@@ -153,6 +153,18 @@ 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 by first switching on the global knob:
+
+ echo Y > /sys/module/zswap/par meters/shrinker_enabled
+
+When the kernel is compiled with CONFIG_MEMCG_KMEM, user needs to further turn
+it on for each cgroup that the shrinker should target:
+
+ echo 1 > /sys/fs/cgroup/<cgroup-name>/memory.zswap.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.
@@ -229,6 +229,7 @@ struct mem_cgroup {
#if defined(CONFIG_MEMCG_KMEM) && defined(CONFIG_ZSWAP)
unsigned long zswap_max;
+ atomic_t zswap_shrinker_enabled;
#endif
unsigned long soft_limit;
@@ -637,6 +637,20 @@ struct lruvec {
#ifdef CONFIG_MEMCG
struct pglist_data *pgdat;
#endif
+#ifdef CONFIG_ZSWAP
+ /*
+ * 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.
+ */
+ unsigned long nr_zswap_protected;
+#endif
};
/* Isolate unmapped pages */
@@ -5350,6 +5350,8 @@ mem_cgroup_css_alloc(struct cgroup_subsys_state *parent_css)
WRITE_ONCE(memcg->soft_limit, PAGE_COUNTER_MAX);
#if defined(CONFIG_MEMCG_KMEM) && defined(CONFIG_ZSWAP)
memcg->zswap_max = PAGE_COUNTER_MAX;
+ /* Disable the shrinker by default */
+ atomic_set(&memcg->zswap_shrinker_enabled, 0);
#endif
page_counter_set_high(&memcg->swap, PAGE_COUNTER_MAX);
if (parent) {
@@ -7867,6 +7869,31 @@ static ssize_t zswap_max_write(struct kernfs_open_file *of,
return nbytes;
}
+static int zswap_shrinker_enabled_show(struct seq_file *m, void *v)
+{
+ struct mem_cgroup *memcg = mem_cgroup_from_seq(m);
+
+ seq_printf(m, "%d\n", atomic_read(&memcg->zswap_shrinker_enabled));
+ return 0;
+}
+
+static ssize_t zswap_shrinker_enabled_write(struct kernfs_open_file *of,
+ char *buf, size_t nbytes, loff_t off)
+{
+ struct mem_cgroup *memcg = mem_cgroup_from_css(of_css(of));
+ int zswap_shrinker_enabled;
+ ssize_t parse_ret = kstrtoint(strstrip(buf), 0, &zswap_shrinker_enabled);
+
+ if (parse_ret)
+ return parse_ret;
+
+ if (zswap_shrinker_enabled < 0 || zswap_shrinker_enabled > 1)
+ return -ERANGE;
+
+ atomic_set(&memcg->zswap_shrinker_enabled, zswap_shrinker_enabled);
+ return nbytes;
+}
+
static struct cftype zswap_files[] = {
{
.name = "zswap.current",
@@ -7879,6 +7906,12 @@ static struct cftype zswap_files[] = {
.seq_show = zswap_max_show,
.write = zswap_max_write,
},
+ {
+ .name = "zswap.shrinker.enabled",
+ .flags = CFTYPE_NOT_ON_ROOT,
+ .seq_show = zswap_shrinker_enabled_show,
+ .write = zswap_shrinker_enabled_write,
+ },
{ } /* terminate */
};
#endif /* CONFIG_MEMCG_KMEM && CONFIG_ZSWAP */
@@ -618,6 +618,22 @@ static unsigned long swapin_nr_pages(unsigned long offset)
return pages;
}
+#ifdef CONFIG_ZSWAP
+/*
+ * Refault is an indication that warmer pages are not resident in memory.
+ * Increase the size of zswap's protected area.
+ */
+static void inc_nr_protected(struct page *page)
+{
+ struct lruvec *lruvec = folio_lruvec(page_folio(page));
+ unsigned long flags;
+
+ spin_lock_irqsave(&lruvec->lru_lock, flags);
+ lruvec->nr_zswap_protected++;
+ spin_unlock_irqrestore(&lruvec->lru_lock, flags);
+}
+#endif
+
/**
* swap_cluster_readahead - swap in pages in hope we need them soon
* @entry: swap entry of this memory
@@ -686,7 +702,12 @@ struct page *swap_cluster_readahead(swp_entry_t entry, gfp_t gfp_mask,
lru_add_drain(); /* Push any new pages onto the LRU now */
skip:
/* The page was likely read above, so no need for plugging here */
- return read_swap_cache_async(entry, gfp_mask, vma, addr, NULL);
+ page = read_swap_cache_async(entry, gfp_mask, vma, addr, NULL);
+#ifdef CONFIG_ZSWAP
+ if (page)
+ inc_nr_protected(page);
+#endif
+ return page;
}
int init_swap_address_space(unsigned int type, unsigned long nr_pages)
@@ -853,8 +874,12 @@ static struct page *swap_vma_readahead(swp_entry_t fentry, gfp_t gfp_mask,
lru_add_drain();
skip:
/* The page was likely read above, so no need for plugging here */
- return read_swap_cache_async(fentry, gfp_mask, vma, vmf->address,
- NULL);
+ page = read_swap_cache_async(fentry, gfp_mask, vma, vmf->address, NULL);
+#ifdef CONFIG_ZSWAP
+ if (page)
+ inc_nr_protected(page);
+#endif
+ return page;
}
/**
@@ -145,6 +145,26 @@ module_param_named(exclusive_loads, zswap_exclusive_loads_enabled, bool, 0644);
/* Number of zpools in zswap_pool (empirically determined for scalability) */
#define ZSWAP_NR_ZPOOLS 32
+/*
+ * Global flag to enable/disable memory pressure-based shrinker for all memcgs.
+ * If CONFIG_MEMCG_KMEM is on, we can further selectively disable
+ * the shrinker for each memcg.
+ */
+static bool zswap_shrinker_enabled;
+module_param_named(shrinker_enabled, zswap_shrinker_enabled, bool, 0644);
+#ifdef CONFIG_MEMCG_KMEM
+static bool is_shrinker_enabled(struct mem_cgroup *memcg)
+{
+ return zswap_shrinker_enabled &&
+ atomic_read(&memcg->zswap_shrinker_enabled);
+}
+#else
+static bool is_shrinker_enabled(struct mem_cgroup *memcg)
+{
+ return zswap_shrinker_enabled;
+}
+#endif
+
/*********************************
* data structures
**********************************/
@@ -174,6 +194,7 @@ struct zswap_pool {
char tfm_name[CRYPTO_MAX_ALG_NAME];
struct list_lru list_lru;
struct mem_cgroup *next_shrink;
+ struct shrinker shrinker;
};
/*
@@ -318,8 +339,23 @@ static bool zswap_lru_add(struct list_lru *list_lru, struct zswap_entry *entry)
{
struct mem_cgroup *memcg = entry->objcg ?
get_mem_cgroup_from_objcg(entry->objcg) : NULL;
+ struct lruvec *lruvec = mem_cgroup_lruvec(memcg, NODE_DATA(entry->nid));
bool added = __list_lru_add(list_lru, &entry->lru, entry->nid, memcg);
+ unsigned long flags, lru_size;
+ if (added) {
+ lru_size = list_lru_count_one(list_lru, entry->nid, memcg);
+ spin_lock_irqsave(&lruvec->lru_lock, flags);
+ lruvec->nr_zswap_protected++;
+
+ /*
+ * Decay to avoid overflow and adapt to changing workloads.
+ * This is based on LRU reclaim cost decaying heuristics.
+ */
+ if (lruvec->nr_zswap_protected > lru_size / 4)
+ lruvec->nr_zswap_protected /= 2;
+ spin_unlock_irqrestore(&lruvec->lru_lock, flags);
+ }
mem_cgroup_put(memcg);
return added;
}
@@ -461,6 +497,89 @@ static struct zswap_entry *zswap_entry_find_get(struct rb_root *root,
return entry;
}
+/*********************************
+* 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 zswap_pool *pool = container_of(shrinker, typeof(*pool), shrinker);
+ unsigned long shrink_ret, nr_zswap_protected, flags,
+ lru_size = list_lru_shrink_count(&pool->list_lru, sc);
+ struct lruvec *lruvec = mem_cgroup_lruvec(sc->memcg, NODE_DATA(sc->nid));
+ bool encountered_page_in_swapcache = false;
+
+ spin_lock_irqsave(&lruvec->lru_lock, flags);
+ nr_zswap_protected = lruvec->nr_zswap_protected;
+ spin_unlock_irqrestore(&lruvec->lru_lock, flags);
+
+ /*
+ * Abort if the shrinker is disabled or if we are shrinking into the
+ * protected region.
+ */
+ if (!is_shrinker_enabled(sc->memcg) ||
+ nr_zswap_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 = container_of(shrinker, typeof(*pool), shrinker);
+ 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, flags;
+
+ 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);
+
+ if (!is_shrinker_enabled(memcg) || !nr_stored)
+ return 0;
+
+ 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.
+ */
+ spin_lock_irqsave(&lruvec->lru_lock, flags);
+ nr_freeable = nr_freeable > lruvec->nr_zswap_protected ?
+ nr_freeable - lruvec->nr_zswap_protected : 0;
+ spin_unlock_irqrestore(&lruvec->lru_lock, flags);
+
+ /*
+ * 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 int zswap_prealloc_shrinker(struct zswap_pool *pool)
+{
+ pool->shrinker.scan_objects = zswap_shrinker_scan;
+ pool->shrinker.count_objects = zswap_shrinker_count;
+ pool->shrinker.batch = 0;
+ pool->shrinker.seeks = DEFAULT_SEEKS;
+ pool->shrinker.flags = SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE;
+
+ return prealloc_shrinker(&pool->shrinker, "mm-zswap");
+}
+
/*********************************
* per-cpu code
**********************************/
@@ -656,11 +775,14 @@ static enum lru_status shrink_memcg_cb(struct list_head *item, struct list_lru_o
spinlock_t *lock, void *arg)
{
struct zswap_entry *entry = container_of(item, struct zswap_entry, lru);
+ bool *encountered_page_in_swapcache = (bool *)arg;
struct mem_cgroup *memcg;
struct zswap_tree *tree;
+ struct lruvec *lruvec;
pgoff_t swpoffset;
enum lru_status ret = LRU_REMOVED_RETRY;
int writeback_result;
+ unsigned long flags;
/*
* Once the lru lock is dropped, the entry might get freed. The
@@ -696,8 +818,24 @@ static enum lru_status shrink_memcg_cb(struct list_head *item, struct list_lru_o
/* we cannot use zswap_lru_add here, because it increments node's lru count */
list_lru_putback(&entry->pool->list_lru, item, entry->nid, memcg);
spin_unlock(lock);
- mem_cgroup_put(memcg);
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;
+ }
+ lruvec = mem_cgroup_lruvec(memcg, NODE_DATA(entry->nid));
+ spin_lock_irqsave(&lruvec->lru_lock, flags);
+ /* Increment the protection area to account for the LRU rotation. */
+ lruvec->nr_zswap_protected++;
+ spin_unlock_irqrestore(&lruvec->lru_lock, flags);
+
+ mem_cgroup_put(memcg);
goto put_unlock;
}
@@ -828,6 +966,10 @@ static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
&pool->node);
if (ret)
goto error;
+
+ if (zswap_prealloc_shrinker(pool))
+ goto error;
+
pr_debug("using %s compressor\n", pool->tfm_name);
/* being the current pool takes 1 ref; this func expects the
@@ -836,12 +978,17 @@ static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
kref_init(&pool->kref);
INIT_LIST_HEAD(&pool->list);
INIT_WORK(&pool->shrink_work, shrink_worker);
- list_lru_init_memcg(&pool->list_lru, NULL);
+ if (list_lru_init_memcg(&pool->list_lru, &pool->shrinker))
+ goto lru_fail;
+ register_shrinker_prepared(&pool->shrinker);
zswap_pool_debug("created", pool);
return pool;
+lru_fail:
+ list_lru_destroy(&pool->list_lru);
+ free_prealloced_shrinker(&pool->shrinker);
error:
if (pool->acomp_ctx)
free_percpu(pool->acomp_ctx);
@@ -899,6 +1046,7 @@ static void zswap_pool_destroy(struct zswap_pool *pool)
zswap_pool_debug("destroying", pool);
+ unregister_shrinker(&pool->shrinker);
cpuhp_state_remove_instance(CPUHP_MM_ZSWP_POOL_PREPARE, &pool->node);
free_percpu(pool->acomp_ctx);
list_lru_destroy(&pool->list_lru);
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. On a benchmark that we have run: (without the shrinker) real -- mean: 153.27s, median: 153.199s sys -- mean: 541.652s, median: 541.903s user -- mean: 4384.9673999999995s, median: 4385.471s (with the shrinker) real -- mean: 151.4956s, median: 151.456s sys -- mean: 461.14639999999997s, median: 465.656s user -- mean: 4384.7118s, median: 4384.675s We observed a 14-15% reduction in kernel CPU time, which translated to over 1% reduction in real time. On another benchmark, where there was a lot more cold memory residing in zswap, we observed even more pronounced gains: (without the shrinker) real -- mean: 157.52519999999998s, median: 157.281s sys -- mean: 769.3082s, median: 780.545s user -- mean: 4378.1622s, median: 4378.286s (with the shrinker) real -- mean: 152.9608s, median: 152.845s sys -- mean: 517.4446s, median: 506.749s user -- mean: 4387.694s, median: 4387.935s Here, we saw around 32-35% reduction in kernel CPU time, which translated to 2.8% reduction in real time. These results confirm our hypothesis that the shrinker is more helpful the more cold memory we have. Suggested-by: Johannes Weiner <hannes@cmpxchg.org> Signed-off-by: Nhat Pham <nphamcs@gmail.com> --- Documentation/admin-guide/mm/zswap.rst | 12 ++ include/linux/memcontrol.h | 1 + include/linux/mmzone.h | 14 +++ mm/memcontrol.c | 33 ++++++ mm/swap_state.c | 31 ++++- mm/zswap.c | 152 ++++++++++++++++++++++++- 6 files changed, 238 insertions(+), 5 deletions(-)