@@ -170,6 +170,22 @@ promotes hot pages. If the scan was done cacheline efficiently, it
adds the PMD entry pointing to the PTE table to the Bloom filter. This
forms a feedback loop between the eviction and the aging.
+Bloom Filters
+-------------
+Bloom filters are a space and memory efficient data structure for set
+membership test, i.e., test if an element is not in the set or may be
+in the set.
+
+In the eviction path, specifically, in ``lru_gen_look_around()``, if a
+PMD has a sufficient number of hot pages, its address is placed in the
+filter. In the aging path, set membership means that the PTE range
+will be scanned for young pages.
+
+Note that Bloom filters are probabilistic on set membership. If a test
+is false positive, the cost is an additional scan of a range of PTEs,
+which may yield hot pages anyway. Parameters of the filter itself can
+control the false positive rate in the limit.
+
Summary
-------
The multi-gen LRU can be disassembled into the following parts:
@@ -3233,6 +3233,98 @@ static bool __maybe_unused seq_is_valid(struct lruvec *lruvec)
get_nr_gens(lruvec, LRU_GEN_ANON) <= MAX_NR_GENS;
}
+/******************************************************************************
+ * Bloom filters
+ ******************************************************************************/
+
+/*
+ * Bloom filters with m=1<<15, k=2 and the false positive rates of ~1/5 when
+ * n=10,000 and ~1/2 when n=20,000, where, conventionally, m is the number of
+ * bits in a bitmap, k is the number of hash functions and n is the number of
+ * inserted items.
+ *
+ * Page table walkers use one of the two filters to reduce their search space.
+ * To get rid of non-leaf entries that no longer have enough leaf entries, the
+ * aging uses the double-buffering technique to flip to the other filter each
+ * time it produces a new generation. For non-leaf entries that have enough
+ * leaf entries, the aging carries them over to the next generation in
+ * walk_pmd_range(); the eviction also report them when walking the rmap
+ * in lru_gen_look_around().
+ *
+ * For future optimizations:
+ * 1. It's not necessary to keep both filters all the time. The spare one can be
+ * freed after the RCU grace period and reallocated if needed again.
+ * 2. And when reallocating, it's worth scaling its size according to the number
+ * of inserted entries in the other filter, to reduce the memory overhead on
+ * small systems and false positives on large systems.
+ * 3. Jenkins' hash function is an alternative to Knuth's.
+ */
+#define BLOOM_FILTER_SHIFT 15
+
+static inline int filter_gen_from_seq(unsigned long seq)
+{
+ return seq % NR_BLOOM_FILTERS;
+}
+
+static void get_item_key(void *item, int *key)
+{
+ u32 hash = hash_ptr(item, BLOOM_FILTER_SHIFT * 2);
+
+ BUILD_BUG_ON(BLOOM_FILTER_SHIFT * 2 > BITS_PER_TYPE(u32));
+
+ key[0] = hash & (BIT(BLOOM_FILTER_SHIFT) - 1);
+ key[1] = hash >> BLOOM_FILTER_SHIFT;
+}
+
+static bool test_bloom_filter(struct lruvec *lruvec, unsigned long seq, void *item)
+{
+ int key[2];
+ unsigned long *filter;
+ int gen = filter_gen_from_seq(seq);
+
+ filter = READ_ONCE(lruvec->mm_state.filters[gen]);
+ if (!filter)
+ return true;
+
+ get_item_key(item, key);
+
+ return test_bit(key[0], filter) && test_bit(key[1], filter);
+}
+
+static void update_bloom_filter(struct lruvec *lruvec, unsigned long seq, void *item)
+{
+ int key[2];
+ unsigned long *filter;
+ int gen = filter_gen_from_seq(seq);
+
+ filter = READ_ONCE(lruvec->mm_state.filters[gen]);
+ if (!filter)
+ return;
+
+ get_item_key(item, key);
+
+ if (!test_bit(key[0], filter))
+ set_bit(key[0], filter);
+ if (!test_bit(key[1], filter))
+ set_bit(key[1], filter);
+}
+
+static void reset_bloom_filter(struct lruvec *lruvec, unsigned long seq)
+{
+ unsigned long *filter;
+ int gen = filter_gen_from_seq(seq);
+
+ filter = lruvec->mm_state.filters[gen];
+ if (filter) {
+ bitmap_clear(filter, 0, BIT(BLOOM_FILTER_SHIFT));
+ return;
+ }
+
+ filter = bitmap_zalloc(BIT(BLOOM_FILTER_SHIFT),
+ __GFP_HIGH | __GFP_NOMEMALLOC | __GFP_NOWARN);
+ WRITE_ONCE(lruvec->mm_state.filters[gen], filter);
+}
+
/******************************************************************************
* mm_struct list
******************************************************************************/
@@ -3352,94 +3444,6 @@ void lru_gen_migrate_mm(struct mm_struct *mm)
}
#endif
-/*
- * Bloom filters with m=1<<15, k=2 and the false positive rates of ~1/5 when
- * n=10,000 and ~1/2 when n=20,000, where, conventionally, m is the number of
- * bits in a bitmap, k is the number of hash functions and n is the number of
- * inserted items.
- *
- * Page table walkers use one of the two filters to reduce their search space.
- * To get rid of non-leaf entries that no longer have enough leaf entries, the
- * aging uses the double-buffering technique to flip to the other filter each
- * time it produces a new generation. For non-leaf entries that have enough
- * leaf entries, the aging carries them over to the next generation in
- * walk_pmd_range(); the eviction also report them when walking the rmap
- * in lru_gen_look_around().
- *
- * For future optimizations:
- * 1. It's not necessary to keep both filters all the time. The spare one can be
- * freed after the RCU grace period and reallocated if needed again.
- * 2. And when reallocating, it's worth scaling its size according to the number
- * of inserted entries in the other filter, to reduce the memory overhead on
- * small systems and false positives on large systems.
- * 3. Jenkins' hash function is an alternative to Knuth's.
- */
-#define BLOOM_FILTER_SHIFT 15
-
-static inline int filter_gen_from_seq(unsigned long seq)
-{
- return seq % NR_BLOOM_FILTERS;
-}
-
-static void get_item_key(void *item, int *key)
-{
- u32 hash = hash_ptr(item, BLOOM_FILTER_SHIFT * 2);
-
- BUILD_BUG_ON(BLOOM_FILTER_SHIFT * 2 > BITS_PER_TYPE(u32));
-
- key[0] = hash & (BIT(BLOOM_FILTER_SHIFT) - 1);
- key[1] = hash >> BLOOM_FILTER_SHIFT;
-}
-
-static void reset_bloom_filter(struct lruvec *lruvec, unsigned long seq)
-{
- unsigned long *filter;
- int gen = filter_gen_from_seq(seq);
-
- filter = lruvec->mm_state.filters[gen];
- if (filter) {
- bitmap_clear(filter, 0, BIT(BLOOM_FILTER_SHIFT));
- return;
- }
-
- filter = bitmap_zalloc(BIT(BLOOM_FILTER_SHIFT),
- __GFP_HIGH | __GFP_NOMEMALLOC | __GFP_NOWARN);
- WRITE_ONCE(lruvec->mm_state.filters[gen], filter);
-}
-
-static void update_bloom_filter(struct lruvec *lruvec, unsigned long seq, void *item)
-{
- int key[2];
- unsigned long *filter;
- int gen = filter_gen_from_seq(seq);
-
- filter = READ_ONCE(lruvec->mm_state.filters[gen]);
- if (!filter)
- return;
-
- get_item_key(item, key);
-
- if (!test_bit(key[0], filter))
- set_bit(key[0], filter);
- if (!test_bit(key[1], filter))
- set_bit(key[1], filter);
-}
-
-static bool test_bloom_filter(struct lruvec *lruvec, unsigned long seq, void *item)
-{
- int key[2];
- unsigned long *filter;
- int gen = filter_gen_from_seq(seq);
-
- filter = READ_ONCE(lruvec->mm_state.filters[gen]);
- if (!filter)
- return true;
-
- get_item_key(item, key);
-
- return test_bit(key[0], filter) && test_bit(key[1], filter);
-}
-
static void reset_mm_stats(struct lruvec *lruvec, struct lru_gen_mm_walk *walk, bool last)
{
int i;
Move Bloom filters code into a dedicated section. Improve the design doc to explain Bloom filter usage and connection between aging and eviction in their use. Signed-off-by: T.J. Alumbaugh <talumbau@google.com> --- Documentation/mm/multigen_lru.rst | 16 +++ mm/vmscan.c | 180 +++++++++++++++--------------- 2 files changed, 108 insertions(+), 88 deletions(-)