zsmalloc: rework zspage chain size selection
Patch series "zsmalloc: make zspage chain size configurable". Computers are bad at division. We currently decide the best zspage chain size (max number of physical pages per-zspage) by looking at a `used percentage` value. This is not enough as we lose precision during usage percentage calculations For example, let's look at size class 208: pages per zspage wasted bytes used% 1 144 96 2 80 99 3 16 99 4 160 99 Current algorithm will select 2 page per zspage configuration, as it's the first one to reach 99%. However, 3 pages per zspage waste less memory. Change algorithm and select zspage configuration that has lowest wasted value. Link: https://lkml.kernel.org/r/20230118005210.2814763-1-senozhatsky@chromium.org Link: https://lkml.kernel.org/r/20230118005210.2814763-2-senozhatsky@chromium.org Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org> Acked-by: Minchan Kim <minchan@kernel.org> Cc: Mike Kravetz <mike.kravetz@oracle.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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@ -822,42 +822,6 @@ out:
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return newfg;
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}
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/*
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* We have to decide on how many pages to link together
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* to form a zspage for each size class. This is important
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* to reduce wastage due to unusable space left at end of
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* each zspage which is given as:
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* wastage = Zp % class_size
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* usage = Zp - wastage
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* where Zp = zspage size = k * PAGE_SIZE where k = 1, 2, ...
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*
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* For example, for size class of 3/8 * PAGE_SIZE, we should
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* link together 3 PAGE_SIZE sized pages to form a zspage
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* since then we can perfectly fit in 8 such objects.
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*/
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static int get_pages_per_zspage(int class_size)
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{
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int i, max_usedpc = 0;
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/* zspage order which gives maximum used size per KB */
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int max_usedpc_order = 1;
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for (i = 1; i <= ZS_MAX_PAGES_PER_ZSPAGE; i++) {
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int zspage_size;
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int waste, usedpc;
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zspage_size = i * PAGE_SIZE;
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waste = zspage_size % class_size;
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usedpc = (zspage_size - waste) * 100 / zspage_size;
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if (usedpc > max_usedpc) {
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max_usedpc = usedpc;
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max_usedpc_order = i;
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}
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}
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return max_usedpc_order;
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}
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static struct zspage *get_zspage(struct page *page)
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{
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struct zspage *zspage = (struct zspage *)page_private(page);
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@ -2401,6 +2365,24 @@ static int zs_register_shrinker(struct zs_pool *pool)
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pool->name);
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}
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static int calculate_zspage_chain_size(int class_size)
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{
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int i, min_waste = INT_MAX;
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int chain_size = 1;
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for (i = 1; i <= ZS_MAX_PAGES_PER_ZSPAGE; i++) {
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int waste;
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waste = (i * PAGE_SIZE) % class_size;
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if (waste < min_waste) {
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min_waste = waste;
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chain_size = i;
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}
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}
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return chain_size;
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}
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/**
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* zs_create_pool - Creates an allocation pool to work from.
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* @name: pool name to be created
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@ -2445,7 +2427,7 @@ struct zs_pool *zs_create_pool(const char *name)
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size = ZS_MIN_ALLOC_SIZE + i * ZS_SIZE_CLASS_DELTA;
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if (size > ZS_MAX_ALLOC_SIZE)
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size = ZS_MAX_ALLOC_SIZE;
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pages_per_zspage = get_pages_per_zspage(size);
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pages_per_zspage = calculate_zspage_chain_size(size);
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objs_per_zspage = pages_per_zspage * PAGE_SIZE / size;
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/*
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