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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. > > > > ... > > > > --- a/include/linux/math64.h > > +++ b/include/linux/math64.h > > @@ -281,4 +281,6 @@ 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) div64_u64((ll) + (d) - 1, (d)) > > This macro references arg `d' more than once. That can cause problems > if the passed expression has side-effects and is poor practice. Can > we please redo this with a temporary? Argh, the original DIV_ROUND_UP can't be fixed this way, as it's used in array's size declarations. So, below is the patch for the new DIV64_U64_ROUND_UP macro only. Thanks! -- >From d8237d3df222e6c5a98a74baa04bc52edf8a3677 Mon Sep 17 00:00:00 2001 From: Roman Gushchin Date: Wed, 29 Aug 2018 14:14:48 -0700 Subject: [PATCH] math64: prevent double calculation of DIV64_U64_ROUND_UP() arguments Cause the DIV64_U64_ROUND_UP(ll, d) macro to cache the result of (d) expression in a local variable to avoid double calculation, which might bring unexpected side effects. Signed-off-by: Roman Gushchin --- include/linux/math64.h | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/include/linux/math64.h b/include/linux/math64.h index 94af3d9c73e7..bb2c84afb80c 100644 --- a/include/linux/math64.h +++ b/include/linux/math64.h @@ -281,6 +281,7 @@ 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) div64_u64((ll) + (d) - 1, (d)) +#define DIV64_U64_ROUND_UP(ll, d) \ + ({ u64 _tmp = (d); div64_u64((ll) + _tmp - 1, _tmp); }) #endif /* _LINUX_MATH64_H */ -- 2.17.1