The patch titled
Subject: mm: don't miss the last page because of round-off error
has been added to the -mm tree. Its filename is
mm-dont-miss-the-last-page-because-of-round-off-error.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/mm-dont-miss-the-last-page-because-of-round-off-error.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/mm-dont-miss-the-last-page-because-of-round-off-error.patch
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------------------------------------------------------
From: Roman Gushchin <guro@xxxxxx>
Subject: mm: don't miss the last page because of round-off error
I've noticed, that dying memory cgroups are often pinned in memory by a
single pagecache page. Even under moderate memory pressure they sometimes
stayed in such state for a long time. That looked strange.
My investigation showed that the problem is caused by applying the LRU
pressure balancing math:
scan = div64_u64(scan * fraction[lru], denominator),
where
denominator = fraction[anon] + fraction[file] + 1.
Because fraction[lru] is always less than denominator, if the initial scan
size is 1, the result is always 0.
This means the last page is not scanned and has
no chances to be reclaimed.
Fix this by rounding up the result of the division.
In practice this change significantly improves the speed of dying cgroups
reclaim.
Link: http://lkml.kernel.org/r/20180827162621.30187-3-guro@xxxxxx
Signed-off-by: Roman Gushchin <guro@xxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxxx>
Cc: Konstantin Khlebnikov <koct9i@xxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/math64.h | 2 ++
mm/vmscan.c | 6 ++++--
2 files changed, 6 insertions(+), 2 deletions(-)
--- a/include/linux/math64.h~mm-dont-miss-the-last-page-because-of-round-off-error
+++ a/include/linux/math64.h
@@ -281,4 +281,6 @@ static inline u64 mul_u64_u32_div(u64 a,
}
#endif /* mul_u64_u32_div */
+#define DIV64_U64_ROUND_UP(ll, d) div64_u64((ll) + (d) - 1, (d))
+
#endif /* _LINUX_MATH64_H */
--- a/mm/vmscan.c~mm-dont-miss-the-last-page-because-of-round-off-error
+++ a/mm/vmscan.c
@@ -2446,9 +2446,11 @@ out:
/*
* Scan types proportional to swappiness and
* their relative recent reclaim efficiency.
+ * Make sure we don't miss the last page
+ * because of a round-off error.
*/
- scan = div64_u64(scan * fraction[file],
- denominator);
+ scan = DIV64_U64_ROUND_UP(scan * fraction[file],
+ denominator);
break;
case SCAN_FILE:
case SCAN_ANON:
_
Patches currently in -mm which might be from guro@xxxxxx are
mm-rework-memcg-kernel-stack-accounting.patch
mm-drain-memcg-stocks-on-css-offlining.patch
mm-dont-miss-the-last-page-because-of-round-off-error.patch
