[RFC PATCH v2 0/5] Refault distance checking for MGLRU

[RFC PATCH v2 0/5] Refault distance checking for MGLRU

[Kairui Song]

From: Kairui Song <kasong@tencent.com>

Hi, linux-mm

I noticed MGLRU not working very well on certain workflows, which is
observed on some workloads with heavy memory stress.

After some debugging, I found this was related to refault distance
detection, when the file page workingset size exceeds total memory,
and the access distance (the left-shift time of a page before it gets
activated or promoted, considering LRU starts from right) of file pages
are larger than total memory. All file pages are stuck on the oldest
generation and getting read-in then evicted permutably, few get activated
and stay in memory.

This series tries to fix this problem by rework the refault distance
based activation to better fit MGLRU, and also tries to use a unified
algorithm for both MGLRU and Inactive/Active LRU, the performance almost
doubled for the workloads that are not working well previously.

Patch 1/5 reworked the refault distance detection model for
Inactive/Active LRU.

Patch 2/5 updated the comments.

Patch 3/5 and 4/5 are simplification and prepare.

Patch 4/4 applies the modified refault distance detection
for MGLRU.

Following benchmark showed 5x improvement:
To simulate the workflow, I setup a 3-replicated mongodb cluster using
docker, each in a standalone cgroup, set to use 5GB of wiretiger cache
and 10g of oplog, on a 32G VM. The benchmark is done using
https://github.com/apavlo/py-tpcc.git, modified to run STOCK_LEVEL
query only, for simulating slow query and get a stable result.

Before the patch (with 10G swap, the result won't change whether
swap is on or not):
$ tpcc.py --config=mongodb.config mongodb --duration=900 --warehouses=500 --clients=30
==================================================================
Execution Results after 904 seconds
------------------------------------------------------------------
                  Executed        Time (µs)       Rate
  STOCK_LEVEL     503             27150226136.4   0.02 txn/s
------------------------------------------------------------------
  TOTAL           503             27150226136.4   0.02 txn/s

$ cat /proc/vmstat | grep working
workingset_nodes 53391
workingset_refault_anon 0
workingset_refault_file 23856735
workingset_activate_anon 0
workingset_activate_file 23845737
workingset_restore_anon 0
workingset_restore_file 18280692
workingset_nodereclaim 1024

$ free -m
              total        used        free      shared  buff/cache   available
Mem:          31837        6752         379          23       24706       24607
Swap:         10239           0       10239

After the patch (with 10G swap on same disk, similar result using ZRAM):
$ tpcc.py --config=mongodb.config mongodb --duration=900 --warehouses=500 --clients=30
==================================================================
Execution Results after 903 seconds
------------------------------------------------------------------
                  Executed        Time (µs)       Rate
  STOCK_LEVEL     2575            27094953498.8   0.10 txn/s
------------------------------------------------------------------
  TOTAL           2575            27094953498.8   0.10 txn/s

$ cat /proc/vmstat | grep working
workingset_nodes 78249
workingset_refault_anon 10139
workingset_refault_file 23001863
workingset_activate_anon 7238
workingset_activate_file 6718032
workingset_restore_anon 7432
workingset_restore_file 6719406
workingset_nodereclaim 9747

$ free -m
              total        used        free      shared  buff/cache   available
Mem:          31837        7376         320           3       24140       24014
Swap:         10239        1662        8577

The performance is 5x times better than before, and the idle anon pages
now can get swapped out as expected. Testing with lower stress also shows
a improvement.

I also checked the benchmark with memtier/memcached and fio and some
other benchmarks, they looked OK so far, the results are in each commits.

Sending out as RFC, I'm still trying to do more test on it, since this changed
a frequently used algorithm and not really sure if there is any performance
regression, it should improvement the performance for file pages in general,
since it saved some operations.

Update from V1:
- Removed the fls operations which previously used in patch 1 for
  protecting active pages by expontial ratio, simply compare with number of
  inactive pages seems good enough.
- Update some benchmarks results, test result that are basically
  identical as before are not updated.

Kairui Song (5):
  workingset: simplify and use a more intuitive model
  workingset: update comment in workingset.c
  workingset: simplify lru_gen_test_recent
  lru_gen: convert avg_total and avg_refaulted to atomic
  workingset, lru_gen: apply refault-distance based re-activation

 include/linux/mmzone.h |   4 +-
 include/linux/swap.h   |   2 -
 mm/swap.c              |   1 -
 mm/vmscan.c            |  18 +-
 mm/workingset.c        | 411 +++++++++++++++++++++--------------------
 5 files changed, 221 insertions(+), 215 deletions(-)

-- 
2.41.0

[RFC PATCH v2 1/5] workingset: simplify and use a more intuitive model

[Kairui Song]

From: Kairui Song <kasong@tencent.com>

This basically removed workingset_activation and reduced calls to
workingset_age_nonresident.

The idea behind this change is a new way to calculate the refault
distance and prepare for adapting refault distance based re-activation
for multi-gen LRU.

Currently, refault distance re-activation is based on two assumptions:
1. Activation of an inactive page will left-shift LRU pages (considering
   LRU starts from right).
2. Eviction of an inactive page will left-shift LRU pages.

Assumption 2 is correct, but assumption 1 is not always true, an activated
page could be anywhere in the LRU list (through mark_page_accessed), it
only left-shift the pages on its right.

And besides, one page can get activate/deactivated for multiple times.

And multi-gen LRU doesn't fit with this model well, pages are getting
aged and activated constantly as the generation sliding window slides.

So instead we introduce a simpler idea here: Just presume the evicted
pages are still in memory, each has an eviction sequence like before.
Let the `nonresistence_age` still be NA and get increased for each
eviction, so we get a "Shadow LRU" here of one evicted page:

  Let SP = ((NA's reading @ current) - (NA's reading @ eviction))

                           +-memory available to cache-+
                           |                           |
 +-------------------------+===============+===========+
 | *   shadows  O O  O     |   INACTIVE    |   ACTIVE  |
 +-+-----------------------+===============+===========+
   |                       |
   +-----------------------+
   |         SP
 fault page          O -> Hole left by previously faulted in pages
                     * -> The page corresponding to SP

It can be easily seen that SP stands for how far the current workflow
could push a page out of available memory. Since all evicted page was
once head of INACTIVE list, the page could have such an access distance:

  SP + NR_INACTIVE

It *may* get re-activated before getting evicted again if:

  SP + NR_INACTIVE < NR_INACTIVE + NR_ACTIVE

Which can be simplified to:

  SP < NR_ACTIVE

Then the page is worth getting re-activated to start from ACTIVE part,
since the access distance is shorter than the total memory to make it
stay. The calculation is same as before, just dropped the assumption 1
above.

And since this is only an estimation, based on several hypotheses, and
it could break the ability of LRU to distinguish a workingset out of
caches, so throttle this by two factors:

1. Notice previously re-faulted in pages may leave "holes" on the shadow
   part of LRU, that part is left unhandled on purpose to decrease
   re-activate rate for pages that have a large SP value (the larger
   SP value a page has, the more likely it will be affected by such
   holes).
2. When the ACTIVE part of LRU is long enough, chanllaging ACTIVE pages
   by re-activating a one-time faulted previously INACTIVE page may not
   be a good idea, so throttle the re-activation when ACTIVE > INACTIVE
   by comparing with INACTIVE instead.

Combined all above, we have:
Upon refault, if any of following conditions is met, mark page as active:

- If ACTIVE LRU is low (NR_ACTIVE < NR_INACTIVE), check if:
  SP < NR_ACTIVE

- If ACTIVE LRU is high (NR_ACTIVE >= NR_INACTIVE), check if:
  SP < NR_INACTIVE

The code is almose same but simpler than before, since no longer need to
do lruvec statistic update when activating a page. A few benchmarks
showed a similar or better result. And when combined with multi-gen
LRU (in later commits) it shows a measurable performance gain for some
workloads.

Using memtier and fio test from commit ac35a4902374 but scaled down
to fit in my test environment, and some other test results:

  memtier test (with 16G ramdisk as swap and 2G memcg limit on an i7-9700):
  memcached -u nobody -m 16384 -s /tmp/memcached.socket \
    -a 0700 -t 12 -B binary &
  memtier_benchmark -S /tmp/memcached.socket -P memcache_binary -n allkeys\
    --key-minimum=1 --key-maximum=24000000 --key-pattern=P:P -c 1 \
    -t 12 --ratio 1:0 --pipeline 8 -d 2000 -x 6

  fio test 1 (with 16G ramdisk on 28G VM on an i7-9700):
  fio -name=refault --numjobs=12 --directory=/mnt --size=1024m \
    --buffered=1 --ioengine=io_uring --iodepth=128 \
    --iodepth_batch_submit=32 --iodepth_batch_complete=32 \
    --rw=randread --random_distribution=random --norandommap \
    --time_based --ramp_time=5m --runtime=5m --group_reporting

  fio test 2 (with 16G ramdisk on 28G VM on an i7-9700):
  fio -name=mglru --numjobs=10 --directory=/mnt --size=1536m \
    --buffered=1 --ioengine=io_uring --iodepth=128 \
    --iodepth_batch_submit=32 --iodepth_batch_complete=32 \
    --rw=randread --random_distribution=zipf:1.2 --norandommap \
    --time_based --ramp_time=10m --runtime=5m --group_reporting

  mysql (using oltp_read_only from sysbench, with 12G of buffer pool
  in a 10G memcg):
  sysbench /usr/share/sysbench/oltp_read_only.lua <auth and db params> \
    --mysql-db=sb --tables=36 --table-size=2000000 --threads=12 --time=1800

Before (Average of 6 test run):
fio: IOPS=5213.7k
fio2: IOPS=7315.3k
memcached: 49493.75 ops/s
mysql: 6237.45 tps

After (Average of 6 test run):
fio: IOPS=5230.5k
fio2: IOPS=7349.3k
memcached: 49912.79 ops/s
mysql: 6240.62 tps

Signed-off-by: Kairui Song <kasong@tencent.com>
---
 include/linux/swap.h |   2 -
 mm/swap.c            |   1 -
 mm/vmscan.c          |   2 -
 mm/workingset.c      | 215 +++++++++++++++++++++----------------------
 4 files changed, 106 insertions(+), 114 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 493487ed7c38..ca51d79842b7 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -344,10 +344,8 @@ static inline swp_entry_t page_swap_entry(struct page *page)
 
 /* linux/mm/workingset.c */
 bool workingset_test_recent(void *shadow, bool file, bool *workingset);
-void workingset_age_nonresident(struct lruvec *lruvec, unsigned long nr_pages);
 void *workingset_eviction(struct folio *folio, struct mem_cgroup *target_memcg);
 void workingset_refault(struct folio *folio, void *shadow);
-void workingset_activation(struct folio *folio);
 
 /* Only track the nodes of mappings with shadow entries */
 void workingset_update_node(struct xa_node *node);
diff --git a/mm/swap.c b/mm/swap.c
index cd8f0150ba3a..685b446fd4f9 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -482,7 +482,6 @@ void folio_mark_accessed(struct folio *folio)
 		else
 			__lru_cache_activate_folio(folio);
 		folio_clear_referenced(folio);
-		workingset_activation(folio);
 	}
 	if (folio_test_idle(folio))
 		folio_clear_idle(folio);
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 6f13394b112e..3f4de75e5186 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2539,8 +2539,6 @@ static unsigned int move_folios_to_lru(struct lruvec *lruvec,
 		lruvec_add_folio(lruvec, folio);
 		nr_pages = folio_nr_pages(folio);
 		nr_moved += nr_pages;
-		if (folio_test_active(folio))
-			workingset_age_nonresident(lruvec, nr_pages);
 	}
 
 	/*
diff --git a/mm/workingset.c b/mm/workingset.c
index da58a26d0d4d..babda11601ea 100644
--- a/mm/workingset.c
+++ b/mm/workingset.c
@@ -180,9 +180,10 @@
  */
 
 #define WORKINGSET_SHIFT 1
-#define EVICTION_SHIFT	((BITS_PER_LONG - BITS_PER_XA_VALUE) +	\
+#define EVICTION_SHIFT	((BITS_PER_LONG - BITS_PER_XA_VALUE) + \
 			 WORKINGSET_SHIFT + NODES_SHIFT + \
 			 MEM_CGROUP_ID_SHIFT)
+#define EVICTION_BITS	(BITS_PER_LONG - (EVICTION_SHIFT))
 #define EVICTION_MASK	(~0UL >> EVICTION_SHIFT)
 
 /*
@@ -226,8 +227,103 @@ static void unpack_shadow(void *shadow, int *memcgidp, pg_data_t **pgdat,
 	*workingsetp = workingset;
 }
 
-#ifdef CONFIG_LRU_GEN
+/*
+ * Get the distance reading at eviction time.
+ */
+static inline unsigned long lru_eviction(struct lruvec *lruvec,
+					 int bits, int bucket_order)
+{
+	unsigned long eviction = atomic_long_read(&lruvec->nonresident_age);
+
+	eviction >>= bucket_order;
+	eviction &= ~0UL >> (BITS_PER_LONG - bits);
+
+	return eviction;
+}
+
+/*
+ * Calculate and test refault distance
+ */
+static inline bool lru_refault(struct mem_cgroup *memcg,
+			       struct lruvec *lruvec,
+			       unsigned long eviction, bool file,
+			       int bits, int bucket_order)
+{
+	unsigned long refault, distance;
+	unsigned long workingset, active, inactive, inactive_file, inactive_anon = 0;
+
+	eviction <<= bucket_order;
+	refault = atomic_long_read(&lruvec->nonresident_age);
+
+	/*
+	 * The unsigned subtraction here gives an accurate distance
+	 * across nonresident_age overflows in most cases. There is a
+	 * special case: usually, shadow entries have a short lifetime
+	 * and are either refaulted or reclaimed along with the inode
+	 * before they get too old.  But it is not impossible for the
+	 * nonresident_age to lap a shadow entry in the field, which
+	 * can then result in a false small refault distance, leading
+	 * to a false activation should this old entry actually
+	 * refault again.  However, earlier kernels used to deactivate
+	 * unconditionally with *every* reclaim invocation for the
+	 * longest time, so the occasional inappropriate activation
+	 * leading to pressure on the active list is not a problem.
+	 */
+	distance = (refault - eviction) & (~0UL >> (BITS_PER_LONG - bits));
 
+	active = lruvec_page_state(lruvec, NR_ACTIVE_FILE);
+	inactive_file = lruvec_page_state(lruvec, NR_INACTIVE_FILE);
+	if (mem_cgroup_get_nr_swap_pages(memcg) > 0) {
+		active += lruvec_page_state(lruvec, NR_ACTIVE_ANON);
+		inactive_anon = lruvec_page_state(lruvec, NR_INACTIVE_ANON);
+	}
+
+	/*
+	 * Compare the distance to the existing workingset size. We
+	 * don't activate pages that couldn't stay resident even if
+	 * all the memory was available to the workingset. Whether
+	 * workingset competition needs to consider anon or not depends
+	 * on having free swap space.
+	 *
+	 * When there are already enough active pages, be less aggressive
+	 * on reactivating pages, challenge an already established set of
+	 * active pages with one time refaulted page may not be a good idea.
+	 */
+	if (active >= (inactive_anon + inactive_file))
+		return distance < inactive_anon + inactive_file;
+	else
+		return distance < active + (file ? inactive_anon : inactive_file);
+}
+
+/**
+ * workingset_age_nonresident - age non-resident entries as LRU ages
+ * @lruvec: the lruvec that was aged
+ * @nr_pages: the number of pages to count
+ *
+ * As in-memory pages are aged, non-resident pages need to be aged as
+ * well, in order for the refault distances later on to be comparable
+ * to the in-memory dimensions. This function allows reclaim and LRU
+ * operations to drive the non-resident aging along in parallel.
+ */
+static void workingset_age_nonresident(struct lruvec *lruvec, unsigned long nr_pages)
+{
+	/*
+	 * Reclaiming a cgroup means reclaiming all its children in a
+	 * round-robin fashion. That means that each cgroup has an LRU
+	 * order that is composed of the LRU orders of its child
+	 * cgroups; and every page has an LRU position not just in the
+	 * cgroup that owns it, but in all of that group's ancestors.
+	 *
+	 * So when the physical inactive list of a leaf cgroup ages,
+	 * the virtual inactive lists of all its parents, including
+	 * the root cgroup's, age as well.
+	 */
+	do {
+		atomic_long_add(nr_pages, &lruvec->nonresident_age);
+	} while ((lruvec = parent_lruvec(lruvec)));
+}
+
+#ifdef CONFIG_LRU_GEN
 static void *lru_gen_eviction(struct folio *folio)
 {
 	int hist;
@@ -342,34 +438,6 @@ static void lru_gen_refault(struct folio *folio, void *shadow)
 
 #endif /* CONFIG_LRU_GEN */
 
-/**
- * workingset_age_nonresident - age non-resident entries as LRU ages
- * @lruvec: the lruvec that was aged
- * @nr_pages: the number of pages to count
- *
- * As in-memory pages are aged, non-resident pages need to be aged as
- * well, in order for the refault distances later on to be comparable
- * to the in-memory dimensions. This function allows reclaim and LRU
- * operations to drive the non-resident aging along in parallel.
- */
-void workingset_age_nonresident(struct lruvec *lruvec, unsigned long nr_pages)
-{
-	/*
-	 * Reclaiming a cgroup means reclaiming all its children in a
-	 * round-robin fashion. That means that each cgroup has an LRU
-	 * order that is composed of the LRU orders of its child
-	 * cgroups; and every page has an LRU position not just in the
-	 * cgroup that owns it, but in all of that group's ancestors.
-	 *
-	 * So when the physical inactive list of a leaf cgroup ages,
-	 * the virtual inactive lists of all its parents, including
-	 * the root cgroup's, age as well.
-	 */
-	do {
-		atomic_long_add(nr_pages, &lruvec->nonresident_age);
-	} while ((lruvec = parent_lruvec(lruvec)));
-}
-
 /**
  * workingset_eviction - note the eviction of a folio from memory
  * @target_memcg: the cgroup that is causing the reclaim
@@ -396,11 +464,11 @@ void *workingset_eviction(struct folio *folio, struct mem_cgroup *target_memcg)
 	lruvec = mem_cgroup_lruvec(target_memcg, pgdat);
 	/* XXX: target_memcg can be NULL, go through lruvec */
 	memcgid = mem_cgroup_id(lruvec_memcg(lruvec));
-	eviction = atomic_long_read(&lruvec->nonresident_age);
-	eviction >>= bucket_order;
+
+	eviction = lru_eviction(lruvec, EVICTION_BITS, bucket_order);
 	workingset_age_nonresident(lruvec, folio_nr_pages(folio));
 	return pack_shadow(memcgid, pgdat, eviction,
-				folio_test_workingset(folio));
+			   folio_test_workingset(folio));
 }
 
 /**
@@ -418,9 +486,6 @@ bool workingset_test_recent(void *shadow, bool file, bool *workingset)
 {
 	struct mem_cgroup *eviction_memcg;
 	struct lruvec *eviction_lruvec;
-	unsigned long refault_distance;
-	unsigned long workingset_size;
-	unsigned long refault;
 	int memcgid;
 	struct pglist_data *pgdat;
 	unsigned long eviction;
@@ -429,7 +494,6 @@ bool workingset_test_recent(void *shadow, bool file, bool *workingset)
 		return lru_gen_test_recent(shadow, file, &eviction_lruvec, &eviction, workingset);
 
 	unpack_shadow(shadow, &memcgid, &pgdat, &eviction, workingset);
-	eviction <<= bucket_order;
 
 	/*
 	 * Look up the memcg associated with the stored ID. It might
@@ -450,50 +514,10 @@ bool workingset_test_recent(void *shadow, bool file, bool *workingset)
 	eviction_memcg = mem_cgroup_from_id(memcgid);
 	if (!mem_cgroup_disabled() && !eviction_memcg)
 		return false;
-
 	eviction_lruvec = mem_cgroup_lruvec(eviction_memcg, pgdat);
-	refault = atomic_long_read(&eviction_lruvec->nonresident_age);
-
-	/*
-	 * Calculate the refault distance
-	 *
-	 * The unsigned subtraction here gives an accurate distance
-	 * across nonresident_age overflows in most cases. There is a
-	 * special case: usually, shadow entries have a short lifetime
-	 * and are either refaulted or reclaimed along with the inode
-	 * before they get too old.  But it is not impossible for the
-	 * nonresident_age to lap a shadow entry in the field, which
-	 * can then result in a false small refault distance, leading
-	 * to a false activation should this old entry actually
-	 * refault again.  However, earlier kernels used to deactivate
-	 * unconditionally with *every* reclaim invocation for the
-	 * longest time, so the occasional inappropriate activation
-	 * leading to pressure on the active list is not a problem.
-	 */
-	refault_distance = (refault - eviction) & EVICTION_MASK;
 
-	/*
-	 * Compare the distance to the existing workingset size. We
-	 * don't activate pages that couldn't stay resident even if
-	 * all the memory was available to the workingset. Whether
-	 * workingset competition needs to consider anon or not depends
-	 * on having free swap space.
-	 */
-	workingset_size = lruvec_page_state(eviction_lruvec, NR_ACTIVE_FILE);
-	if (!file) {
-		workingset_size += lruvec_page_state(eviction_lruvec,
-						     NR_INACTIVE_FILE);
-	}
-	if (mem_cgroup_get_nr_swap_pages(eviction_memcg) > 0) {
-		workingset_size += lruvec_page_state(eviction_lruvec,
-						     NR_ACTIVE_ANON);
-		if (file) {
-			workingset_size += lruvec_page_state(eviction_lruvec,
-						     NR_INACTIVE_ANON);
-		}
-	}
-
-	return refault_distance <= workingset_size;
+	return lru_refault(eviction_memcg, eviction_lruvec, eviction, file,
+			   EVICTION_BITS, bucket_order);
 }
 
 /**
@@ -543,7 +567,6 @@ void workingset_refault(struct folio *folio, void *shadow)
 		goto out;
 
 	folio_set_active(folio);
-	workingset_age_nonresident(lruvec, nr);
 	mod_lruvec_state(lruvec, WORKINGSET_ACTIVATE_BASE + file, nr);
 
 	/* Folio was active prior to eviction */
@@ -560,30 +583,6 @@ void workingset_refault(struct folio *folio, void *shadow)
 	rcu_read_unlock();
 }
 
-/**
- * workingset_activation - note a page activation
- * @folio: Folio that is being activated.
- */
-void workingset_activation(struct folio *folio)
-{
-	struct mem_cgroup *memcg;
-
-	rcu_read_lock();
-	/*
-	 * Filter non-memcg pages here, e.g. unmap can call
-	 * mark_page_accessed() on VDSO pages.
-	 *
-	 * XXX: See workingset_refault() - this should return
-	 * root_mem_cgroup even for !CONFIG_MEMCG.
-	 */
-	memcg = folio_memcg_rcu(folio);
-	if (!mem_cgroup_disabled() && !memcg)
-		goto out;
-	workingset_age_nonresident(folio_lruvec(folio), folio_nr_pages(folio));
-out:
-	rcu_read_unlock();
-}
-
 /*
  * Shadow entries reflect the share of the working set that does not
  * fit into memory, so their number depends on the access pattern of
@@ -778,7 +777,6 @@ static struct lock_class_key shadow_nodes_key;
 
 static int __init workingset_init(void)
 {
-	unsigned int timestamp_bits;
 	unsigned int max_order;
 	int ret;
 
@@ -790,12 +788,11 @@ static int __init workingset_init(void)
 	 * some more pages at runtime, so keep working with up to
 	 * double the initial memory by using totalram_pages as-is.
 	 */
-	timestamp_bits = BITS_PER_LONG - EVICTION_SHIFT;
 	max_order = fls_long(totalram_pages() - 1);
-	if (max_order > timestamp_bits)
-		bucket_order = max_order - timestamp_bits;
+	if (max_order > EVICTION_BITS)
+		bucket_order = max_order - EVICTION_BITS;
 	pr_info("workingset: timestamp_bits=%d max_order=%d bucket_order=%u\n",
-	       timestamp_bits, max_order, bucket_order);
+	       EVICTION_BITS, max_order, bucket_order);
 
 	ret = prealloc_shrinker(&workingset_shadow_shrinker, "mm-shadow");
 	if (ret)
-- 
2.41.0

[RFC PATCH v2 2/5] workingset: update comment in workingset.c

[Kairui Song]

From: Kairui Song <kasong@tencent.com>

Update the comments according to new implementation.

Signed-off-by: Kairui Song <kasong@tencent.com>
---
 mm/workingset.c | 98 ++++++++++++++++++++++---------------------------
 1 file changed, 44 insertions(+), 54 deletions(-)

diff --git a/mm/workingset.c b/mm/workingset.c
index babda11601ea..b5c565a5a959 100644
--- a/mm/workingset.c
+++ b/mm/workingset.c
@@ -64,74 +64,64 @@
  * thrashing on the inactive list, after which refaulting pages can be
  * activated optimistically to compete with the existing active pages.
  *
- * Approximating inactive page access frequency - Observations:
+ * For such approximation, we introduce a counter `nonresistence_age` (NA)
+ * here. This counter increases each time a page is evicted, and each evicted
+ * page will have a shadow that stores the counter reading at the eviction
+ * time as a timestamp. So when an evicted page was faulted again, we have:
  *
- * 1. When a page is accessed for the first time, it is added to the
- *    head of the inactive list, slides every existing inactive page
- *    towards the tail by one slot, and pushes the current tail page
- *    out of memory.
+ *   Let SP = ((NA's reading @ current) - (NA's reading @ eviction))
  *
- * 2. When a page is accessed for the second time, it is promoted to
- *    the active list, shrinking the inactive list by one slot.  This
- *    also slides all inactive pages that were faulted into the cache
- *    more recently than the activated page towards the tail of the
- *    inactive list.
+ *                            +-memory available to cache-+
+ *                            |                           |
+ *  +-------------------------+===============+===========+
+ *  | *   shadows  O O  O     |   INACTIVE    |   ACTIVE  |
+ *  +-+-----------------------+===============+===========+
+ *    |                       |
+ *    +-----------------------+
+ *    |         SP
+ *  fault page          O -> Hole left by previously faulted in pages
+ *                      * -> The page corresponding to SP
  *
- * Thus:
+ * Here SP can stands for how far the current workflow could push a page
+ * out of available memory. Since all evicted page was once head of
+ * INACTIVE list, the page could have such an access distance of:
  *
- * 1. The sum of evictions and activations between any two points in
- *    time indicate the minimum number of inactive pages accessed in
- *    between.
+ *   SP + NR_INACTIVE
  *
- * 2. Moving one inactive page N page slots towards the tail of the
- *    list requires at least N inactive page accesses.
+ * So if:
  *
- * Combining these:
+ *   SP + NR_INACTIVE < NR_INACTIVE + NR_ACTIVE
  *
- * 1. When a page is finally evicted from memory, the number of
- *    inactive pages accessed while the page was in cache is at least
- *    the number of page slots on the inactive list.
+ * Which can be simplified to:
  *
- * 2. In addition, measuring the sum of evictions and activations (E)
- *    at the time of a page's eviction, and comparing it to another
- *    reading (R) at the time the page faults back into memory tells
- *    the minimum number of accesses while the page was not cached.
- *    This is called the refault distance.
+ *   SP < NR_ACTIVE
  *
- * Because the first access of the page was the fault and the second
- * access the refault, we combine the in-cache distance with the
- * out-of-cache distance to get the complete minimum access distance
- * of this page:
+ * Then the page is worth getting re-activated to start from ACTIVE part,
+ * since the access distance is shorter than total memory to make it stay.
  *
- *      NR_inactive + (R - E)
+ * And since this is only an estimation, based on several hypotheses, and
+ * it could break the ability of LRU to distinguish a workingset out of
+ * caches, so throttle this by two factors:
  *
- * And knowing the minimum access distance of a page, we can easily
- * tell if the page would be able to stay in cache assuming all page
- * slots in the cache were available:
+ * 1. Notice that re-faulted in pages may leave "holes" on the shadow
+ *    part of LRU, that part is left unhandled on purpose to decrease
+ *    re-activate rate for pages that have a large SP value (the larger
+ *    SP value a page have, the more likely it will be affected by such
+ *    holes).
+ * 2. When the ACTIVE part of LRU is long enough, chanllaging ACTIVE pages
+ *    by re-activating a one-time faulted previously INACTIVE page may not
+ *    be a good idea, so throttle the re-activation when ACTIVE > INACTIVE
+ *    by comparing with INACTIVE instead.
  *
- *   NR_inactive + (R - E) <= NR_inactive + NR_active
+ * Combined all above, we have:
+ * Upon refault, if any of the following conditions is met, mark the page
+ * as active:
  *
- * If we have swap we should consider about NR_inactive_anon and
- * NR_active_anon, so for page cache and anonymous respectively:
- *
- *   NR_inactive_file + (R - E) <= NR_inactive_file + NR_active_file
- *   + NR_inactive_anon + NR_active_anon
- *
- *   NR_inactive_anon + (R - E) <= NR_inactive_anon + NR_active_anon
- *   + NR_inactive_file + NR_active_file
- *
- * Which can be further simplified to:
- *
- *   (R - E) <= NR_active_file + NR_inactive_anon + NR_active_anon
- *
- *   (R - E) <= NR_active_anon + NR_inactive_file + NR_active_file
- *
- * Put into words, the refault distance (out-of-cache) can be seen as
- * a deficit in inactive list space (in-cache).  If the inactive list
- * had (R - E) more page slots, the page would not have been evicted
- * in between accesses, but activated instead.  And on a full system,
- * the only thing eating into inactive list space is active pages.
+ * - If ACTIVE LRU is low (NR_ACTIVE < NR_INACTIVE), check if:
+ *   SP < NR_ACTIVE
  *
+ * - If ACTIVE LRU is high (NR_ACTIVE >= NR_INACTIVE), check if:
+ *   SP < NR_INACTIVE
  *
  *		Refaulting inactive pages
  *
-- 
2.41.0

[RFC PATCH v2 3/5] workingset: simplify lru_gen_test_recent

[Kairui Song]

From: Kairui Song <kasong@tencent.com>

Simplify the code, move some common path into its caller, prepare for
following commits.

Signed-off-by: Kairui Song <kasong@tencent.com>
---
 mm/workingset.c | 30 +++++++++++++-----------------
 1 file changed, 13 insertions(+), 17 deletions(-)

diff --git a/mm/workingset.c b/mm/workingset.c
index b5c565a5a959..ff7587456b7f 100644
--- a/mm/workingset.c
+++ b/mm/workingset.c
@@ -345,42 +345,38 @@ static void *lru_gen_eviction(struct folio *folio)
  * Tests if the shadow entry is for a folio that was recently evicted.
  * Fills in @lruvec, @token, @workingset with the values unpacked from shadow.
  */
-static bool lru_gen_test_recent(void *shadow, bool file, struct lruvec **lruvec,
-				unsigned long *token, bool *workingset)
+static bool lru_gen_test_recent(struct lruvec *lruvec, bool file,
+				unsigned long token)
 {
-	int memcg_id;
 	unsigned long min_seq;
-	struct mem_cgroup *memcg;
-	struct pglist_data *pgdat;
 
-	unpack_shadow(shadow, &memcg_id, &pgdat, token, workingset);
-
-	memcg = mem_cgroup_from_id(memcg_id);
-	*lruvec = mem_cgroup_lruvec(memcg, pgdat);
-
-	min_seq = READ_ONCE((*lruvec)->lrugen.min_seq[file]);
-	return (*token >> LRU_REFS_WIDTH) == (min_seq & (EVICTION_MASK >> LRU_REFS_WIDTH));
+	min_seq = READ_ONCE(lruvec->lrugen.min_seq[file]);
+	return (token >> LRU_REFS_WIDTH) == (min_seq & (EVICTION_MASK >> LRU_REFS_WIDTH));
 }
 
 static void lru_gen_refault(struct folio *folio, void *shadow)
 {
+	int memcgid;
 	bool recent;
-	int hist, tier, refs;
 	bool workingset;
 	unsigned long token;
+	int hist, tier, refs;
 	struct lruvec *lruvec;
+	struct pglist_data *pgdat;
 	struct lru_gen_folio *lrugen;
 	int type = folio_is_file_lru(folio);
 	int delta = folio_nr_pages(folio);
 
 	rcu_read_lock();
 
-	recent = lru_gen_test_recent(shadow, type, &lruvec, &token, &workingset);
+	unpack_shadow(shadow, &memcgid, &pgdat, &token, &workingset);
+	lruvec = mem_cgroup_lruvec(mem_cgroup_from_id(memcgid), pgdat);
 	if (lruvec != folio_lruvec(folio))
 		goto unlock;
 
 	mod_lruvec_state(lruvec, WORKINGSET_REFAULT_BASE + type, delta);
 
+	recent = lru_gen_test_recent(lruvec, type, token);
 	if (!recent)
 		goto unlock;
 
@@ -480,9 +476,6 @@ bool workingset_test_recent(void *shadow, bool file, bool *workingset)
 	struct pglist_data *pgdat;
 	unsigned long eviction;
 
-	if (lru_gen_enabled())
-		return lru_gen_test_recent(shadow, file, &eviction_lruvec, &eviction, workingset);
-
 	unpack_shadow(shadow, &memcgid, &pgdat, &eviction, workingset);
 
 	/*
@@ -506,6 +499,9 @@ bool workingset_test_recent(void *shadow, bool file, bool *workingset)
 		return false;
 	eviction_lruvec = mem_cgroup_lruvec(eviction_memcg, pgdat);
 
+	if (lru_gen_enabled())
+		return lru_gen_test_recent(eviction_lruvec, file, eviction);
+
 	return lru_refault(eviction_memcg, eviction_lruvec, eviction, file,
 			   EVICTION_BITS, bucket_order);
 }
-- 
2.41.0

[RFC PATCH v2 4/5] lru_gen: convert avg_total and avg_refaulted to atomic

[Kairui Song]

From: Kairui Song <kasong@tencent.com>

No feature change, prepare for later patch.

Signed-off-by: Kairui Song <kasong@tencent.com>
---
 include/linux/mmzone.h |  4 ++--
 mm/vmscan.c            | 16 ++++++++--------
 2 files changed, 10 insertions(+), 10 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 4106fbc5b4b3..d944987b67d3 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -425,9 +425,9 @@ struct lru_gen_folio {
 	/* the multi-gen LRU sizes, eventually consistent */
 	long nr_pages[MAX_NR_GENS][ANON_AND_FILE][MAX_NR_ZONES];
 	/* the exponential moving average of refaulted */
-	unsigned long avg_refaulted[ANON_AND_FILE][MAX_NR_TIERS];
+	atomic_long_t avg_refaulted[ANON_AND_FILE][MAX_NR_TIERS];
 	/* the exponential moving average of evicted+protected */
-	unsigned long avg_total[ANON_AND_FILE][MAX_NR_TIERS];
+	atomic_long_t avg_total[ANON_AND_FILE][MAX_NR_TIERS];
 	/* the first tier doesn't need protection, hence the minus one */
 	unsigned long protected[NR_HIST_GENS][ANON_AND_FILE][MAX_NR_TIERS - 1];
 	/* can be modified without holding the LRU lock */
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 3f4de75e5186..82acc1934c86 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3705,9 +3705,9 @@ static void read_ctrl_pos(struct lruvec *lruvec, int type, int tier, int gain,
 	struct lru_gen_folio *lrugen = &lruvec->lrugen;
 	int hist = lru_hist_from_seq(lrugen->min_seq[type]);
 
-	pos->refaulted = lrugen->avg_refaulted[type][tier] +
+	pos->refaulted = atomic_long_read(&lrugen->avg_refaulted[type][tier]) +
 			 atomic_long_read(&lrugen->refaulted[hist][type][tier]);
-	pos->total = lrugen->avg_total[type][tier] +
+	pos->total = atomic_long_read(&lrugen->avg_total[type][tier]) +
 		     atomic_long_read(&lrugen->evicted[hist][type][tier]);
 	if (tier)
 		pos->total += lrugen->protected[hist][type][tier - 1];
@@ -3732,15 +3732,15 @@ static void reset_ctrl_pos(struct lruvec *lruvec, int type, bool carryover)
 		if (carryover) {
 			unsigned long sum;
 
-			sum = lrugen->avg_refaulted[type][tier] +
+			sum = atomic_long_read(&lrugen->avg_refaulted[type][tier]) +
 			      atomic_long_read(&lrugen->refaulted[hist][type][tier]);
-			WRITE_ONCE(lrugen->avg_refaulted[type][tier], sum / 2);
+			atomic_long_set(&lrugen->avg_refaulted[type][tier], sum / 2);
 
-			sum = lrugen->avg_total[type][tier] +
+			sum = atomic_long_read(&lrugen->avg_total[type][tier]) +
 			      atomic_long_read(&lrugen->evicted[hist][type][tier]);
 			if (tier)
 				sum += lrugen->protected[hist][type][tier - 1];
-			WRITE_ONCE(lrugen->avg_total[type][tier], sum / 2);
+			atomic_long_set(&lrugen->avg_total[type][tier], sum / 2);
 		}
 
 		if (clear) {
@@ -5885,8 +5885,8 @@ static void lru_gen_seq_show_full(struct seq_file *m, struct lruvec *lruvec,
 
 			if (seq == max_seq) {
 				s = "RT ";
-				n[0] = READ_ONCE(lrugen->avg_refaulted[type][tier]);
-				n[1] = READ_ONCE(lrugen->avg_total[type][tier]);
+				n[0] = atomic_long_read(&lrugen->avg_refaulted[type][tier]);
+				n[1] = atomic_long_read(&lrugen->avg_total[type][tier]);
 			} else if (seq == min_seq[type] || NR_HIST_GENS > 1) {
 				s = "rep";
 				n[0] = atomic_long_read(&lrugen->refaulted[hist][type][tier]);
-- 
2.41.0

[RFC PATCH v2 5/5] workingset, lru_gen: apply refault-distance based re-activation

[Kairui Song]

From: Kairui Song <kasong@tencent.com>

I noticed MGLRU not working very well on certain workflows, which is
observed on some heavily stressed databases. That is when the file
page workingset size exceeds total memory, and the access distance
(the left-shift time of a page before it gets activated, considering
LRU starts from right) of file pages also larger than total memory.
All file pages are stuck on the oldest generation and getting
read-in then evicted permutably. Despite anon pages being idle,
they never get aged. PID controller didn't kickin until there are some
minor access pattern changes. And file pages are not promoted
or reused.

Even though the memory can't cover the whole workingset, the
refault-distance based re-activation can help hold part of the
workingset in-memory to help reduce the IO workload significantly.

So apply it for MGLRU as well. The updated refault-distance model
fits well for MGLRU in most cases, if we just consider the last two
generation as the inactive LRU and the first two generations as
active LRU.

Some adjustment is done to fit the logic better, also make the
refault-distance contributed to page tiering and PID refault detection
of MGLRU:

- If a tier-0 page have a qualified refault-distance, just promote
  it to higher tier, send it to second oldest gen.
- If a tier >= 1 page have a qualified refault-distance, mark it as
  active and send it to youngest gen.
- Increase the reference of every page that have a qualified refault-distance
  and increase the PID countroled refault rate of the updated tier.

Following benchmark showed improvement. To simulate the workflow, I
setup a 3-replicated mongodb cluster, each use 5 gb of cache and 10g of
oplog, on a 32G VM. The benchmark is done using
https://github.com/apavlo/py-tpcc.git, modified to run STOCK_LEVEL
query only, for simulating slow query and get a stable result.

Before the patch (with 10G swap, the result won't change whether
swap is on or not):
$ tpcc.py --config=mongodb.config mongodb --duration=900 --warehouses=500 --clients=30
==================================================================
Execution Results after 904 seconds
------------------------------------------------------------------
                  Executed        Time (µs)       Rate
  STOCK_LEVEL     503             27150226136.4   0.02 txn/s
------------------------------------------------------------------
  TOTAL           503             27150226136.4   0.02 txn/s

$ cat /proc/vmstat | grep working
workingset_nodes 53391
workingset_refault_anon 0
workingset_refault_file 23856735
workingset_activate_anon 0
workingset_activate_file 23845737
workingset_restore_anon 0
workingset_restore_file 18280692
workingset_nodereclaim 1024

$ free -m
              total        used        free      shared  buff/cache   available
Mem:          31837        6752         379          23       24706       24607
Swap:         10239           0       10239

After the patch (with 10G swap on same disk, similiar result using ZRAM):
$ tpcc.py --config=mongodb.config mongodb --duration=900 --warehouses=500 --clients=30
==================================================================
Execution Results after 903 seconds
------------------------------------------------------------------
                  Executed        Time (µs)       Rate
  STOCK_LEVEL     2575            27094953498.8   0.10 txn/s
------------------------------------------------------------------
  TOTAL           2575            27094953498.8   0.10 txn/s

$ cat /proc/vmstat | grep working
workingset_nodes 78249
workingset_refault_anon 10139
workingset_refault_file 23001863
workingset_activate_anon 7238
workingset_activate_file 6718032
workingset_restore_anon 7432
workingset_restore_file 6719406
workingset_nodereclaim 9747

$ free -m
              total        used        free      shared  buff/cache   available
Mem:          31837        7376         320           3       24140       24014
Swap:         10239        1662        8577

The performance is 5x times better than before, and the idle anon pages
now can get swapped out as expected. The result is also better with
lower test stress, testing with lower stress also shows a improvement.

I also checked the benchmark with memtier/memcached and fio,
using similar setup as in commit ac35a4902374 but scaled down to fit in
my test environment:

  memtier test (16G ramdisk as swap, 2G memcg limit, VM on a EPYC 7K62):
  memcached -u nobody -m 16384 -s /tmp/memcached.socket -a 0766 \
    -t 12 -B binary &
  memtier_benchmark -S /tmp/memcached.socket -P memcache_binary -n allkeys\
    --key-minimum=1 --key-maximum=24000000 --key-pattern=P:P -c 1 \
    -t 12 --ratio 1:0 --pipeline 8 -d 2000 -x 6

  fio test (16G ramdisk on /mnt, 4G memcg limit, VM on a EPYC 7K62):
  fio -name=refault --numjobs=14 --directory=/mnt --size=1024m \
    --buffered=1 --ioengine=io_uring --iodepth=128 \
    --iodepth_batch_submit=32 --iodepth_batch_complete=32 \
    --rw=randread --random_distribution=random --norandommap \
    --time_based --ramp_time=5m --runtime=5m --group_reporting

  mysql test (15G buffer pool with 16G memcg limit, VM on a EPYC 7K62):
    sysbench /usr/share/sysbench/oltp_read_only.lua <auth and db params> \
      --tables=48 --table-size=2000000 --threads=32 --time=1800

Before this patch:
memtier: 379329.77 op/s
fio: 5786.8k iops
mysql: 150190.43 qps

After this patch:
memtier: 373877.41 op/s
fio: 5805.5k iops
mysql: 150220.93 qps

The test looks ok except a bit extra overhead introduced by atomic
operations introduced, there seems to be no LRU accuracy drop.

Signed-off-by: Kairui Song <kasong@tencent.com>
---
 mm/workingset.c | 78 +++++++++++++++++++++++++++++++++----------------
 1 file changed, 53 insertions(+), 25 deletions(-)

diff --git a/mm/workingset.c b/mm/workingset.c
index ff7587456b7f..1fa336054528 100644
--- a/mm/workingset.c
+++ b/mm/workingset.c
@@ -175,6 +175,7 @@
 			 MEM_CGROUP_ID_SHIFT)
 #define EVICTION_BITS	(BITS_PER_LONG - (EVICTION_SHIFT))
 #define EVICTION_MASK	(~0UL >> EVICTION_SHIFT)
+#define LRU_GEN_EVICTION_BITS	(EVICTION_BITS - LRU_REFS_WIDTH - LRU_GEN_WIDTH)
 
 /*
  * Eviction timestamps need to be able to cover the full range of
@@ -185,6 +186,7 @@
  * evictions into coarser buckets by shaving off lower timestamp bits.
  */
 static unsigned int bucket_order __read_mostly;
+static unsigned int lru_gen_bucket_order __read_mostly;
 
 static void *pack_shadow(int memcgid, pg_data_t *pgdat, unsigned long eviction,
 			 bool workingset)
@@ -240,7 +242,7 @@ static inline bool lru_refault(struct mem_cgroup *memcg,
 			       int bits, int bucket_order)
 {
 	unsigned long refault, distance;
-	unsigned long workingset, active, inactive, inactive_file, inactive_anon = 0;
+	unsigned long active, inactive_file, inactive_anon = 0;
 
 	eviction <<= bucket_order;
 	refault = atomic_long_read(&lruvec->nonresident_age);
@@ -280,7 +282,7 @@ static inline bool lru_refault(struct mem_cgroup *memcg,
 	 * active pages with one time refaulted page may not be a good idea.
 	 */
 	if (active >= (inactive_anon + inactive_file))
-		return distance < inactive_anon + inactive_file;
+		return distance < (inactive_anon + inactive_file);
 	else
 		return distance < active + (file ? inactive_anon : inactive_file);
 }
@@ -333,10 +335,14 @@ static void *lru_gen_eviction(struct folio *folio)
 	lruvec = mem_cgroup_lruvec(memcg, pgdat);
 	lrugen = &lruvec->lrugen;
 	min_seq = READ_ONCE(lrugen->min_seq[type]);
+
 	token = (min_seq << LRU_REFS_WIDTH) | max(refs - 1, 0);
+	token <<= LRU_GEN_EVICTION_BITS;
+	token |= lru_eviction(lruvec, LRU_GEN_EVICTION_BITS, lru_gen_bucket_order);
 
 	hist = lru_hist_from_seq(min_seq);
 	atomic_long_add(delta, &lrugen->evicted[hist][type][tier]);
+	workingset_age_nonresident(lruvec, folio_nr_pages(folio));
 
 	return pack_shadow(mem_cgroup_id(memcg), pgdat, token, refs);
 }
@@ -351,44 +357,55 @@ static bool lru_gen_test_recent(struct lruvec *lruvec, bool file,
 	unsigned long min_seq;
 
 	min_seq = READ_ONCE(lruvec->lrugen.min_seq[file]);
+	token >>= LRU_GEN_EVICTION_BITS;
 	return (token >> LRU_REFS_WIDTH) == (min_seq & (EVICTION_MASK >> LRU_REFS_WIDTH));
 }
 
 static void lru_gen_refault(struct folio *folio, void *shadow)
 {
 	int memcgid;
-	bool recent;
+	bool refault;
 	bool workingset;
 	unsigned long token;
+	bool recent = false;
+	int refault_tier = 0;
 	int hist, tier, refs;
 	struct lruvec *lruvec;
+	struct mem_cgroup *memcg;
 	struct pglist_data *pgdat;
 	struct lru_gen_folio *lrugen;
 	int type = folio_is_file_lru(folio);
 	int delta = folio_nr_pages(folio);
 
-	rcu_read_lock();
-
 	unpack_shadow(shadow, &memcgid, &pgdat, &token, &workingset);
-	lruvec = mem_cgroup_lruvec(mem_cgroup_from_id(memcgid), pgdat);
-	if (lruvec != folio_lruvec(folio))
-		goto unlock;
+	memcg = mem_cgroup_from_id(memcgid);
+	lruvec = mem_cgroup_lruvec(memcg, pgdat);
+	/* memcg can be NULL, go through lruvec */
+	memcg = lruvec_memcg(lruvec);
 
 	mod_lruvec_state(lruvec, WORKINGSET_REFAULT_BASE + type, delta);
-
-	recent = lru_gen_test_recent(lruvec, type, token);
-	if (!recent)
-		goto unlock;
+	refault = lru_refault(memcg, lruvec, token, type,
+			      LRU_GEN_EVICTION_BITS, lru_gen_bucket_order);
+	if (lruvec == folio_lruvec(folio))
+		recent = lru_gen_test_recent(lruvec, type, token);
+	if (!recent && !refault)
+		return;
 
 	lrugen = &lruvec->lrugen;
-
 	hist = lru_hist_from_seq(READ_ONCE(lrugen->min_seq[type]));
 	/* see the comment in folio_lru_refs() */
+	token >>= LRU_GEN_EVICTION_BITS;
 	refs = (token & (BIT(LRU_REFS_WIDTH) - 1)) + workingset;
 	tier = lru_tier_from_refs(refs);
-
-	atomic_long_add(delta, &lrugen->refaulted[hist][type][tier]);
-	mod_lruvec_state(lruvec, WORKINGSET_ACTIVATE_BASE + type, delta);
+	refault_tier = tier;
+
+	if (refault) {
+		if (refs)
+			folio_set_active(folio);
+		if (refs != BIT(LRU_REFS_WIDTH))
+			refault_tier = lru_tier_from_refs(refs + 1);
+		mod_lruvec_state(lruvec, WORKINGSET_ACTIVATE_BASE + type, delta);
+	}
 
 	/*
 	 * Count the following two cases as stalls:
@@ -397,12 +414,17 @@ static void lru_gen_refault(struct folio *folio, void *shadow)
 	 * 2. For pages accessed multiple times through file descriptors,
 	 *    numbers of accesses might have been out of the range.
 	 */
-	if (lru_gen_in_fault() || refs == BIT(LRU_REFS_WIDTH)) {
+	if (refault || lru_gen_in_fault() || refs == BIT(LRU_REFS_WIDTH)) {
 		folio_set_workingset(folio);
 		mod_lruvec_state(lruvec, WORKINGSET_RESTORE_BASE + type, delta);
 	}
-unlock:
-	rcu_read_unlock();
+
+	if (recent && refault_tier == tier) {
+		atomic_long_add(delta, &lrugen->refaulted[hist][type][tier]);
+	} else {
+		atomic_long_add(delta, &lrugen->avg_total[type][refault_tier]);
+		atomic_long_add(delta, &lrugen->avg_refaulted[type][refault_tier]);
+	}
 }
 
 #else /* !CONFIG_LRU_GEN */
@@ -524,16 +546,15 @@ void workingset_refault(struct folio *folio, void *shadow)
 	bool workingset;
 	long nr;
 
-	if (lru_gen_enabled()) {
-		lru_gen_refault(folio, shadow);
-		return;
-	}
-
 	/* Flush stats (and potentially sleep) before holding RCU read lock */
 	mem_cgroup_flush_stats_ratelimited();
-
 	rcu_read_lock();
 
+	if (lru_gen_enabled()) {
+		lru_gen_refault(folio, shadow);
+		goto out;
+	}
+
 	/*
 	 * The activation decision for this folio is made at the level
 	 * where the eviction occurred, as that is where the LRU order
@@ -780,6 +801,13 @@ static int __init workingset_init(void)
 	pr_info("workingset: timestamp_bits=%d max_order=%d bucket_order=%u\n",
 	       EVICTION_BITS, max_order, bucket_order);
 
+#ifdef CONFIG_LRU_GEN
+	if (max_order > LRU_GEN_EVICTION_BITS)
+		lru_gen_bucket_order = max_order - LRU_GEN_EVICTION_BITS;
+	pr_info("workingset: lru_gen_timestamp_bits=%d lru_gen_bucket_order=%u\n",
+		LRU_GEN_EVICTION_BITS, lru_gen_bucket_order);
+#endif
+
 	ret = prealloc_shrinker(&workingset_shadow_shrinker, "mm-shadow");
 	if (ret)
 		goto err;
-- 
2.41.0

Re: [RFC PATCH v2 1/5] workingset: simplify and use a more intuitive model

[Johannes Weiner]

Hello Kairui,

On Wed, Sep 13, 2023 at 02:45:07AM +0800, Kairui Song wrote:
> @@ -226,8 +227,103 @@ static void unpack_shadow(void *shadow, int *memcgidp, pg_data_t **pgdat,
>  	*workingsetp = workingset;
>  }
>  
> -#ifdef CONFIG_LRU_GEN
> +/*
> + * Get the distance reading at eviction time.
> + */
> +static inline unsigned long lru_eviction(struct lruvec *lruvec,
> +					 int bits, int bucket_order)
> +{
> +	unsigned long eviction = atomic_long_read(&lruvec->nonresident_age);
> +
> +	eviction >>= bucket_order;
> +	eviction &= ~0UL >> (BITS_PER_LONG - bits);
> +
> +	return eviction;
> +}
> +
> +/*
> + * Calculate and test refault distance
> + */
> +static inline bool lru_refault(struct mem_cgroup *memcg,
> +			       struct lruvec *lruvec,
> +			       unsigned long eviction, bool file,
> +			       int bits, int bucket_order)
> +{

This patch changes the refault decision making at the same time as it
moves the functions around in preparation for use by MGLRU. This makes
it difficult to review the exact changes to the current algorithm.

Can you please separate those two components?

Step 1: Change the existing logic in its place to the new
algorithm. Please also update the big comment on how everything works
at the same time (IOW, merge patch #2 into this one).

Step 2: Add a new patch to refactor for MGLRU right before the patch
that will use those functions. As far as I can see, that's the last
patch in the series. So the refactor should be second-to-last.

Thanks

Re: [RFC PATCH v2 1/5] workingset: simplify and use a more intuitive model

[Kairui Song]

Johannes Weiner <hannes@cmpxchg.org> 于2023年9月13日周三 03:47写道：
>
> Hello Kairui,
>
> On Wed, Sep 13, 2023 at 02:45:07AM +0800, Kairui Song wrote:
> > @@ -226,8 +227,103 @@ static void unpack_shadow(void *shadow, int *memcgidp, pg_data_t **pgdat,
> >       *workingsetp = workingset;
> >  }
> >
> > -#ifdef CONFIG_LRU_GEN
> > +/*
> > + * Get the distance reading at eviction time.
> > + */
> > +static inline unsigned long lru_eviction(struct lruvec *lruvec,
> > +                                      int bits, int bucket_order)
> > +{
> > +     unsigned long eviction = atomic_long_read(&lruvec->nonresident_age);
> > +
> > +     eviction >>= bucket_order;
> > +     eviction &= ~0UL >> (BITS_PER_LONG - bits);
> > +
> > +     return eviction;
> > +}
> > +
> > +/*
> > + * Calculate and test refault distance
> > + */
> > +static inline bool lru_refault(struct mem_cgroup *memcg,
> > +                            struct lruvec *lruvec,
> > +                            unsigned long eviction, bool file,
> > +                            int bits, int bucket_order)
> > +{
>
> This patch changes the refault decision making at the same time as it
> moves the functions around in preparation for use by MGLRU. This makes
> it difficult to review the exact changes to the current algorithm.
>
> Can you please separate those two components?

Hi, thanks for the review.

>
> Step 1: Change the existing logic in its place to the new
> algorithm. Please also update the big comment on how everything works
> at the same time (IOW, merge patch #2 into this one).
>
> Step 2: Add a new patch to refactor for MGLRU right before the patch
> that will use those functions. As far as I can see, that's the last
> patch in the series. So the refactor should be second-to-last.
>

Good suggestion, I'll split these parts into individual patches and
merge the comments.

> Thanks