254 lines
9 KiB
C
254 lines
9 KiB
C
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/*
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* Workingset detection
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*
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* Copyright (C) 2013 Red Hat, Inc., Johannes Weiner
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*/
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#include <linux/memcontrol.h>
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#include <linux/writeback.h>
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#include <linux/pagemap.h>
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#include <linux/atomic.h>
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#include <linux/module.h>
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#include <linux/swap.h>
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#include <linux/fs.h>
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#include <linux/mm.h>
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/*
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* Double CLOCK lists
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*
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* Per zone, two clock lists are maintained for file pages: the
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* inactive and the active list. Freshly faulted pages start out at
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* the head of the inactive list and page reclaim scans pages from the
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* tail. Pages that are accessed multiple times on the inactive list
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* are promoted to the active list, to protect them from reclaim,
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* whereas active pages are demoted to the inactive list when the
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* active list grows too big.
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*
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* fault ------------------------+
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* |
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* +--------------+ | +-------------+
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* reclaim <- | inactive | <-+-- demotion | active | <--+
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* +--------------+ +-------------+ |
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* | |
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* +-------------- promotion ------------------+
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*
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*
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* Access frequency and refault distance
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*
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* A workload is thrashing when its pages are frequently used but they
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* are evicted from the inactive list every time before another access
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* would have promoted them to the active list.
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*
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* In cases where the average access distance between thrashing pages
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* is bigger than the size of memory there is nothing that can be
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* done - the thrashing set could never fit into memory under any
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* circumstance.
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*
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* However, the average access distance could be bigger than the
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* inactive list, yet smaller than the size of memory. In this case,
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* the set could fit into memory if it weren't for the currently
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* active pages - which may be used more, hopefully less frequently:
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*
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* +-memory available to cache-+
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* | |
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* +-inactive------+-active----+
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* a b | c d e f g h i | J K L M N |
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* +---------------+-----------+
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*
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* It is prohibitively expensive to accurately track access frequency
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* of pages. But a reasonable approximation can be made to measure
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* thrashing on the inactive list, after which refaulting pages can be
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* activated optimistically to compete with the existing active pages.
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*
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* Approximating inactive page access frequency - Observations:
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*
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* 1. When a page is accessed for the first time, it is added to the
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* head of the inactive list, slides every existing inactive page
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* towards the tail by one slot, and pushes the current tail page
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* out of memory.
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*
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* 2. When a page is accessed for the second time, it is promoted to
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* the active list, shrinking the inactive list by one slot. This
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* also slides all inactive pages that were faulted into the cache
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* more recently than the activated page towards the tail of the
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* inactive list.
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*
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* Thus:
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*
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* 1. The sum of evictions and activations between any two points in
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* time indicate the minimum number of inactive pages accessed in
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* between.
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*
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* 2. Moving one inactive page N page slots towards the tail of the
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* list requires at least N inactive page accesses.
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*
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* Combining these:
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*
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* 1. When a page is finally evicted from memory, the number of
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* inactive pages accessed while the page was in cache is at least
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* the number of page slots on the inactive list.
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*
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* 2. In addition, measuring the sum of evictions and activations (E)
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* at the time of a page's eviction, and comparing it to another
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* reading (R) at the time the page faults back into memory tells
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* the minimum number of accesses while the page was not cached.
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* This is called the refault distance.
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*
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* Because the first access of the page was the fault and the second
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* access the refault, we combine the in-cache distance with the
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* out-of-cache distance to get the complete minimum access distance
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* of this page:
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*
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* NR_inactive + (R - E)
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*
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* And knowing the minimum access distance of a page, we can easily
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* tell if the page would be able to stay in cache assuming all page
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* slots in the cache were available:
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*
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* NR_inactive + (R - E) <= NR_inactive + NR_active
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*
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* which can be further simplified to
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*
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* (R - E) <= NR_active
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*
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* Put into words, the refault distance (out-of-cache) can be seen as
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* a deficit in inactive list space (in-cache). If the inactive list
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* had (R - E) more page slots, the page would not have been evicted
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* in between accesses, but activated instead. And on a full system,
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* the only thing eating into inactive list space is active pages.
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*
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*
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* Activating refaulting pages
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*
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* All that is known about the active list is that the pages have been
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* accessed more than once in the past. This means that at any given
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* time there is actually a good chance that pages on the active list
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* are no longer in active use.
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*
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* So when a refault distance of (R - E) is observed and there are at
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* least (R - E) active pages, the refaulting page is activated
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* optimistically in the hope that (R - E) active pages are actually
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* used less frequently than the refaulting page - or even not used at
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* all anymore.
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*
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* If this is wrong and demotion kicks in, the pages which are truly
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* used more frequently will be reactivated while the less frequently
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* used once will be evicted from memory.
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*
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* But if this is right, the stale pages will be pushed out of memory
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* and the used pages get to stay in cache.
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*
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*
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* Implementation
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*
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* For each zone's file LRU lists, a counter for inactive evictions
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* and activations is maintained (zone->inactive_age).
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*
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* On eviction, a snapshot of this counter (along with some bits to
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* identify the zone) is stored in the now empty page cache radix tree
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* slot of the evicted page. This is called a shadow entry.
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*
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* On cache misses for which there are shadow entries, an eligible
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* refault distance will immediately activate the refaulting page.
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*/
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static void *pack_shadow(unsigned long eviction, struct zone *zone)
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{
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eviction = (eviction << NODES_SHIFT) | zone_to_nid(zone);
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eviction = (eviction << ZONES_SHIFT) | zone_idx(zone);
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eviction = (eviction << RADIX_TREE_EXCEPTIONAL_SHIFT);
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return (void *)(eviction | RADIX_TREE_EXCEPTIONAL_ENTRY);
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}
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static void unpack_shadow(void *shadow,
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struct zone **zone,
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unsigned long *distance)
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{
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unsigned long entry = (unsigned long)shadow;
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unsigned long eviction;
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unsigned long refault;
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unsigned long mask;
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int zid, nid;
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entry >>= RADIX_TREE_EXCEPTIONAL_SHIFT;
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zid = entry & ((1UL << ZONES_SHIFT) - 1);
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entry >>= ZONES_SHIFT;
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nid = entry & ((1UL << NODES_SHIFT) - 1);
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entry >>= NODES_SHIFT;
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eviction = entry;
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*zone = NODE_DATA(nid)->node_zones + zid;
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refault = atomic_long_read(&(*zone)->inactive_age);
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mask = ~0UL >> (NODES_SHIFT + ZONES_SHIFT +
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RADIX_TREE_EXCEPTIONAL_SHIFT);
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/*
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* The unsigned subtraction here gives an accurate distance
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* across inactive_age overflows in most cases.
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*
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* There is a special case: usually, shadow entries have a
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* short lifetime and are either refaulted or reclaimed along
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* with the inode before they get too old. But it is not
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* impossible for the inactive_age to lap a shadow entry in
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* the field, which can then can result in a false small
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* refault distance, leading to a false activation should this
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* old entry actually refault again. However, earlier kernels
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* used to deactivate unconditionally with *every* reclaim
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* invocation for the longest time, so the occasional
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* inappropriate activation leading to pressure on the active
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* list is not a problem.
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*/
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*distance = (refault - eviction) & mask;
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}
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/**
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* workingset_eviction - note the eviction of a page from memory
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* @mapping: address space the page was backing
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* @page: the page being evicted
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*
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* Returns a shadow entry to be stored in @mapping->page_tree in place
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* of the evicted @page so that a later refault can be detected.
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*/
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void *workingset_eviction(struct address_space *mapping, struct page *page)
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{
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struct zone *zone = page_zone(page);
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unsigned long eviction;
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eviction = atomic_long_inc_return(&zone->inactive_age);
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return pack_shadow(eviction, zone);
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}
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/**
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* workingset_refault - evaluate the refault of a previously evicted page
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* @shadow: shadow entry of the evicted page
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*
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* Calculates and evaluates the refault distance of the previously
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* evicted page in the context of the zone it was allocated in.
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*
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* Returns %true if the page should be activated, %false otherwise.
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*/
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bool workingset_refault(void *shadow)
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{
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unsigned long refault_distance;
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struct zone *zone;
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unpack_shadow(shadow, &zone, &refault_distance);
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inc_zone_state(zone, WORKINGSET_REFAULT);
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if (refault_distance <= zone_page_state(zone, NR_ACTIVE_FILE)) {
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inc_zone_state(zone, WORKINGSET_ACTIVATE);
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return true;
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}
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return false;
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}
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/**
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* workingset_activation - note a page activation
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* @page: page that is being activated
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*/
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void workingset_activation(struct page *page)
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{
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atomic_long_inc(&page_zone(page)->inactive_age);
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}
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