On Mon, Aug 27, 2018 at 02:04:32PM -0700, Andrew Morton wrote: > On Mon, 27 Aug 2018 09:26:21 -0700 Roman Gushchin <guro@xxxxxx> wrote: > > > 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. > > > > ... > > > > --- 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? Sure. This was copy-pasted to match the existing DIV_ROUND_UP (probably, not the best idea). So let me fix them both in a separate patch. Thanks!
