From: Roman Gushchin guro@fb.com
[ Upstream commit 68600f623d69da428c6163275f97ca126e1a8ec5 ]
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.
[guro@fb.com: prevent double calculation of DIV64_U64_ROUND_UP() arguments] Link: http://lkml.kernel.org/r/20180829213311.GA13501@castle Link: http://lkml.kernel.org/r/20180827162621.30187-3-guro@fb.com Signed-off-by: Roman Gushchin guro@fb.com Reviewed-by: Andrew Morton akpm@linux-foundation.org Cc: Johannes Weiner hannes@cmpxchg.org Cc: Michal Hocko mhocko@kernel.org Cc: Tejun Heo tj@kernel.org Cc: Rik van Riel riel@surriel.com Cc: Konstantin Khlebnikov koct9i@gmail.com Cc: Matthew Wilcox willy@infradead.org Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Sasha Levin sashal@kernel.org --- include/linux/math64.h | 3 +++ mm/vmscan.c | 6 ++++-- 2 files changed, 7 insertions(+), 2 deletions(-)
diff --git a/include/linux/math64.h b/include/linux/math64.h index 082de345b73c..3a7a14062668 100644 --- a/include/linux/math64.h +++ b/include/linux/math64.h @@ -254,4 +254,7 @@ static inline u64 mul_u64_u32_div(u64 a, u32 mul, u32 divisor) } #endif /* mul_u64_u32_div */
+#define DIV64_U64_ROUND_UP(ll, d) \ + ({ u64 _tmp = (d); div64_u64((ll) + _tmp - 1, _tmp); }) + #endif /* _LINUX_MATH64_H */ diff --git a/mm/vmscan.c b/mm/vmscan.c index be56e2e1931e..9734e62654fa 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -2367,9 +2367,11 @@ static void get_scan_count(struct lruvec *lruvec, struct mem_cgroup *memcg, /* * 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: