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

Minchan Kim
2014-06-16T23:45:44Z

On Mon, Jun 16, 2014 at 08:57:54PM +0800, Chen Yucong wrote:
> On Mon, 2014-06-09 at 21:27 +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;
> >
> Hi Andrew Morton,
>
> I think the patch
>
> [PATCH]
> mm-vmscanc-avoid-recording-the-original-scan-targets-in-shrink_lruvec-fix.patch
>
> which I committed should be discarded. Because It have some critical
> defects.
> 1) If we want to solve the divide-by-zero and unfair problems, it
> needs to two variables for recording the ratios.
>
> 2) For "x = nr_file - ratio * nr_anon", the "x" is likely to be a
> negative number. we can assume:
>
> nr[LRU_ACTIVE_FILE] = 30
> nr[LRU_INACTIVE_FILE] = 30
> nr[LRU_ACTIVE_ANON] = 0
> nr[LRU_INACTIVE_ANON] = 40
>
> ratio = 60/40 = 3/2
>
> When the value of (nr_reclaimed < nr_to_reclaim) become false, there are
> the following results:
> nr[LRU_ACTIVE_FILE] = 15
> nr[LRU_INACTIVE_FILE] = 15
> nr[LRU_ACTIVE_ANON] = 0
> nr[LRU_INACTIVE_ANON] = 25
>
> nr_file = 30
> nr_anon = 25
>
> x = 30 - 25 * (3/2) = 30 - 37.5 = -7.5.
>
> The result is too terrible.
>
> 3) This method is less accurate than the original, especially for the
> qualitative difference between FILE and ANON that is very small.

Yes, 3 changed old behavior. I'm ashamed but wanted to clean it up.
Is it worth to clean it up?

>From aedf8288e28a07bdd6c459a403f21cc2615ecc4e Mon Sep 17 00:00:00 2001
From: Minchan Kim <minchan@kernel.org>
Date: Tue, 17 Jun 2014 08:36:56 +0900
Subject: [PATCH] mm: proportional scanning cleanup

It aims for clean up, not changing behaivor so if anyone doesn't
looks it's more readable or not enough for readability, it should
really drop.

Signed-off-by: Minchan Kim <minchan@kernel.org>
---
mm/vmscan.c | 69 ++++++++++++++++++++++++++++++++-----------------------------
1 file changed, 36 insertions(+), 33 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 0f16ffe8eb67..acc29315bad0 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2054,19 +2054,18 @@ 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 remains[NR_LRU_LISTS];
enum lru_list lru;
unsigned long nr_reclaimed = 0;
unsigned long nr_to_reclaim = sc->nr_to_reclaim;
struct blk_plug plug;
bool scan_adjusted;

- get_scan_count(lruvec, sc, nr);
+ get_scan_count(lruvec, sc, targets);

- /* Record the original scan target for proportional adjustments later */
- memcpy(targets, nr, sizeof(nr));
+ /* Keep the original scan target for proportional adjustments later */
+ memcpy(remains, targets, sizeof(targets));

/*
* Global reclaiming within direct reclaim at DEF_PRIORITY is a normal
@@ -2083,19 +2082,21 @@ static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
sc->priority == DEF_PRIORITY);

blk_start_plug(&plug);
- while (nr[LRU_INACTIVE_ANON] || nr[LRU_ACTIVE_FILE] ||
- nr[LRU_INACTIVE_FILE]) {
- unsigned long nr_anon, nr_file, percentage;
- unsigned long nr_scanned;
+ while (remains[LRU_INACTIVE_ANON] || remains[LRU_ACTIVE_FILE] ||
+ remains[LRU_INACTIVE_FILE]) {
+ unsigned long target, remain_anon, remain_file;
+ unsigned long percentage;
+ unsigned long nr_scanned, nr_to_scan;

for_each_evictable_lru(lru) {
- if (nr[lru]) {
- nr_to_scan = min(nr[lru], SWAP_CLUSTER_MAX);
- nr[lru] -= nr_to_scan;
+ if (!remains[lru])
+ continue;

- nr_reclaimed += shrink_list(lru, nr_to_scan,
- lruvec, sc);
- }
+ nr_to_scan = min(remains[lru], SWAP_CLUSTER_MAX);
+ remains[lru] -= nr_to_scan;
+
+ nr_reclaimed += shrink_list(lru, nr_to_scan,
+ lruvec, sc);
}

if (nr_reclaimed < nr_to_reclaim || scan_adjusted)
@@ -2108,8 +2109,10 @@ static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
* stop reclaiming one LRU and reduce the amount scanning
* proportional to the original scan target.
*/
- nr_file = nr[LRU_INACTIVE_FILE] + nr[LRU_ACTIVE_FILE];
- nr_anon = nr[LRU_INACTIVE_ANON] + nr[LRU_ACTIVE_ANON];
+ remain_file = remains[LRU_INACTIVE_FILE] +
+ remains[LRU_ACTIVE_FILE];
+ remain_anon = remains[LRU_INACTIVE_ANON] +
+ remains[LRU_ACTIVE_ANON];

/*
* It's just vindictive to attack the larger once the smaller
@@ -2117,38 +2120,38 @@ static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
* smaller below, this makes sure that we only make one nudge
* towards proportionality once we've got nr_to_reclaim.
*/
- if (!nr_file || !nr_anon)
+ if (!remain_file || !remain_anon)
break;

- if (nr_file > nr_anon) {
- unsigned long scan_target = targets[LRU_INACTIVE_ANON] +
- targets[LRU_ACTIVE_ANON] + 1;
+ if (remain_file > remain_anon) {
+ target = targets[LRU_ACTIVE_ANON] +
+ targets[LRU_INACTIVE_ANON] + 1;
+ percentage = 100 * (target - remain_anon) / target;
lru = LRU_BASE;
- percentage = nr_anon * 100 / scan_target;
} else {
- unsigned long scan_target = targets[LRU_INACTIVE_FILE] +
- targets[LRU_ACTIVE_FILE] + 1;
+ target = targets[LRU_ACTIVE_FILE] +
+ targets[LRU_INACTIVE_FILE] + 1;
+ percentage = 100 * (target - remain_file) / target;
lru = LRU_FILE;
- percentage = nr_file * 100 / scan_target;
}

/* Stop scanning the smaller of the LRU */
- nr[lru] = 0;
- nr[lru + LRU_ACTIVE] = 0;
+ remains[lru] = 0;
+ remains[lru + LRU_ACTIVE] = 0;

/*
* Recalculate the other LRU scan count based on its original
* 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);
+ nr_scanned = targets[lru] - remains[lru];
+ remains[lru] = targets[lru] * percentage / 100;
+ remains[lru] -= min(remains[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_scanned = targets[lru] - remains[lru];
+ remains[lru] = targets[lru] * percentage / 100;
+ remains[lru] -= min(remains[lru], nr_scanned);

scan_adjusted = true;
}
--
2.0.0

--
Kind regards,
Minchan Kim