The patch titled
Subject: zswap: shrinks zswap pool based on memory pressure
has been added to the -mm mm-unstable branch. Its filename is
zswap-shrinks-zswap-pool-based-on-memory-pressure.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/zswap-shrinks-zswap-pool-based-on-memory-pressure.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Nhat Pham <nphamcs@xxxxxxxxx>
Subject: zswap: shrinks zswap pool based on memory pressure
Date: Mon, 27 Nov 2023 11:37:03 -0800
Currently, we only shrink the zswap pool when the user-defined limit is
hit. This means that if we set the limit too high, cold data that are
unlikely to be used again will reside in the pool, wasting precious
memory. It is hard to predict how much zswap space will be needed ahead
of time, as this depends on the workload (specifically, on factors such as
memory access patterns and compressibility of the memory pages).
This patch implements a memcg- and NUMA-aware shrinker for zswap, that is
initiated when there is memory pressure. The shrinker does not have any
parameter that must be tuned by the user, and can be opted in or out on a
per-memcg basis.
Furthermore, to make it more robust for many workloads and prevent
overshrinking (i.e evicting warm pages that might be refaulted into
memory), we build in the following heuristics:
* Estimate the number of warm pages residing in zswap, and attempt to
protect this region of the zswap LRU.
* Scale the number of freeable objects by an estimate of the memory
saving factor. The better zswap compresses the data, the fewer pages
we will evict to swap (as we will otherwise incur IO for relatively
small memory saving).
* During reclaim, if the shrinker encounters a page that is also being
brought into memory, the shrinker will cautiously terminate its
shrinking action, as this is a sign that it is touching the warmer
region of the zswap LRU.
As a proof of concept, we ran the following synthetic benchmark: build the
linux kernel in a memory-limited cgroup, and allocate some cold data in
tmpfs to see if the shrinker could write them out and improved the overall
performance. Depending on the amount of cold data generated, we observe
from 14% to 35% reduction in kernel CPU time used in the kernel builds.
Link: https://lkml.kernel.org/r/20231127193703.1980089-7-nphamcs@xxxxxxxxx
Signed-off-by: Nhat Pham <nphamcs@xxxxxxxxx>
Cc: Chris Li <chrisl@xxxxxxxxxx>
Cc: Dan Streetman <ddstreet@xxxxxxxx>
Cc: Domenico Cerasuolo <cerasuolodomenico@xxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Muchun Song <muchun.song@xxxxxxxxx>
Cc: Roman Gushchin <roman.gushchin@xxxxxxxxx>
Cc: Seth Jennings <sjenning@xxxxxxxxxx>
Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>
Cc: Shuah Khan <shuah@xxxxxxxxxx>
Cc: Vitaly Wool <vitaly.wool@xxxxxxxxxxxx>
Cc: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
Documentation/admin-guide/mm/zswap.rst | 7
include/linux/mmzone.h | 2
include/linux/zswap.h | 25 ++-
mm/mmzone.c | 1
mm/swap_state.c | 2
mm/zswap.c | 177 ++++++++++++++++++++++-
6 files changed, 208 insertions(+), 6 deletions(-)
--- a/Documentation/admin-guide/mm/zswap.rst~zswap-shrinks-zswap-pool-based-on-memory-pressure
+++ a/Documentation/admin-guide/mm/zswap.rst
@@ -153,6 +153,13 @@ attribute, e. g.::
Setting this parameter to 100 will disable the hysteresis.
+When there is a sizable amount of cold memory residing in the zswap pool, it
+can be advantageous to proactively write these cold pages to swap and reclaim
+the memory for other use cases. By default, the zswap shrinker is disabled.
+User can enable it as follows:
+
+ echo Y > /sys/module/zswap/parameters/shrinker_enabled
+
A debugfs interface is provided for various statistic about pool size, number
of pages stored, same-value filled pages and various counters for the reasons
pages are rejected.
--- a/include/linux/mmzone.h~zswap-shrinks-zswap-pool-based-on-memory-pressure
+++ a/include/linux/mmzone.h
@@ -22,6 +22,7 @@
#include <linux/mm_types.h>
#include <linux/page-flags.h>
#include <linux/local_lock.h>
+#include <linux/zswap.h>
#include <asm/page.h>
/* Free memory management - zoned buddy allocator. */
@@ -641,6 +642,7 @@ struct lruvec {
#ifdef CONFIG_MEMCG
struct pglist_data *pgdat;
#endif
+ struct zswap_lruvec_state zswap_lruvec_state;
};
/* Isolate for asynchronous migration */
--- a/include/linux/zswap.h~zswap-shrinks-zswap-pool-based-on-memory-pressure
+++ a/include/linux/zswap.h
@@ -5,20 +5,40 @@
#include <linux/types.h>
#include <linux/mm_types.h>
+struct lruvec;
+
extern u64 zswap_pool_total_size;
extern atomic_t zswap_stored_pages;
#ifdef CONFIG_ZSWAP
+struct zswap_lruvec_state {
+ /*
+ * Number of pages in zswap that should be protected from the shrinker.
+ * This number is an estimate of the following counts:
+ *
+ * a) Recent page faults.
+ * b) Recent insertion to the zswap LRU. This includes new zswap stores,
+ * as well as recent zswap LRU rotations.
+ *
+ * These pages are likely to be warm, and might incur IO if the are written
+ * to swap.
+ */
+ atomic_long_t nr_zswap_protected;
+};
+
bool zswap_store(struct folio *folio);
bool zswap_load(struct folio *folio);
void zswap_invalidate(int type, pgoff_t offset);
void zswap_swapon(int type);
void zswap_swapoff(int type);
void zswap_memcg_offline_cleanup(struct mem_cgroup *memcg);
-
+void zswap_lruvec_state_init(struct lruvec *lruvec);
+void zswap_lruvec_swapin(struct page *page);
#else
+struct zswap_lruvec_state {};
+
static inline bool zswap_store(struct folio *folio)
{
return false;
@@ -33,7 +53,8 @@ static inline void zswap_invalidate(int
static inline void zswap_swapon(int type) {}
static inline void zswap_swapoff(int type) {}
static inline void zswap_memcg_offline_cleanup(struct mem_cgroup *memcg) {}
-
+static inline void zswap_lruvec_init(struct lruvec *lruvec) {}
+static inline void zswap_lruvec_swapin(struct page *page) {}
#endif
#endif /* _LINUX_ZSWAP_H */
--- a/mm/mmzone.c~zswap-shrinks-zswap-pool-based-on-memory-pressure
+++ a/mm/mmzone.c
@@ -78,6 +78,7 @@ void lruvec_init(struct lruvec *lruvec)
memset(lruvec, 0, sizeof(struct lruvec));
spin_lock_init(&lruvec->lru_lock);
+ zswap_lruvec_state_init(lruvec);
for_each_lru(lru)
INIT_LIST_HEAD(&lruvec->lists[lru]);
--- a/mm/swap_state.c~zswap-shrinks-zswap-pool-based-on-memory-pressure
+++ a/mm/swap_state.c
@@ -687,6 +687,7 @@ skip:
&page_allocated, false);
if (unlikely(page_allocated))
swap_readpage(page, false, NULL);
+ zswap_lruvec_swapin(page);
return page;
}
@@ -862,6 +863,7 @@ skip:
&page_allocated, false);
if (unlikely(page_allocated))
swap_readpage(page, false, NULL);
+ zswap_lruvec_swapin(page);
return page;
}
--- a/mm/zswap.c~zswap-shrinks-zswap-pool-based-on-memory-pressure
+++ a/mm/zswap.c
@@ -148,6 +148,10 @@ module_param_named(exclusive_loads, zswa
/* Number of zpools in zswap_pool (empirically determined for scalability) */
#define ZSWAP_NR_ZPOOLS 32
+/* Enable/disable memory pressure-based shrinker. */
+static bool zswap_shrinker_enabled;
+module_param_named(shrinker_enabled, zswap_shrinker_enabled, bool, 0644);
+
/*********************************
* data structures
**********************************/
@@ -177,6 +181,8 @@ struct zswap_pool {
char tfm_name[CRYPTO_MAX_ALG_NAME];
struct list_lru list_lru;
struct mem_cgroup *next_shrink;
+ struct shrinker *shrinker;
+ atomic_t nr_stored;
};
/*
@@ -275,17 +281,26 @@ static bool zswap_can_accept(void)
DIV_ROUND_UP(zswap_pool_total_size, PAGE_SIZE);
}
+static u64 get_zswap_pool_size(struct zswap_pool *pool)
+{
+ u64 pool_size = 0;
+ int i;
+
+ for (i = 0; i < ZSWAP_NR_ZPOOLS; i++)
+ pool_size += zpool_get_total_size(pool->zpools[i]);
+
+ return pool_size;
+}
+
static void zswap_update_total_size(void)
{
struct zswap_pool *pool;
u64 total = 0;
- int i;
rcu_read_lock();
list_for_each_entry_rcu(pool, &zswap_pools, list)
- for (i = 0; i < ZSWAP_NR_ZPOOLS; i++)
- total += zpool_get_total_size(pool->zpools[i]);
+ total += get_zswap_pool_size(pool);
rcu_read_unlock();
@@ -339,12 +354,33 @@ static void zswap_entry_cache_free(struc
}
/*********************************
+* zswap lruvec functions
+**********************************/
+void zswap_lruvec_state_init(struct lruvec *lruvec)
+{
+ atomic_long_set(&lruvec->zswap_lruvec_state.nr_zswap_protected, 0);
+}
+
+void zswap_lruvec_swapin(struct page *page)
+{
+ struct lruvec *lruvec;
+
+ if (page) {
+ lruvec = folio_lruvec(page_folio(page));
+ atomic_long_inc(&lruvec->zswap_lruvec_state.nr_zswap_protected);
+ }
+}
+
+/*********************************
* lru functions
**********************************/
static void zswap_lru_add(struct list_lru *list_lru, struct zswap_entry *entry)
{
+ atomic_long_t *nr_zswap_protected;
+ unsigned long lru_size, old, new;
int nid = entry_to_nid(entry);
struct mem_cgroup *memcg;
+ struct lruvec *lruvec;
/*
* Note that it is safe to use rcu_read_lock() here, even in the face of
@@ -362,6 +398,19 @@ static void zswap_lru_add(struct list_lr
memcg = mem_cgroup_from_entry(entry);
/* will always succeed */
list_lru_add(list_lru, &entry->lru, nid, memcg);
+
+ /* Update the protection area */
+ lru_size = list_lru_count_one(list_lru, nid, memcg);
+ lruvec = mem_cgroup_lruvec(memcg, NODE_DATA(nid));
+ nr_zswap_protected = &lruvec->zswap_lruvec_state.nr_zswap_protected;
+ old = atomic_long_inc_return(nr_zswap_protected);
+ /*
+ * Decay to avoid overflow and adapt to changing workloads.
+ * This is based on LRU reclaim cost decaying heuristics.
+ */
+ do {
+ new = old > lru_size / 4 ? old / 2 : old;
+ } while (!atomic_long_try_cmpxchg(nr_zswap_protected, &old, new));
rcu_read_unlock();
}
@@ -383,6 +432,7 @@ static void zswap_lru_putback(struct lis
int nid = entry_to_nid(entry);
spinlock_t *lock = &list_lru->node[nid].lock;
struct mem_cgroup *memcg;
+ struct lruvec *lruvec;
rcu_read_lock();
memcg = mem_cgroup_from_entry(entry);
@@ -390,6 +440,10 @@ static void zswap_lru_putback(struct lis
/* we cannot use list_lru_add here, because it increments node's lru count */
list_lru_putback(list_lru, &entry->lru, nid, memcg);
spin_unlock(lock);
+
+ lruvec = mem_cgroup_lruvec(memcg, NODE_DATA(entry_to_nid(entry)));
+ /* increment the protection area to account for the LRU rotation. */
+ atomic_long_inc(&lruvec->zswap_lruvec_state.nr_zswap_protected);
rcu_read_unlock();
}
@@ -479,6 +533,7 @@ static void zswap_free_entry(struct zswa
else {
zswap_lru_del(&entry->pool->list_lru, entry);
zpool_free(zswap_find_zpool(entry), entry->handle);
+ atomic_dec(&entry->pool->nr_stored);
zswap_pool_put(entry->pool);
}
zswap_entry_cache_free(entry);
@@ -521,6 +576,95 @@ static struct zswap_entry *zswap_entry_f
}
/*********************************
+* shrinker functions
+**********************************/
+static enum lru_status shrink_memcg_cb(struct list_head *item, struct list_lru_one *l,
+ spinlock_t *lock, void *arg);
+
+static unsigned long zswap_shrinker_scan(struct shrinker *shrinker,
+ struct shrink_control *sc)
+{
+ struct lruvec *lruvec = mem_cgroup_lruvec(sc->memcg, NODE_DATA(sc->nid));
+ unsigned long shrink_ret, nr_protected, lru_size;
+ struct zswap_pool *pool = shrinker->private_data;
+ bool encountered_page_in_swapcache = false;
+
+ nr_protected =
+ atomic_long_read(&lruvec->zswap_lruvec_state.nr_zswap_protected);
+ lru_size = list_lru_shrink_count(&pool->list_lru, sc);
+
+ /*
+ * Abort if the shrinker is disabled or if we are shrinking into the
+ * protected region.
+ */
+ if (!zswap_shrinker_enabled || nr_protected >= lru_size - sc->nr_to_scan) {
+ sc->nr_scanned = 0;
+ return SHRINK_STOP;
+ }
+
+ shrink_ret = list_lru_shrink_walk(&pool->list_lru, sc, &shrink_memcg_cb,
+ &encountered_page_in_swapcache);
+
+ if (encountered_page_in_swapcache)
+ return SHRINK_STOP;
+
+ return shrink_ret ? shrink_ret : SHRINK_STOP;
+}
+
+static unsigned long zswap_shrinker_count(struct shrinker *shrinker,
+ struct shrink_control *sc)
+{
+ struct zswap_pool *pool = shrinker->private_data;
+ struct mem_cgroup *memcg = sc->memcg;
+ struct lruvec *lruvec = mem_cgroup_lruvec(memcg, NODE_DATA(sc->nid));
+ unsigned long nr_backing, nr_stored, nr_freeable, nr_protected;
+
+#ifdef CONFIG_MEMCG_KMEM
+ cgroup_rstat_flush(memcg->css.cgroup);
+ nr_backing = memcg_page_state(memcg, MEMCG_ZSWAP_B) >> PAGE_SHIFT;
+ nr_stored = memcg_page_state(memcg, MEMCG_ZSWAPPED);
+#else
+ /* use pool stats instead of memcg stats */
+ nr_backing = get_zswap_pool_size(pool) >> PAGE_SHIFT;
+ nr_stored = atomic_read(&pool->nr_stored);
+#endif
+
+ if (!zswap_shrinker_enabled || !nr_stored)
+ return 0;
+
+ nr_protected =
+ atomic_long_read(&lruvec->zswap_lruvec_state.nr_zswap_protected);
+ nr_freeable = list_lru_shrink_count(&pool->list_lru, sc);
+ /*
+ * Subtract the lru size by an estimate of the number of pages
+ * that should be protected.
+ */
+ nr_freeable = nr_freeable > nr_protected ? nr_freeable - nr_protected : 0;
+
+ /*
+ * Scale the number of freeable pages by the memory saving factor.
+ * This ensures that the better zswap compresses memory, the fewer
+ * pages we will evict to swap (as it will otherwise incur IO for
+ * relatively small memory saving).
+ */
+ return mult_frac(nr_freeable, nr_backing, nr_stored);
+}
+
+static void zswap_alloc_shrinker(struct zswap_pool *pool)
+{
+ pool->shrinker =
+ shrinker_alloc(SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE, "mm-zswap");
+ if (!pool->shrinker)
+ return;
+
+ pool->shrinker->private_data = pool;
+ pool->shrinker->scan_objects = zswap_shrinker_scan;
+ pool->shrinker->count_objects = zswap_shrinker_count;
+ pool->shrinker->batch = 0;
+ pool->shrinker->seeks = DEFAULT_SEEKS;
+}
+
+/*********************************
* per-cpu code
**********************************/
static DEFINE_PER_CPU(u8 *, zswap_dstmem);
@@ -715,6 +859,7 @@ static enum lru_status shrink_memcg_cb(s
spinlock_t *lock, void *arg)
{
struct zswap_entry *entry = container_of(item, struct zswap_entry, lru);
+ bool *encountered_page_in_swapcache = (bool *)arg;
struct zswap_tree *tree;
pgoff_t swpoffset;
enum lru_status ret = LRU_REMOVED_RETRY;
@@ -750,6 +895,17 @@ static enum lru_status shrink_memcg_cb(s
zswap_reject_reclaim_fail++;
zswap_lru_putback(&entry->pool->list_lru, entry);
ret = LRU_RETRY;
+
+ /*
+ * Encountering a page already in swap cache is a sign that we are shrinking
+ * into the warmer region. We should terminate shrinking (if we're in the dynamic
+ * shrinker context).
+ */
+ if (writeback_result == -EEXIST && encountered_page_in_swapcache) {
+ ret = LRU_SKIP;
+ *encountered_page_in_swapcache = true;
+ }
+
goto put_unlock;
}
zswap_written_back_pages++;
@@ -889,6 +1045,11 @@ static struct zswap_pool *zswap_pool_cre
&pool->node);
if (ret)
goto error;
+
+ zswap_alloc_shrinker(pool);
+ if (!pool->shrinker)
+ goto error;
+
pr_debug("using %s compressor\n", pool->tfm_name);
/* being the current pool takes 1 ref; this func expects the
@@ -896,13 +1057,19 @@ static struct zswap_pool *zswap_pool_cre
*/
kref_init(&pool->kref);
INIT_LIST_HEAD(&pool->list);
- list_lru_init_memcg(&pool->list_lru, NULL);
+ if (list_lru_init_memcg(&pool->list_lru, pool->shrinker))
+ goto lru_fail;
+ shrinker_register(pool->shrinker);
INIT_WORK(&pool->shrink_work, shrink_worker);
+ atomic_set(&pool->nr_stored, 0);
zswap_pool_debug("created", pool);
return pool;
+lru_fail:
+ list_lru_destroy(&pool->list_lru);
+ shrinker_free(pool->shrinker);
error:
if (pool->acomp_ctx)
free_percpu(pool->acomp_ctx);
@@ -960,6 +1127,7 @@ static void zswap_pool_destroy(struct zs
zswap_pool_debug("destroying", pool);
+ shrinker_free(pool->shrinker);
cpuhp_state_remove_instance(CPUHP_MM_ZSWP_POOL_PREPARE, &pool->node);
free_percpu(pool->acomp_ctx);
list_lru_destroy(&pool->list_lru);
@@ -1511,6 +1679,7 @@ insert_entry:
if (entry->length) {
INIT_LIST_HEAD(&entry->lru);
zswap_lru_add(&entry->pool->list_lru, entry);
+ atomic_inc(&entry->pool->nr_stored);
}
spin_unlock(&tree->lock);
_
Patches currently in -mm which might be from nphamcs@xxxxxxxxx are
list_lru-allows-explicit-memcg-and-numa-node-selection.patch
memcontrol-allows-mem_cgroup_iter-to-check-for-onlineness.patch
zswap-shrinks-zswap-pool-based-on-memory-pressure.patch
