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

Minchan Kim
2014-06-09T23:24:59Z

Hello,

On Mon, Jun 09, 2014 at 09:27:16PM +0800, Chen Yucong 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;

Nice cleanup!

Below one nitpick.

>
> Signed-off-by: Chen Yucong <slaoub@gmail.com>
> ---
> mm/vmscan.c | 28 +++++++++-------------------
> 1 file changed, 9 insertions(+), 19 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index a8ffe4e..daaf89c 100644
> --- 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;
> 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;

If both nr_file and nr_anon are zero, then the nr_anon could be zero
if HugePage are reclaimed so that it could pass the below check

if (nr_reclaimed < nr_to_reclaim || scan_adjusted)

> lru = LRU_FILE;
> - percentage = nr_file * 100 / scan_target;
> }
>
> /* Stop scanning the smaller of the LRU */
> @@ -2143,14 +2139,8 @@ static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
> * scan target and the percentage scanning already complete
> */
> lru = (lru == LRU_FILE) ? LRU_BASE : LRU_FILE;
> - nr_scanned = targets[lru] - nr[lru];
> - nr[lru] = targets[lru] * (100 - percentage) / 100;
> - nr[lru] -= min(nr[lru], nr_scanned);
> -
> - lru += LRU_ACTIVE;
> - nr_scanned = targets[lru] - nr[lru];
> - nr[lru] = targets[lru] * (100 - percentage) / 100;
> - nr[lru] -= min(nr[lru], nr_scanned);
> + nr[lru] = nr_to_scan * percentage / 100;
> + nr[lru + LRU_ACTIVE] = nr_to_scan - nr[lru];
>
> scan_adjusted = true;
> }
> --
> 1.7.10.4
>
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--
Kind regards,
Minchan Kim