Re: [PATCH] mm/vmscan.c: avoid recording the original scan targets in shrink_lruvec()

Andrew Morton
2014-06-10T23:33:41Z

On Mon, 9 Jun 2014 21:27:16 +0800 Chen Yucong <slaoub@gmail.com> wrote:

> Via https://lkml.org/lkml/2013/4/10/334 , we can find that recording the
> original scan targets introduces extra 40 bytes on the stack. This patch
> is able to avoid this situation and the call to memcpy(). At the same time,
> it does not change the relative design idea.
>
> ratio = original_nr_file / original_nr_anon;
>
> If (nr_file > nr_anon), then ratio = (nr_file - x) / nr_anon.
> x = nr_file - ratio * nr_anon;
>
> if (nr_file <= nr_anon), then ratio = nr_file / (nr_anon - x).
> x = nr_anon - nr_file / ratio;
>
> ...
>

Are you sure this is an equivalent-to-before change? If so, then I
can't immediately see why :(

> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2057,8 +2057,7 @@ out:
> static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
> {
> unsigned long nr[NR_LRU_LISTS];
> - unsigned long targets[NR_LRU_LISTS];
> - unsigned long nr_to_scan;
> + unsigned long nr_to_scan, ratio;
> enum lru_list lru;
> unsigned long nr_reclaimed = 0;
> unsigned long nr_to_reclaim = sc->nr_to_reclaim;
> @@ -2067,8 +2066,8 @@ static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
>
> get_scan_count(lruvec, sc, nr);
>
> - /* Record the original scan target for proportional adjustments later */
> - memcpy(targets, nr, sizeof(nr));
> + ratio = (nr[LRU_INACTIVE_FILE] + nr[LRU_ACTIVE_FILE] + 1) /
> + (nr[LRU_INACTIVE_ANON] + nr[LRU_ACTIVE_ANON] + 1);
>
> /*
> * Global reclaiming within direct reclaim at DEF_PRIORITY is a normal
> @@ -2088,7 +2087,6 @@ static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
> while (nr[LRU_INACTIVE_ANON] || nr[LRU_ACTIVE_FILE] ||
> nr[LRU_INACTIVE_FILE]) {
> unsigned long nr_anon, nr_file, percentage;
> - unsigned long nr_scanned;
>
> for_each_evictable_lru(lru) {
> if (nr[lru]) {
> @@ -2123,15 +2121,13 @@ static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
> break;
>
> if (nr_file > nr_anon) {
> - unsigned long scan_target = targets[LRU_INACTIVE_ANON] +
> - targets[LRU_ACTIVE_ANON] + 1;
> + nr_to_scan = nr_file - ratio * nr_anon;
> + percentage = nr[LRU_FILE] * 100 / nr_file;

here, nr_file and nr_anon are derived from the contents of nr[]. But
nr[] was modified in the for_each_evictable_lru() loop, so its contents
now may differ from what was in targets[]?

> lru = LRU_BASE;
> - percentage = nr_anon * 100 / scan_target;
> } else {
> - unsigned long scan_target = targets[LRU_INACTIVE_FILE] +
> - targets[LRU_ACTIVE_FILE] + 1;
> + nr_to_scan = nr_anon - nr_file / ratio;
> + percentage = nr[LRU_BASE] * 100 / nr_anon;
> lru = LRU_FILE;
> - percentage = nr_file * 100 / scan_target;
> }
>
> /* Stop scanning the smaller of the LRU */
> ...