this change significantly improves the speed of dying cgroups reclaim. Signed-off-by: Roman Gushchin <guro@xxxxxx> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Cc: Johannes Weiner <hannes@xxxxxxxxxxx> Cc: Michal Hocko <mhocko@xxxxxxxxxx> Cc: Tejun Heo <tj@xxxxxxxxxx> Cc: Rik van Riel <riel@xxxxxxxxxxx> Cc: Konstantin Khlebnikov <koct9i@xxxxxxxxx> --- mm/vmscan.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) diff --git a/mm/vmscan.c b/mm/vmscan.c index 03822f86f288..f85c5ec01886 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -2287,9 +2287,12 @@ static void get_scan_count(struct lruvec *lruvec, struct mem_cgroup *memcg, /* * Scan types proportional to swappiness and * their relative recent reclaim efficiency. + * Make sure we don't miss the last page + * because of a round-off error. */ - scan = div64_u64(scan * fraction[file], - denominator); + if (scan > 1) + scan = div64_u64(scan * fraction[file], + denominator); break; case SCAN_FILE: case SCAN_ANON: -- 2.17.1
