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

Chen Yucong
2014-06-16T06:22:49Z

On Sun, 2014-06-15 at 17:47 -0700, Hugh Dickins wrote:
> On Wed, 11 Jun 2014, Chen Yucong wrote:
> > On Tue, 2014-06-10 at 16:33 -0700, Andrew Morton wrote:
> > > > 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[]?
> >
> > nr_to_scan is used for recording the number of pages that should be
> > scanned to keep original *ratio*.
> >
> > We can assume that the value of (nr_file > nr_anon) is true, nr_to_scan
> > should be distribute to nr[LRU_ACTIVE_FILE] and nr[LRU_INACTIVE_FILE] in
> > proportion.
> >
> > nr_file = nr[LRU_ACTIVE_FILE] + nr[LRU_INACTIVE_FILE];
> > percentage = nr[LRU_FILE] / nr_file;
> >
> > Note that in comparison with *old* percentage, the "new" percentage has
> > the different meaning. It is just used to divide nr_so_scan pages
> > appropriately.
>
> [PATCH] mm-vmscanc-avoid-recording-the-original-scan-targets-in-shrink_lruvec-fix.patch
>
> I have not reviewed your logic at all, but soon hit a divide-by-zero
> crash on mmotm: it needs some such fix as below.
>
> Signed-off-by: Hugh Dickins <hughd@google.com>
> ---
>
> mm/vmscan.c | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
> --- mmotm/mm/vmscan.c 2014-06-12 17:46:36.632008452 -0700
> +++ linux/mm/vmscan.c 2014-06-12 18:55:18.832425713 -0700
> @@ -2122,11 +2122,12 @@ static void shrink_lruvec(struct lruvec
> nr_to_scan = nr_file - ratio * nr_anon;
> percentage = nr[LRU_FILE] * 100 / nr_file;
> lru = LRU_BASE;
> - } else {
> + } else if (ratio) {
> nr_to_scan = nr_anon - nr_file / ratio;
> percentage = nr[LRU_BASE] * 100 / nr_anon;
> lru = LRU_FILE;
> - }
> + } else
> + break;
>
> /* Stop scanning the smaller of the LRU */
> nr[lru] = 0;

I think I made a terrible mistake. If the value of
(nr[LRU_INACTIVE_FILE] + nr[LRU_ACTIVE_FILE]) <
(nr[LRU_INACTIVE_ANON] + nr[LRU_ACTIVE_ANON])
is true , the ratio will always be zero in original patch. This is too
terrible. It is unfair for anon list. Although the above fix can avoid
hitting a divide-by-zero crash, it can not solve the problem of
fairness.

The following fix can solve divide-by-zero and unfair problems
simultaneously. But it needs to introduce a new variable for saving the
ratio of anon to file and relative operations.

thx!
cyc

Signed-off-by: Chen Yucong <slaoub@gmail.com>
---
mm/vmscan.c | 30 +++++++++++-------------------
1 file changed, 11 insertions(+), 19 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index a8ffe4e..cf8d0a3 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_file_to_anon, ratio_anon_to_file;
enum lru_list lru;
unsigned long nr_reclaimed = 0;
unsigned long nr_to_reclaim = sc->nr_to_reclaim;
@@ -2067,8 +2066,10 @@ 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_file_to_anon = (nr[LRU_INACTIVE_FILE] + nr[LRU_ACTIVE_FILE] +
1) /
+ (nr[LRU_INACTIVE_ANON] + nr[LRU_ACTIVE_ANON] + 1);
+ ratio_anon_to_file = (nr[LRU_INACTIVE_ANON] + nr[LRU_ACTIVE_ANON] +
1) /
+ (nr[LRU_INACTIVE_FILE] + nr[LRU_ACTIVE_FILE] + 1);

/*
* Global reclaiming within direct reclaim at DEF_PRIORITY is a normal
@@ -2088,7 +2089,6 @@ 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 +2123,13 @@ 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_file_to_anon * 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 - ratio_anon_to_file * nr_file;
+ percentage = nr[LRU_BASE] * 100 / nr_anon;
lru = LRU_FILE;
- percentage = nr_file * 100 / scan_target;
}

/* Stop scanning the smaller of the LRU */
@@ -2143,14 +2141,8 @@ 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