On Mon, Aug 27, 2018 at 02:04:32PM -0700, Andrew Morton wrote:

On Mon, 27 Aug 2018 09:26:21 -0700 Roman Gushchin [off-list ref] wrote:

quoted

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?

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!

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