On Mon 17-12-12 13:12:33, Johannes Weiner wrote:
> A swappiness of 0 has a slightly different meaning for global reclaim
> (may swap if file cache really low) and memory cgroup reclaim (never
> swap, ever).
>
> In addition, global reclaim at highest priority will scan all LRU
> lists equal to their size and ignore other balancing heuristics.
> UNLESS swappiness forbids swapping, then the lists are balanced based
> on recent reclaim effectiveness. UNLESS file cache is running low,
> then anonymous pages are force-scanned.
>
> This (total mess of a) behaviour is implicit and not obvious from the
> way the code is organized. At least make it apparent in the code flow
> and document the conditions. It will be it easier to come up with
> sane semantics later.
>
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Reviewed-by: Michal Hocko <mhocko@xxxxxxx>
Thanks!
> ---
> mm/vmscan.c | 39 ++++++++++++++++++++++++++++++---------
> 1 file changed, 30 insertions(+), 9 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 648a4db..c37deaf 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1644,7 +1644,6 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
> struct zone_reclaim_stat *reclaim_stat = &lruvec->reclaim_stat;
> u64 fraction[2], denominator;
> enum lru_list lru;
> - int noswap = 0;
> bool force_scan = false;
> struct zone *zone = lruvec_zone(lruvec);
>
> @@ -1665,13 +1664,38 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
>
> /* If we have no swap space, do not bother scanning anon pages. */
> if (!sc->may_swap || (nr_swap_pages <= 0)) {
> - noswap = 1;
> fraction[0] = 0;
> fraction[1] = 1;
> denominator = 1;
> goto out;
> }
>
> + /*
> + * Global reclaim will swap to prevent OOM even with no
> + * swappiness, but memcg users want to use this knob to
> + * disable swapping for individual groups completely when
> + * using the memory controller's swap limit feature would be
> + * too expensive.
> + */
> + if (!global_reclaim(sc) && !vmscan_swappiness(sc)) {
> + fraction[0] = 0;
> + fraction[1] = 1;
> + denominator = 1;
> + goto out;
> + }
> +
> + /*
> + * Do not apply any pressure balancing cleverness when the
> + * system is close to OOM, scan both anon and file equally
> + * (unless the swappiness setting disagrees with swapping).
> + */
> + if (!sc->priority && vmscan_swappiness(sc)) {
> + fraction[0] = 1;
> + fraction[1] = 1;
> + denominator = 1;
> + goto out;
> + }
> +
> anon = get_lru_size(lruvec, LRU_ACTIVE_ANON) +
> get_lru_size(lruvec, LRU_INACTIVE_ANON);
> file = get_lru_size(lruvec, LRU_ACTIVE_FILE) +
> @@ -1753,13 +1777,10 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
> unsigned long scan;
>
> size = get_lru_size(lruvec, lru);
> - if (sc->priority || noswap || !vmscan_swappiness(sc)) {
> - scan = size >> sc->priority;
> - if (!scan && force_scan)
> - scan = min(size, SWAP_CLUSTER_MAX);
> - scan = div64_u64(scan * fraction[file], denominator);
> - } else
> - scan = size;
> + scan = size >> sc->priority;
> + if (!scan && force_scan)
> + scan = min(size, SWAP_CLUSTER_MAX);
> + scan = div64_u64(scan * fraction[file], denominator);
> nr[lru] = scan;
> }
> }
> --
> 1.7.11.7
>
--
Michal Hocko
SUSE Labs
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