[PATCH 0/2] mm/zswap: optimize for dynamic zswap_pools

[PATCH 0/2] mm/zswap: optimize for dynamic zswap_pools

[Chengming Zhou]

Dynamic pool creation has been supported for a long time, which maybe
not used so much in practice. But with the per-memcg lru merged, the
current structure of zswap_pool's lru and shrinker become less optimal.

In the current structure, each zswap_pool has its own lru, shrinker and
shrink_work, but only the latest zswap_pool will be the current used.

1. When memory has pressure, all shrinkers of zswap_pools will try to
   shrink its lru list, there is no order between them.

2. When zswap limit hit, only the last zswap_pool's shrink_work will
   try to shrink its own lru, which is inefficient.

A more natural way is to have a global zswap lru shared between all
zswap_pools, and so is the shrinker. The code becomes much simpler too.

Another optimization is changing zswap_pool kref to percpu_ref, which
will be taken reference by every zswap entry. So the scalability is
better.

Testing kernel build in tmpfs with memory.max=2GB (zswap shrinker and
writeback enabled with one 50GB swapfile).

        mm-unstable  zswap-global-lru
real    63.20        63.12
user    1061.75      1062.95
sys     268.74       264.44

Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
---
Chengming Zhou (2):
      mm/zswap: global lru and shrinker shared by all zswap_pools
      mm/zswap: change zswap_pool kref to percpu_ref

 mm/zswap.c | 183 +++++++++++++++++++++++++------------------------------------
 1 file changed, 76 insertions(+), 107 deletions(-)
---
base-commit: 191d97734e41a5c9f90a2f6636fdd335ae1d435d
change-id: 20240210-zswap-global-lru-94d49316178b

Best regards,
-- 
Chengming Zhou <zhouchengming@bytedance.com>

[PATCH 1/2] mm/zswap: global lru and shrinker shared by all zswap_pools

[Chengming Zhou]

Dynamic zswap_pool creation may create/reuse to have multiple
zswap_pools in a list, only the first will be current used.

Each zswap_pool has its own lru and shrinker, which is not
necessary and has its problem:

1. When memory has pressure, all shrinker of zswap_pools will
   try to shrink its own lru, there is no order between them.

2. When zswap limit hit, only the last zswap_pool's shrink_work
   will try to shrink its lru, which is inefficient.

Anyway, having a global lru and shrinker shared by all zswap_pools
is better and efficient.

Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
---
 mm/zswap.c | 153 ++++++++++++++++++++++---------------------------------------
 1 file changed, 55 insertions(+), 98 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index 62fe307521c9..7668db8c10e3 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -176,14 +176,17 @@ struct zswap_pool {
 	struct kref kref;
 	struct list_head list;
 	struct work_struct release_work;
-	struct work_struct shrink_work;
 	struct hlist_node node;
 	char tfm_name[CRYPTO_MAX_ALG_NAME];
+};
+
+struct {
 	struct list_lru list_lru;
-	struct mem_cgroup *next_shrink;
-	struct shrinker *shrinker;
 	atomic_t nr_stored;
-};
+	struct shrinker *shrinker;
+	struct work_struct shrink_work;
+	struct mem_cgroup *next_shrink;
+} zswap;
 
 /*
  * struct zswap_entry
@@ -301,9 +304,6 @@ static void zswap_update_total_size(void)
 * pool functions
 **********************************/
 
-static void zswap_alloc_shrinker(struct zswap_pool *pool);
-static void shrink_worker(struct work_struct *w);
-
 static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
 {
 	int i;
@@ -353,30 +353,16 @@ static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
 	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
 	 * caller to always add the new pool as the current pool
 	 */
 	kref_init(&pool->kref);
 	INIT_LIST_HEAD(&pool->list);
-	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);
@@ -434,15 +420,8 @@ static void zswap_pool_destroy(struct zswap_pool *pool)
 
 	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);
-
-	spin_lock(&zswap_pools_lock);
-	mem_cgroup_iter_break(NULL, pool->next_shrink);
-	pool->next_shrink = NULL;
-	spin_unlock(&zswap_pools_lock);
 
 	for (i = 0; i < ZSWAP_NR_ZPOOLS; i++)
 		zpool_destroy_pool(pool->zpools[i]);
@@ -529,24 +508,6 @@ static struct zswap_pool *zswap_pool_current_get(void)
 	return pool;
 }
 
-static struct zswap_pool *zswap_pool_last_get(void)
-{
-	struct zswap_pool *pool, *last = NULL;
-
-	rcu_read_lock();
-
-	list_for_each_entry_rcu(pool, &zswap_pools, list)
-		last = pool;
-	WARN_ONCE(!last && zswap_has_pool,
-		  "%s: no page storage pool!\n", __func__);
-	if (!zswap_pool_get(last))
-		last = NULL;
-
-	rcu_read_unlock();
-
-	return last;
-}
-
 /* type and compressor must be null-terminated */
 static struct zswap_pool *zswap_pool_find_get(char *type, char *compressor)
 {
@@ -816,14 +777,10 @@ void zswap_folio_swapin(struct folio *folio)
 
 void zswap_memcg_offline_cleanup(struct mem_cgroup *memcg)
 {
-	struct zswap_pool *pool;
-
-	/* lock out zswap pools list modification */
+	/* lock out zswap shrinker walking memcg tree */
 	spin_lock(&zswap_pools_lock);
-	list_for_each_entry(pool, &zswap_pools, list) {
-		if (pool->next_shrink == memcg)
-			pool->next_shrink = mem_cgroup_iter(NULL, pool->next_shrink, NULL);
-	}
+	if (zswap.next_shrink == memcg)
+		zswap.next_shrink = mem_cgroup_iter(NULL, zswap.next_shrink, NULL);
 	spin_unlock(&zswap_pools_lock);
 }
 
@@ -923,9 +880,9 @@ static void zswap_entry_free(struct zswap_entry *entry)
 	if (!entry->length)
 		atomic_dec(&zswap_same_filled_pages);
 	else {
-		zswap_lru_del(&entry->pool->list_lru, entry);
+		zswap_lru_del(&zswap.list_lru, entry);
 		zpool_free(zswap_find_zpool(entry), entry->handle);
-		atomic_dec(&entry->pool->nr_stored);
+		atomic_dec(&zswap.nr_stored);
 		zswap_pool_put(entry->pool);
 	}
 	if (entry->objcg) {
@@ -1288,7 +1245,6 @@ static unsigned long zswap_shrinker_scan(struct shrinker *shrinker,
 {
 	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;
 
 	if (!zswap_shrinker_enabled ||
@@ -1299,7 +1255,7 @@ static unsigned long zswap_shrinker_scan(struct shrinker *shrinker,
 
 	nr_protected =
 		atomic_long_read(&lruvec->zswap_lruvec_state.nr_zswap_protected);
-	lru_size = list_lru_shrink_count(&pool->list_lru, sc);
+	lru_size = list_lru_shrink_count(&zswap.list_lru, sc);
 
 	/*
 	 * Abort if we are shrinking into the protected region.
@@ -1316,7 +1272,7 @@ static unsigned long zswap_shrinker_scan(struct shrinker *shrinker,
 		return SHRINK_STOP;
 	}
 
-	shrink_ret = list_lru_shrink_walk(&pool->list_lru, sc, &shrink_memcg_cb,
+	shrink_ret = list_lru_shrink_walk(&zswap.list_lru, sc, &shrink_memcg_cb,
 		&encountered_page_in_swapcache);
 
 	if (encountered_page_in_swapcache)
@@ -1328,7 +1284,6 @@ static unsigned long zswap_shrinker_scan(struct shrinker *shrinker,
 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;
@@ -1343,7 +1298,7 @@ static unsigned long zswap_shrinker_count(struct shrinker *shrinker,
 #else
 	/* use pool stats instead of memcg stats */
 	nr_backing = get_zswap_pool_size(pool) >> PAGE_SHIFT;
-	nr_stored = atomic_read(&pool->nr_stored);
+	nr_stored = atomic_read(&zswap.nr_stored);
 #endif
 
 	if (!nr_stored)
@@ -1351,7 +1306,7 @@ static unsigned long zswap_shrinker_count(struct shrinker *shrinker,
 
 	nr_protected =
 		atomic_long_read(&lruvec->zswap_lruvec_state.nr_zswap_protected);
-	nr_freeable = list_lru_shrink_count(&pool->list_lru, sc);
+	nr_freeable = list_lru_shrink_count(&zswap.list_lru, sc);
 	/*
 	 * Subtract the lru size by an estimate of the number of pages
 	 * that should be protected.
@@ -1367,23 +1322,24 @@ static unsigned long zswap_shrinker_count(struct shrinker *shrinker,
 	return mult_frac(nr_freeable, nr_backing, nr_stored);
 }
 
-static void zswap_alloc_shrinker(struct zswap_pool *pool)
+static struct shrinker *zswap_alloc_shrinker(void)
 {
-	pool->shrinker =
+	struct shrinker *shrinker;
+
+	shrinker =
 		shrinker_alloc(SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE, "mm-zswap");
-	if (!pool->shrinker)
-		return;
+	if (!shrinker)
+		return NULL;
 
-	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;
+	shrinker->scan_objects = zswap_shrinker_scan;
+	shrinker->count_objects = zswap_shrinker_count;
+	shrinker->batch = 0;
+	shrinker->seeks = DEFAULT_SEEKS;
+	return shrinker;
 }
 
 static int shrink_memcg(struct mem_cgroup *memcg)
 {
-	struct zswap_pool *pool;
 	int nid, shrunk = 0;
 
 	if (!mem_cgroup_zswap_writeback_enabled(memcg))
@@ -1396,32 +1352,25 @@ static int shrink_memcg(struct mem_cgroup *memcg)
 	if (memcg && !mem_cgroup_online(memcg))
 		return -ENOENT;
 
-	pool = zswap_pool_current_get();
-	if (!pool)
-		return -EINVAL;
-
 	for_each_node_state(nid, N_NORMAL_MEMORY) {
 		unsigned long nr_to_walk = 1;
 
-		shrunk += list_lru_walk_one(&pool->list_lru, nid, memcg,
+		shrunk += list_lru_walk_one(&zswap.list_lru, nid, memcg,
 					    &shrink_memcg_cb, NULL, &nr_to_walk);
 	}
-	zswap_pool_put(pool);
 	return shrunk ? 0 : -EAGAIN;
 }
 
 static void shrink_worker(struct work_struct *w)
 {
-	struct zswap_pool *pool = container_of(w, typeof(*pool),
-						shrink_work);
 	struct mem_cgroup *memcg;
 	int ret, failures = 0;
 
 	/* global reclaim will select cgroup in a round-robin fashion. */
 	do {
 		spin_lock(&zswap_pools_lock);
-		pool->next_shrink = mem_cgroup_iter(NULL, pool->next_shrink, NULL);
-		memcg = pool->next_shrink;
+		zswap.next_shrink = mem_cgroup_iter(NULL, zswap.next_shrink, NULL);
+		memcg = zswap.next_shrink;
 
 		/*
 		 * We need to retry if we have gone through a full round trip, or if we
@@ -1445,7 +1394,7 @@ static void shrink_worker(struct work_struct *w)
 		if (!mem_cgroup_tryget_online(memcg)) {
 			/* drop the reference from mem_cgroup_iter() */
 			mem_cgroup_iter_break(NULL, memcg);
-			pool->next_shrink = NULL;
+			zswap.next_shrink = NULL;
 			spin_unlock(&zswap_pools_lock);
 
 			if (++failures == MAX_RECLAIM_RETRIES)
@@ -1467,7 +1416,6 @@ static void shrink_worker(struct work_struct *w)
 resched:
 		cond_resched();
 	} while (!zswap_can_accept());
-	zswap_pool_put(pool);
 }
 
 static int zswap_is_page_same_filled(void *ptr, unsigned long *value)
@@ -1508,7 +1456,6 @@ bool zswap_store(struct folio *folio)
 	struct zswap_entry *entry, *dupentry;
 	struct obj_cgroup *objcg = NULL;
 	struct mem_cgroup *memcg = NULL;
-	struct zswap_pool *shrink_pool;
 
 	VM_WARN_ON_ONCE(!folio_test_locked(folio));
 	VM_WARN_ON_ONCE(!folio_test_swapcache(folio));
@@ -1576,7 +1523,7 @@ bool zswap_store(struct folio *folio)
 
 	if (objcg) {
 		memcg = get_mem_cgroup_from_objcg(objcg);
-		if (memcg_list_lru_alloc(memcg, &entry->pool->list_lru, GFP_KERNEL)) {
+		if (memcg_list_lru_alloc(memcg, &zswap.list_lru, GFP_KERNEL)) {
 			mem_cgroup_put(memcg);
 			goto put_pool;
 		}
@@ -1607,8 +1554,8 @@ bool zswap_store(struct folio *folio)
 	}
 	if (entry->length) {
 		INIT_LIST_HEAD(&entry->lru);
-		zswap_lru_add(&entry->pool->list_lru, entry);
-		atomic_inc(&entry->pool->nr_stored);
+		zswap_lru_add(&zswap.list_lru, entry);
+		atomic_inc(&zswap.nr_stored);
 	}
 	spin_unlock(&tree->lock);
 
@@ -1640,9 +1587,7 @@ bool zswap_store(struct folio *folio)
 	return false;
 
 shrink:
-	shrink_pool = zswap_pool_last_get();
-	if (shrink_pool && !queue_work(shrink_wq, &shrink_pool->shrink_work))
-		zswap_pool_put(shrink_pool);
+	queue_work(shrink_wq, &zswap.shrink_work);
 	goto reject;
 }
 
@@ -1804,6 +1749,21 @@ static int zswap_setup(void)
 	if (ret)
 		goto hp_fail;
 
+	shrink_wq = alloc_workqueue("zswap-shrink",
+			WQ_UNBOUND|WQ_MEM_RECLAIM, 1);
+	if (!shrink_wq)
+		goto hp_fail;
+
+	zswap.shrinker = zswap_alloc_shrinker();
+	if (!zswap.shrinker)
+		goto shrinker_fail;
+	if (list_lru_init_memcg(&zswap.list_lru, zswap.shrinker))
+		goto lru_fail;
+	shrinker_register(zswap.shrinker);
+
+	INIT_WORK(&zswap.shrink_work, shrink_worker);
+	atomic_set(&zswap.nr_stored, 0);
+
 	pool = __zswap_pool_create_fallback();
 	if (pool) {
 		pr_info("loaded using pool %s/%s\n", pool->tfm_name,
@@ -1815,19 +1775,16 @@ static int zswap_setup(void)
 		zswap_enabled = false;
 	}
 
-	shrink_wq = alloc_workqueue("zswap-shrink",
-			WQ_UNBOUND|WQ_MEM_RECLAIM, 1);
-	if (!shrink_wq)
-		goto fallback_fail;
-
 	if (zswap_debugfs_init())
 		pr_warn("debugfs initialization failed\n");
 	zswap_init_state = ZSWAP_INIT_SUCCEED;
 	return 0;
 
-fallback_fail:
-	if (pool)
-		zswap_pool_destroy(pool);
+lru_fail:
+	list_lru_destroy(&zswap.list_lru);
+	shrinker_free(zswap.shrinker);
+shrinker_fail:
+	destroy_workqueue(shrink_wq);
 hp_fail:
 	kmem_cache_destroy(zswap_entry_cache);
 cache_fail:

-- 
b4 0.10.1

[PATCH 2/2] mm/zswap: change zswap_pool kref to percpu_ref

[Chengming Zhou]

All zswap entries will take a reference of zswap_pool when
zswap_store(), and drop it when free. Change it to use the
percpu_ref is better for scalability performance.

Testing kernel build in tmpfs with memory.max=2GB
(zswap shrinker and writeback enabled with one 50GB swapfile).

        mm-unstable  zswap-global-lru
real    63.20        63.12
user    1061.75      1062.95
sys     268.74       264.44

Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
---
 mm/zswap.c | 30 +++++++++++++++++++++---------
 1 file changed, 21 insertions(+), 9 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index 7668db8c10e3..afb31904fb08 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -173,7 +173,7 @@ struct crypto_acomp_ctx {
 struct zswap_pool {
 	struct zpool *zpools[ZSWAP_NR_ZPOOLS];
 	struct crypto_acomp_ctx __percpu *acomp_ctx;
-	struct kref kref;
+	struct percpu_ref ref;
 	struct list_head list;
 	struct work_struct release_work;
 	struct hlist_node node;
@@ -303,6 +303,7 @@ static void zswap_update_total_size(void)
 /*********************************
 * pool functions
 **********************************/
+static void __zswap_pool_empty(struct percpu_ref *ref);
 
 static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
 {
@@ -356,13 +357,18 @@ static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
 	/* being the current pool takes 1 ref; this func expects the
 	 * caller to always add the new pool as the current pool
 	 */
-	kref_init(&pool->kref);
+	ret = percpu_ref_init(&pool->ref, __zswap_pool_empty,
+			      PERCPU_REF_ALLOW_REINIT, GFP_KERNEL);
+	if (ret)
+		goto ref_fail;
 	INIT_LIST_HEAD(&pool->list);
 
 	zswap_pool_debug("created", pool);
 
 	return pool;
 
+ref_fail:
+	cpuhp_state_remove_instance(CPUHP_MM_ZSWP_POOL_PREPARE, &pool->node);
 error:
 	if (pool->acomp_ctx)
 		free_percpu(pool->acomp_ctx);
@@ -435,8 +441,8 @@ static void __zswap_pool_release(struct work_struct *work)
 
 	synchronize_rcu();
 
-	/* nobody should have been able to get a kref... */
-	WARN_ON(kref_get_unless_zero(&pool->kref));
+	/* nobody should have been able to get a ref... */
+	percpu_ref_exit(&pool->ref);
 
 	/* pool is now off zswap_pools list and has no references. */
 	zswap_pool_destroy(pool);
@@ -444,11 +450,11 @@ static void __zswap_pool_release(struct work_struct *work)
 
 static struct zswap_pool *zswap_pool_current(void);
 
-static void __zswap_pool_empty(struct kref *kref)
+static void __zswap_pool_empty(struct percpu_ref *ref)
 {
 	struct zswap_pool *pool;
 
-	pool = container_of(kref, typeof(*pool), kref);
+	pool = container_of(ref, typeof(*pool), ref);
 
 	spin_lock(&zswap_pools_lock);
 
@@ -467,12 +473,12 @@ static int __must_check zswap_pool_get(struct zswap_pool *pool)
 	if (!pool)
 		return 0;
 
-	return kref_get_unless_zero(&pool->kref);
+	return percpu_ref_tryget(&pool->ref);
 }
 
 static void zswap_pool_put(struct zswap_pool *pool)
 {
-	kref_put(&pool->kref, __zswap_pool_empty);
+	percpu_ref_put(&pool->ref);
 }
 
 static struct zswap_pool *__zswap_pool_current(void)
@@ -602,6 +608,12 @@ static int __zswap_param_set(const char *val, const struct kernel_param *kp,
 
 	if (!pool)
 		pool = zswap_pool_create(type, compressor);
+	else {
+		/* Resurrect percpu_ref to percpu mode. */
+		percpu_ref_resurrect(&pool->ref);
+		/* Drop the ref from zswap_pool_find_get(). */
+		zswap_pool_put(pool);
+	}
 
 	if (pool)
 		ret = param_set_charp(s, kp);
@@ -640,7 +652,7 @@ static int __zswap_param_set(const char *val, const struct kernel_param *kp,
 	 * or the new pool we failed to add
 	 */
 	if (put_pool)
-		zswap_pool_put(put_pool);
+		percpu_ref_kill(&put_pool->ref);
 
 	return ret;
 }

-- 
b4 0.10.1

Re: [PATCH 1/2] mm/zswap: global lru and shrinker shared by all zswap_pools

[kernel test robot]

Hi Chengming,

kernel test robot noticed the following build errors:

[auto build test ERROR on 191d97734e41a5c9f90a2f6636fdd335ae1d435d]

url:    https://github.com/intel-lab-lkp/linux/commits/Chengming-Zhou/mm-zswap-global-lru-and-shrinker-shared-by-all-zswap_pools/20240211-220028
base:   191d97734e41a5c9f90a2f6636fdd335ae1d435d
patch link:    https://lore.kernel.org/r/20240210-zswap-global-lru-v1-1-853473d7b0da%40bytedance.com
patch subject: [PATCH 1/2] mm/zswap: global lru and shrinker shared by all zswap_pools
config: x86_64-randconfig-013-20240211 (https://download.01.org/0day-ci/archive/20240212/202402120226.TK7G37U9-lkp@intel.com/config)
compiler: gcc-12 (Debian 12.2.0-14) 12.2.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20240212/202402120226.TK7G37U9-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202402120226.TK7G37U9-lkp@intel.com/

All errors (new ones prefixed by >>):

   mm/zswap.c: In function 'zswap_shrinker_count':
>> mm/zswap.c:1300:42: error: 'pool' undeclared (first use in this function); did you mean 'zpool'?
    1300 |         nr_backing = get_zswap_pool_size(pool) >> PAGE_SHIFT;
         |                                          ^~~~
         |                                          zpool
   mm/zswap.c:1300:42: note: each undeclared identifier is reported only once for each function it appears in

Kconfig warnings: (for reference only)
   WARNING: unmet direct dependencies detected for DRM_I915_DEBUG_GEM
   Depends on [n]: HAS_IOMEM [=y] && DRM_I915 [=y] && EXPERT [=y] && DRM_I915_WERROR [=n]
   Selected by [y]:
   - DRM_I915_DEBUG [=y] && HAS_IOMEM [=y] && DRM_I915 [=y] && EXPERT [=y] && !COMPILE_TEST [=n]


vim +1300 mm/zswap.c

b5ba474f3f5187 Nhat Pham      2023-11-30  1283  
b5ba474f3f5187 Nhat Pham      2023-11-30  1284  static unsigned long zswap_shrinker_count(struct shrinker *shrinker,
b5ba474f3f5187 Nhat Pham      2023-11-30  1285  		struct shrink_control *sc)
b5ba474f3f5187 Nhat Pham      2023-11-30  1286  {
b5ba474f3f5187 Nhat Pham      2023-11-30  1287  	struct mem_cgroup *memcg = sc->memcg;
b5ba474f3f5187 Nhat Pham      2023-11-30  1288  	struct lruvec *lruvec = mem_cgroup_lruvec(memcg, NODE_DATA(sc->nid));
b5ba474f3f5187 Nhat Pham      2023-11-30  1289  	unsigned long nr_backing, nr_stored, nr_freeable, nr_protected;
b5ba474f3f5187 Nhat Pham      2023-11-30  1290  
501a06fe8e4c18 Nhat Pham      2023-12-07  1291  	if (!zswap_shrinker_enabled || !mem_cgroup_zswap_writeback_enabled(memcg))
b5ba474f3f5187 Nhat Pham      2023-11-30  1292  		return 0;
b5ba474f3f5187 Nhat Pham      2023-11-30  1293  
b5ba474f3f5187 Nhat Pham      2023-11-30  1294  #ifdef CONFIG_MEMCG_KMEM
7d7ef0a4686abe Yosry Ahmed    2023-11-29  1295  	mem_cgroup_flush_stats(memcg);
b5ba474f3f5187 Nhat Pham      2023-11-30  1296  	nr_backing = memcg_page_state(memcg, MEMCG_ZSWAP_B) >> PAGE_SHIFT;
b5ba474f3f5187 Nhat Pham      2023-11-30  1297  	nr_stored = memcg_page_state(memcg, MEMCG_ZSWAPPED);
b5ba474f3f5187 Nhat Pham      2023-11-30  1298  #else
b5ba474f3f5187 Nhat Pham      2023-11-30  1299  	/* use pool stats instead of memcg stats */
b5ba474f3f5187 Nhat Pham      2023-11-30 @1300  	nr_backing = get_zswap_pool_size(pool) >> PAGE_SHIFT;
bf414d6ae81ba2 Chengming Zhou 2024-02-11  1301  	nr_stored = atomic_read(&zswap.nr_stored);
b5ba474f3f5187 Nhat Pham      2023-11-30  1302  #endif
b5ba474f3f5187 Nhat Pham      2023-11-30  1303  
b5ba474f3f5187 Nhat Pham      2023-11-30  1304  	if (!nr_stored)
b5ba474f3f5187 Nhat Pham      2023-11-30  1305  		return 0;
b5ba474f3f5187 Nhat Pham      2023-11-30  1306  
b5ba474f3f5187 Nhat Pham      2023-11-30  1307  	nr_protected =
b5ba474f3f5187 Nhat Pham      2023-11-30  1308  		atomic_long_read(&lruvec->zswap_lruvec_state.nr_zswap_protected);
bf414d6ae81ba2 Chengming Zhou 2024-02-11  1309  	nr_freeable = list_lru_shrink_count(&zswap.list_lru, sc);
b5ba474f3f5187 Nhat Pham      2023-11-30  1310  	/*
b5ba474f3f5187 Nhat Pham      2023-11-30  1311  	 * Subtract the lru size by an estimate of the number of pages
b5ba474f3f5187 Nhat Pham      2023-11-30  1312  	 * that should be protected.
b5ba474f3f5187 Nhat Pham      2023-11-30  1313  	 */
b5ba474f3f5187 Nhat Pham      2023-11-30  1314  	nr_freeable = nr_freeable > nr_protected ? nr_freeable - nr_protected : 0;
b5ba474f3f5187 Nhat Pham      2023-11-30  1315  
b5ba474f3f5187 Nhat Pham      2023-11-30  1316  	/*
b5ba474f3f5187 Nhat Pham      2023-11-30  1317  	 * Scale the number of freeable pages by the memory saving factor.
b5ba474f3f5187 Nhat Pham      2023-11-30  1318  	 * This ensures that the better zswap compresses memory, the fewer
b5ba474f3f5187 Nhat Pham      2023-11-30  1319  	 * pages we will evict to swap (as it will otherwise incur IO for
b5ba474f3f5187 Nhat Pham      2023-11-30  1320  	 * relatively small memory saving).
b5ba474f3f5187 Nhat Pham      2023-11-30  1321  	 */
b5ba474f3f5187 Nhat Pham      2023-11-30  1322  	return mult_frac(nr_freeable, nr_backing, nr_stored);
b5ba474f3f5187 Nhat Pham      2023-11-30  1323  }
b5ba474f3f5187 Nhat Pham      2023-11-30  1324  

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

Re: [PATCH 1/2] mm/zswap: global lru and shrinker shared by all zswap_pools

[Nhat Pham]

On Sun, Feb 11, 2024 at 5:57 AM Chengming Zhou
<zhouchengming@bytedance.com> wrote:
>
> Dynamic zswap_pool creation may create/reuse to have multiple
> zswap_pools in a list, only the first will be current used.
>
> Each zswap_pool has its own lru and shrinker, which is not
> necessary and has its problem:
>
> 1. When memory has pressure, all shrinker of zswap_pools will
>    try to shrink its own lru, there is no order between them.
>
> 2. When zswap limit hit, only the last zswap_pool's shrink_work
>    will try to shrink its lru, which is inefficient.
>
> Anyway, having a global lru and shrinker shared by all zswap_pools
> is better and efficient.
>
> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>

I'll do a careful review later, but IMO this is a good idea :)

Chris pointed out when he reviewed the zswap shrinker patch series
that the reclaim algorithm has to decide which pool to reclaim from,
and I have always thought that it was a bit weird that we have to do
it at all. We should reclaim stored objects by access ordering,
irregardless of which pool it belongs to. Having a shared LRU and
other associated reclaim structures is sound, and saves a bit of space
too while we're at it.

> ---
>  mm/zswap.c | 153 ++++++++++++++++++++++---------------------------------------
>  1 file changed, 55 insertions(+), 98 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 62fe307521c9..7668db8c10e3 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -176,14 +176,17 @@ struct zswap_pool {
>         struct kref kref;
>         struct list_head list;
>         struct work_struct release_work;
> -       struct work_struct shrink_work;
>         struct hlist_node node;
>         char tfm_name[CRYPTO_MAX_ALG_NAME];
> +};
> +
> +struct {
>         struct list_lru list_lru;
> -       struct mem_cgroup *next_shrink;
> -       struct shrinker *shrinker;
>         atomic_t nr_stored;
> -};
> +       struct shrinker *shrinker;
> +       struct work_struct shrink_work;
> +       struct mem_cgroup *next_shrink;
> +} zswap;
>
>  /*
>   * struct zswap_entry
> @@ -301,9 +304,6 @@ static void zswap_update_total_size(void)
>  * pool functions
>  **********************************/
>
> -static void zswap_alloc_shrinker(struct zswap_pool *pool);
> -static void shrink_worker(struct work_struct *w);
> -
>  static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
>  {
>         int i;
> @@ -353,30 +353,16 @@ static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
>         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
>          * caller to always add the new pool as the current pool
>          */
>         kref_init(&pool->kref);
>         INIT_LIST_HEAD(&pool->list);
> -       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);
> @@ -434,15 +420,8 @@ static void zswap_pool_destroy(struct zswap_pool *pool)
>
>         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);
> -
> -       spin_lock(&zswap_pools_lock);
> -       mem_cgroup_iter_break(NULL, pool->next_shrink);
> -       pool->next_shrink = NULL;
> -       spin_unlock(&zswap_pools_lock);
>
>         for (i = 0; i < ZSWAP_NR_ZPOOLS; i++)
>                 zpool_destroy_pool(pool->zpools[i]);
> @@ -529,24 +508,6 @@ static struct zswap_pool *zswap_pool_current_get(void)
>         return pool;
>  }
>
> -static struct zswap_pool *zswap_pool_last_get(void)
> -{
> -       struct zswap_pool *pool, *last = NULL;
> -
> -       rcu_read_lock();
> -
> -       list_for_each_entry_rcu(pool, &zswap_pools, list)
> -               last = pool;
> -       WARN_ONCE(!last && zswap_has_pool,
> -                 "%s: no page storage pool!\n", __func__);
> -       if (!zswap_pool_get(last))
> -               last = NULL;
> -
> -       rcu_read_unlock();
> -
> -       return last;
> -}
> -
>  /* type and compressor must be null-terminated */
>  static struct zswap_pool *zswap_pool_find_get(char *type, char *compressor)
>  {
> @@ -816,14 +777,10 @@ void zswap_folio_swapin(struct folio *folio)
>
>  void zswap_memcg_offline_cleanup(struct mem_cgroup *memcg)
>  {
> -       struct zswap_pool *pool;
> -
> -       /* lock out zswap pools list modification */
> +       /* lock out zswap shrinker walking memcg tree */
>         spin_lock(&zswap_pools_lock);
> -       list_for_each_entry(pool, &zswap_pools, list) {
> -               if (pool->next_shrink == memcg)
> -                       pool->next_shrink = mem_cgroup_iter(NULL, pool->next_shrink, NULL);
> -       }
> +       if (zswap.next_shrink == memcg)
> +               zswap.next_shrink = mem_cgroup_iter(NULL, zswap.next_shrink, NULL);
>         spin_unlock(&zswap_pools_lock);
>  }
>
> @@ -923,9 +880,9 @@ static void zswap_entry_free(struct zswap_entry *entry)
>         if (!entry->length)
>                 atomic_dec(&zswap_same_filled_pages);
>         else {
> -               zswap_lru_del(&entry->pool->list_lru, entry);
> +               zswap_lru_del(&zswap.list_lru, entry);
>                 zpool_free(zswap_find_zpool(entry), entry->handle);
> -               atomic_dec(&entry->pool->nr_stored);
> +               atomic_dec(&zswap.nr_stored);
>                 zswap_pool_put(entry->pool);
>         }
>         if (entry->objcg) {
> @@ -1288,7 +1245,6 @@ static unsigned long zswap_shrinker_scan(struct shrinker *shrinker,
>  {
>         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;
>
>         if (!zswap_shrinker_enabled ||
> @@ -1299,7 +1255,7 @@ static unsigned long zswap_shrinker_scan(struct shrinker *shrinker,
>
>         nr_protected =
>                 atomic_long_read(&lruvec->zswap_lruvec_state.nr_zswap_protected);
> -       lru_size = list_lru_shrink_count(&pool->list_lru, sc);
> +       lru_size = list_lru_shrink_count(&zswap.list_lru, sc);
>
>         /*
>          * Abort if we are shrinking into the protected region.
> @@ -1316,7 +1272,7 @@ static unsigned long zswap_shrinker_scan(struct shrinker *shrinker,
>                 return SHRINK_STOP;
>         }
>
> -       shrink_ret = list_lru_shrink_walk(&pool->list_lru, sc, &shrink_memcg_cb,
> +       shrink_ret = list_lru_shrink_walk(&zswap.list_lru, sc, &shrink_memcg_cb,
>                 &encountered_page_in_swapcache);
>
>         if (encountered_page_in_swapcache)
> @@ -1328,7 +1284,6 @@ static unsigned long zswap_shrinker_scan(struct shrinker *shrinker,
>  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;
> @@ -1343,7 +1298,7 @@ static unsigned long zswap_shrinker_count(struct shrinker *shrinker,
>  #else
>         /* use pool stats instead of memcg stats */
>         nr_backing = get_zswap_pool_size(pool) >> PAGE_SHIFT;
> -       nr_stored = atomic_read(&pool->nr_stored);
> +       nr_stored = atomic_read(&zswap.nr_stored);
>  #endif
>
>         if (!nr_stored)
> @@ -1351,7 +1306,7 @@ static unsigned long zswap_shrinker_count(struct shrinker *shrinker,
>
>         nr_protected =
>                 atomic_long_read(&lruvec->zswap_lruvec_state.nr_zswap_protected);
> -       nr_freeable = list_lru_shrink_count(&pool->list_lru, sc);
> +       nr_freeable = list_lru_shrink_count(&zswap.list_lru, sc);
>         /*
>          * Subtract the lru size by an estimate of the number of pages
>          * that should be protected.
> @@ -1367,23 +1322,24 @@ static unsigned long zswap_shrinker_count(struct shrinker *shrinker,
>         return mult_frac(nr_freeable, nr_backing, nr_stored);
>  }
>
> -static void zswap_alloc_shrinker(struct zswap_pool *pool)
> +static struct shrinker *zswap_alloc_shrinker(void)
>  {
> -       pool->shrinker =
> +       struct shrinker *shrinker;
> +
> +       shrinker =
>                 shrinker_alloc(SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE, "mm-zswap");
> -       if (!pool->shrinker)
> -               return;
> +       if (!shrinker)
> +               return NULL;
>
> -       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;
> +       shrinker->scan_objects = zswap_shrinker_scan;
> +       shrinker->count_objects = zswap_shrinker_count;
> +       shrinker->batch = 0;
> +       shrinker->seeks = DEFAULT_SEEKS;
> +       return shrinker;
>  }
>
>  static int shrink_memcg(struct mem_cgroup *memcg)
>  {
> -       struct zswap_pool *pool;
>         int nid, shrunk = 0;
>
>         if (!mem_cgroup_zswap_writeback_enabled(memcg))
> @@ -1396,32 +1352,25 @@ static int shrink_memcg(struct mem_cgroup *memcg)
>         if (memcg && !mem_cgroup_online(memcg))
>                 return -ENOENT;
>
> -       pool = zswap_pool_current_get();
> -       if (!pool)
> -               return -EINVAL;
> -
>         for_each_node_state(nid, N_NORMAL_MEMORY) {
>                 unsigned long nr_to_walk = 1;
>
> -               shrunk += list_lru_walk_one(&pool->list_lru, nid, memcg,
> +               shrunk += list_lru_walk_one(&zswap.list_lru, nid, memcg,
>                                             &shrink_memcg_cb, NULL, &nr_to_walk);
>         }
> -       zswap_pool_put(pool);
>         return shrunk ? 0 : -EAGAIN;
>  }
>
>  static void shrink_worker(struct work_struct *w)
>  {
> -       struct zswap_pool *pool = container_of(w, typeof(*pool),
> -                                               shrink_work);
>         struct mem_cgroup *memcg;
>         int ret, failures = 0;
>
>         /* global reclaim will select cgroup in a round-robin fashion. */
>         do {
>                 spin_lock(&zswap_pools_lock);
> -               pool->next_shrink = mem_cgroup_iter(NULL, pool->next_shrink, NULL);
> -               memcg = pool->next_shrink;
> +               zswap.next_shrink = mem_cgroup_iter(NULL, zswap.next_shrink, NULL);
> +               memcg = zswap.next_shrink;
>
>                 /*
>                  * We need to retry if we have gone through a full round trip, or if we
> @@ -1445,7 +1394,7 @@ static void shrink_worker(struct work_struct *w)
>                 if (!mem_cgroup_tryget_online(memcg)) {
>                         /* drop the reference from mem_cgroup_iter() */
>                         mem_cgroup_iter_break(NULL, memcg);
> -                       pool->next_shrink = NULL;
> +                       zswap.next_shrink = NULL;
>                         spin_unlock(&zswap_pools_lock);
>
>                         if (++failures == MAX_RECLAIM_RETRIES)
> @@ -1467,7 +1416,6 @@ static void shrink_worker(struct work_struct *w)
>  resched:
>                 cond_resched();
>         } while (!zswap_can_accept());
> -       zswap_pool_put(pool);
>  }
>
>  static int zswap_is_page_same_filled(void *ptr, unsigned long *value)
> @@ -1508,7 +1456,6 @@ bool zswap_store(struct folio *folio)
>         struct zswap_entry *entry, *dupentry;
>         struct obj_cgroup *objcg = NULL;
>         struct mem_cgroup *memcg = NULL;
> -       struct zswap_pool *shrink_pool;
>
>         VM_WARN_ON_ONCE(!folio_test_locked(folio));
>         VM_WARN_ON_ONCE(!folio_test_swapcache(folio));
> @@ -1576,7 +1523,7 @@ bool zswap_store(struct folio *folio)
>
>         if (objcg) {
>                 memcg = get_mem_cgroup_from_objcg(objcg);
> -               if (memcg_list_lru_alloc(memcg, &entry->pool->list_lru, GFP_KERNEL)) {
> +               if (memcg_list_lru_alloc(memcg, &zswap.list_lru, GFP_KERNEL)) {
>                         mem_cgroup_put(memcg);
>                         goto put_pool;
>                 }
> @@ -1607,8 +1554,8 @@ bool zswap_store(struct folio *folio)
>         }
>         if (entry->length) {
>                 INIT_LIST_HEAD(&entry->lru);
> -               zswap_lru_add(&entry->pool->list_lru, entry);
> -               atomic_inc(&entry->pool->nr_stored);
> +               zswap_lru_add(&zswap.list_lru, entry);
> +               atomic_inc(&zswap.nr_stored);
>         }
>         spin_unlock(&tree->lock);
>
> @@ -1640,9 +1587,7 @@ bool zswap_store(struct folio *folio)
>         return false;
>
>  shrink:
> -       shrink_pool = zswap_pool_last_get();
> -       if (shrink_pool && !queue_work(shrink_wq, &shrink_pool->shrink_work))
> -               zswap_pool_put(shrink_pool);
> +       queue_work(shrink_wq, &zswap.shrink_work);
>         goto reject;
>  }
>
> @@ -1804,6 +1749,21 @@ static int zswap_setup(void)
>         if (ret)
>                 goto hp_fail;
>
> +       shrink_wq = alloc_workqueue("zswap-shrink",
> +                       WQ_UNBOUND|WQ_MEM_RECLAIM, 1);
> +       if (!shrink_wq)
> +               goto hp_fail;
> +
> +       zswap.shrinker = zswap_alloc_shrinker();
> +       if (!zswap.shrinker)
> +               goto shrinker_fail;
> +       if (list_lru_init_memcg(&zswap.list_lru, zswap.shrinker))
> +               goto lru_fail;
> +       shrinker_register(zswap.shrinker);
> +
> +       INIT_WORK(&zswap.shrink_work, shrink_worker);
> +       atomic_set(&zswap.nr_stored, 0);
> +
>         pool = __zswap_pool_create_fallback();
>         if (pool) {
>                 pr_info("loaded using pool %s/%s\n", pool->tfm_name,
> @@ -1815,19 +1775,16 @@ static int zswap_setup(void)
>                 zswap_enabled = false;
>         }
>
> -       shrink_wq = alloc_workqueue("zswap-shrink",
> -                       WQ_UNBOUND|WQ_MEM_RECLAIM, 1);
> -       if (!shrink_wq)
> -               goto fallback_fail;
> -
>         if (zswap_debugfs_init())
>                 pr_warn("debugfs initialization failed\n");
>         zswap_init_state = ZSWAP_INIT_SUCCEED;
>         return 0;
>
> -fallback_fail:
> -       if (pool)
> -               zswap_pool_destroy(pool);
> +lru_fail:
> +       list_lru_destroy(&zswap.list_lru);
> +       shrinker_free(zswap.shrinker);
> +shrinker_fail:
> +       destroy_workqueue(shrink_wq);
>  hp_fail:
>         kmem_cache_destroy(zswap_entry_cache);
>  cache_fail:
>
> --
> b4 0.10.1

Re: [PATCH 2/2] mm/zswap: change zswap_pool kref to percpu_ref

[Nhat Pham]

On Sun, Feb 11, 2024 at 5:58 AM Chengming Zhou
<zhouchengming@bytedance.com> wrote:
>
> All zswap entries will take a reference of zswap_pool when
> zswap_store(), and drop it when free. Change it to use the
> percpu_ref is better for scalability performance.
>
> Testing kernel build in tmpfs with memory.max=2GB
> (zswap shrinker and writeback enabled with one 50GB swapfile).
>
>         mm-unstable  zswap-global-lru
> real    63.20        63.12
> user    1061.75      1062.95
> sys     268.74       264.44
>
> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
> ---
>  mm/zswap.c | 30 +++++++++++++++++++++---------
>  1 file changed, 21 insertions(+), 9 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 7668db8c10e3..afb31904fb08 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -173,7 +173,7 @@ struct crypto_acomp_ctx {
>  struct zswap_pool {
>         struct zpool *zpools[ZSWAP_NR_ZPOOLS];
>         struct crypto_acomp_ctx __percpu *acomp_ctx;
> -       struct kref kref;
> +       struct percpu_ref ref;
>         struct list_head list;
>         struct work_struct release_work;
>         struct hlist_node node;
> @@ -303,6 +303,7 @@ static void zswap_update_total_size(void)
>  /*********************************
>  * pool functions
>  **********************************/
> +static void __zswap_pool_empty(struct percpu_ref *ref);
>
>  static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
>  {
> @@ -356,13 +357,18 @@ static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
>         /* being the current pool takes 1 ref; this func expects the
>          * caller to always add the new pool as the current pool
>          */
> -       kref_init(&pool->kref);
> +       ret = percpu_ref_init(&pool->ref, __zswap_pool_empty,
> +                             PERCPU_REF_ALLOW_REINIT, GFP_KERNEL);
> +       if (ret)
> +               goto ref_fail;
>         INIT_LIST_HEAD(&pool->list);
>
>         zswap_pool_debug("created", pool);
>
>         return pool;
>
> +ref_fail:
> +       cpuhp_state_remove_instance(CPUHP_MM_ZSWP_POOL_PREPARE, &pool->node);
>  error:
>         if (pool->acomp_ctx)
>                 free_percpu(pool->acomp_ctx);
> @@ -435,8 +441,8 @@ static void __zswap_pool_release(struct work_struct *work)
>
>         synchronize_rcu();
>
> -       /* nobody should have been able to get a kref... */
> -       WARN_ON(kref_get_unless_zero(&pool->kref));

Do we no longer care about this WARN? IIUC, this is to catch someone
still holding a reference to the pool at release time, which sounds
like a bug. I think we can simulate the similar behavior with:

WARN_ON(percpu_ref_tryget(&pool->ref))

no? percpu_ref_tryget() should fail when the refcnt goes back down to
0. Then we can do percpu_ref_exit() as well.

> +       /* nobody should have been able to get a ref... */
> +       percpu_ref_exit(&pool->ref);
>
>         /* pool is now off zswap_pools list and has no references. */
>         zswap_pool_destroy(pool);
> @@ -444,11 +450,11 @@ static void __zswap_pool_release(struct work_struct *work)
>
>  static struct zswap_pool *zswap_pool_current(void);
>
> -static void __zswap_pool_empty(struct kref *kref)
> +static void __zswap_pool_empty(struct percpu_ref *ref)
>  {
>         struct zswap_pool *pool;
>
> -       pool = container_of(kref, typeof(*pool), kref);
> +       pool = container_of(ref, typeof(*pool), ref);
>
>         spin_lock(&zswap_pools_lock);
>
> @@ -467,12 +473,12 @@ static int __must_check zswap_pool_get(struct zswap_pool *pool)
>         if (!pool)
>                 return 0;
>
> -       return kref_get_unless_zero(&pool->kref);
> +       return percpu_ref_tryget(&pool->ref);
>  }
>
>  static void zswap_pool_put(struct zswap_pool *pool)
>  {
> -       kref_put(&pool->kref, __zswap_pool_empty);
> +       percpu_ref_put(&pool->ref);
>  }
>
>  static struct zswap_pool *__zswap_pool_current(void)
> @@ -602,6 +608,12 @@ static int __zswap_param_set(const char *val, const struct kernel_param *kp,
>
>         if (!pool)
>                 pool = zswap_pool_create(type, compressor);
> +       else {
> +               /* Resurrect percpu_ref to percpu mode. */
> +               percpu_ref_resurrect(&pool->ref);
> +               /* Drop the ref from zswap_pool_find_get(). */
> +               zswap_pool_put(pool);
> +       }
>
>         if (pool)
>                 ret = param_set_charp(s, kp);
> @@ -640,7 +652,7 @@ static int __zswap_param_set(const char *val, const struct kernel_param *kp,
>          * or the new pool we failed to add
>          */
>         if (put_pool)
> -               zswap_pool_put(put_pool);
> +               percpu_ref_kill(&put_pool->ref);
>
>         return ret;
>  }
>
> --
> b4 0.10.1

The rest of the code looks solid to me FWIW. Number seems to indicate
this is a good idea as well.

Re: [PATCH 1/2] mm/zswap: global lru and shrinker shared by all zswap_pools

[kernel test robot]

Hi Chengming,

kernel test robot noticed the following build errors:

[auto build test ERROR on 191d97734e41a5c9f90a2f6636fdd335ae1d435d]

url:    https://github.com/intel-lab-lkp/linux/commits/Chengming-Zhou/mm-zswap-global-lru-and-shrinker-shared-by-all-zswap_pools/20240211-220028
base:   191d97734e41a5c9f90a2f6636fdd335ae1d435d
patch link:    https://lore.kernel.org/r/20240210-zswap-global-lru-v1-1-853473d7b0da%40bytedance.com
patch subject: [PATCH 1/2] mm/zswap: global lru and shrinker shared by all zswap_pools
config: i386-randconfig-054-20240212 (https://download.01.org/0day-ci/archive/20240212/202402120503.HRNkoWyq-lkp@intel.com/config)
compiler: clang version 17.0.6 (https://github.com/llvm/llvm-project 6009708b4367171ccdbf4b5905cb6a803753fe18)
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20240212/202402120503.HRNkoWyq-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202402120503.HRNkoWyq-lkp@intel.com/

All errors (new ones prefixed by >>):

>> mm/zswap.c:1300:35: error: use of undeclared identifier 'pool'
    1300 |         nr_backing = get_zswap_pool_size(pool) >> PAGE_SHIFT;
         |                                          ^
   1 error generated.


vim +/pool +1300 mm/zswap.c

b5ba474f3f5187 Nhat Pham      2023-11-30  1283  
b5ba474f3f5187 Nhat Pham      2023-11-30  1284  static unsigned long zswap_shrinker_count(struct shrinker *shrinker,
b5ba474f3f5187 Nhat Pham      2023-11-30  1285  		struct shrink_control *sc)
b5ba474f3f5187 Nhat Pham      2023-11-30  1286  {
b5ba474f3f5187 Nhat Pham      2023-11-30  1287  	struct mem_cgroup *memcg = sc->memcg;
b5ba474f3f5187 Nhat Pham      2023-11-30  1288  	struct lruvec *lruvec = mem_cgroup_lruvec(memcg, NODE_DATA(sc->nid));
b5ba474f3f5187 Nhat Pham      2023-11-30  1289  	unsigned long nr_backing, nr_stored, nr_freeable, nr_protected;
b5ba474f3f5187 Nhat Pham      2023-11-30  1290  
501a06fe8e4c18 Nhat Pham      2023-12-07  1291  	if (!zswap_shrinker_enabled || !mem_cgroup_zswap_writeback_enabled(memcg))
b5ba474f3f5187 Nhat Pham      2023-11-30  1292  		return 0;
b5ba474f3f5187 Nhat Pham      2023-11-30  1293  
b5ba474f3f5187 Nhat Pham      2023-11-30  1294  #ifdef CONFIG_MEMCG_KMEM
7d7ef0a4686abe Yosry Ahmed    2023-11-29  1295  	mem_cgroup_flush_stats(memcg);
b5ba474f3f5187 Nhat Pham      2023-11-30  1296  	nr_backing = memcg_page_state(memcg, MEMCG_ZSWAP_B) >> PAGE_SHIFT;
b5ba474f3f5187 Nhat Pham      2023-11-30  1297  	nr_stored = memcg_page_state(memcg, MEMCG_ZSWAPPED);
b5ba474f3f5187 Nhat Pham      2023-11-30  1298  #else
b5ba474f3f5187 Nhat Pham      2023-11-30  1299  	/* use pool stats instead of memcg stats */
b5ba474f3f5187 Nhat Pham      2023-11-30 @1300  	nr_backing = get_zswap_pool_size(pool) >> PAGE_SHIFT;
bf414d6ae81ba2 Chengming Zhou 2024-02-11  1301  	nr_stored = atomic_read(&zswap.nr_stored);
b5ba474f3f5187 Nhat Pham      2023-11-30  1302  #endif
b5ba474f3f5187 Nhat Pham      2023-11-30  1303  
b5ba474f3f5187 Nhat Pham      2023-11-30  1304  	if (!nr_stored)
b5ba474f3f5187 Nhat Pham      2023-11-30  1305  		return 0;
b5ba474f3f5187 Nhat Pham      2023-11-30  1306  
b5ba474f3f5187 Nhat Pham      2023-11-30  1307  	nr_protected =
b5ba474f3f5187 Nhat Pham      2023-11-30  1308  		atomic_long_read(&lruvec->zswap_lruvec_state.nr_zswap_protected);
bf414d6ae81ba2 Chengming Zhou 2024-02-11  1309  	nr_freeable = list_lru_shrink_count(&zswap.list_lru, sc);
b5ba474f3f5187 Nhat Pham      2023-11-30  1310  	/*
b5ba474f3f5187 Nhat Pham      2023-11-30  1311  	 * Subtract the lru size by an estimate of the number of pages
b5ba474f3f5187 Nhat Pham      2023-11-30  1312  	 * that should be protected.
b5ba474f3f5187 Nhat Pham      2023-11-30  1313  	 */
b5ba474f3f5187 Nhat Pham      2023-11-30  1314  	nr_freeable = nr_freeable > nr_protected ? nr_freeable - nr_protected : 0;
b5ba474f3f5187 Nhat Pham      2023-11-30  1315  
b5ba474f3f5187 Nhat Pham      2023-11-30  1316  	/*
b5ba474f3f5187 Nhat Pham      2023-11-30  1317  	 * Scale the number of freeable pages by the memory saving factor.
b5ba474f3f5187 Nhat Pham      2023-11-30  1318  	 * This ensures that the better zswap compresses memory, the fewer
b5ba474f3f5187 Nhat Pham      2023-11-30  1319  	 * pages we will evict to swap (as it will otherwise incur IO for
b5ba474f3f5187 Nhat Pham      2023-11-30  1320  	 * relatively small memory saving).
b5ba474f3f5187 Nhat Pham      2023-11-30  1321  	 */
b5ba474f3f5187 Nhat Pham      2023-11-30  1322  	return mult_frac(nr_freeable, nr_backing, nr_stored);
b5ba474f3f5187 Nhat Pham      2023-11-30  1323  }
b5ba474f3f5187 Nhat Pham      2023-11-30  1324  

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

Re: [PATCH 1/2] mm/zswap: global lru and shrinker shared by all zswap_pools

[Chengming Zhou]

On 2024/2/12 03:01, kernel test robot wrote:
> Hi Chengming,
> 
> kernel test robot noticed the following build errors:
> 
> [auto build test ERROR on 191d97734e41a5c9f90a2f6636fdd335ae1d435d]
> 
> url:    https://github.com/intel-lab-lkp/linux/commits/Chengming-Zhou/mm-zswap-global-lru-and-shrinker-shared-by-all-zswap_pools/20240211-220028
> base:   191d97734e41a5c9f90a2f6636fdd335ae1d435d
> patch link:    https://lore.kernel.org/r/20240210-zswap-global-lru-v1-1-853473d7b0da%40bytedance.com
> patch subject: [PATCH 1/2] mm/zswap: global lru and shrinker shared by all zswap_pools
> config: x86_64-randconfig-013-20240211 (https://download.01.org/0day-ci/archive/20240212/202402120226.TK7G37U9-lkp@intel.com/config)
> compiler: gcc-12 (Debian 12.2.0-14) 12.2.0
> reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20240212/202402120226.TK7G37U9-lkp@intel.com/reproduce)
> 
> If you fix the issue in a separate patch/commit (i.e. not just a new version of
> the same patch/commit), kindly add following tags
> | Reported-by: kernel test robot <lkp@intel.com>
> | Closes: https://lore.kernel.org/oe-kbuild-all/202402120226.TK7G37U9-lkp@intel.com/
> 
> All errors (new ones prefixed by >>):
> 
>    mm/zswap.c: In function 'zswap_shrinker_count':
>>> mm/zswap.c:1300:42: error: 'pool' undeclared (first use in this function); did you mean 'zpool'?
>     1300 |         nr_backing = get_zswap_pool_size(pool) >> PAGE_SHIFT;

Forgot to test with !CONFIG_MEMCG_KMEM, need to change to zswap_pool_total_size here.

Thanks.

Re: [PATCH 1/2] mm/zswap: global lru and shrinker shared by all zswap_pools

[Chengming Zhou]

On 2024/2/12 05:04, Nhat Pham wrote:
> On Sun, Feb 11, 2024 at 5:57 AM Chengming Zhou
> <zhouchengming@bytedance.com> wrote:
>>
>> Dynamic zswap_pool creation may create/reuse to have multiple
>> zswap_pools in a list, only the first will be current used.
>>
>> Each zswap_pool has its own lru and shrinker, which is not
>> necessary and has its problem:
>>
>> 1. When memory has pressure, all shrinker of zswap_pools will
>>    try to shrink its own lru, there is no order between them.
>>
>> 2. When zswap limit hit, only the last zswap_pool's shrink_work
>>    will try to shrink its lru, which is inefficient.
>>
>> Anyway, having a global lru and shrinker shared by all zswap_pools
>> is better and efficient.
>>
>> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
> 
> I'll do a careful review later, but IMO this is a good idea :)

Ok, thanks, take your time. :)

> 
> Chris pointed out when he reviewed the zswap shrinker patch series
> that the reclaim algorithm has to decide which pool to reclaim from,
> and I have always thought that it was a bit weird that we have to do
> it at all. We should reclaim stored objects by access ordering,
> irregardless of which pool it belongs to. Having a shared LRU and
> other associated reclaim structures is sound, and saves a bit of space
> too while we're at it.

Right, agree!

Re: [PATCH 2/2] mm/zswap: change zswap_pool kref to percpu_ref

[Chengming Zhou]

On 2024/2/12 05:21, Nhat Pham wrote:
> On Sun, Feb 11, 2024 at 5:58 AM Chengming Zhou
> <zhouchengming@bytedance.com> wrote:
>>
>> All zswap entries will take a reference of zswap_pool when
>> zswap_store(), and drop it when free. Change it to use the
>> percpu_ref is better for scalability performance.
>>
>> Testing kernel build in tmpfs with memory.max=2GB
>> (zswap shrinker and writeback enabled with one 50GB swapfile).
>>
>>         mm-unstable  zswap-global-lru
>> real    63.20        63.12
>> user    1061.75      1062.95
>> sys     268.74       264.44
>>
>> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
>> ---
>>  mm/zswap.c | 30 +++++++++++++++++++++---------
>>  1 file changed, 21 insertions(+), 9 deletions(-)
>>
>> diff --git a/mm/zswap.c b/mm/zswap.c
>> index 7668db8c10e3..afb31904fb08 100644
>> --- a/mm/zswap.c
>> +++ b/mm/zswap.c
>> @@ -173,7 +173,7 @@ struct crypto_acomp_ctx {
>>  struct zswap_pool {
>>         struct zpool *zpools[ZSWAP_NR_ZPOOLS];
>>         struct crypto_acomp_ctx __percpu *acomp_ctx;
>> -       struct kref kref;
>> +       struct percpu_ref ref;
>>         struct list_head list;
>>         struct work_struct release_work;
>>         struct hlist_node node;
>> @@ -303,6 +303,7 @@ static void zswap_update_total_size(void)
>>  /*********************************
>>  * pool functions
>>  **********************************/
>> +static void __zswap_pool_empty(struct percpu_ref *ref);
>>
>>  static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
>>  {
>> @@ -356,13 +357,18 @@ static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
>>         /* being the current pool takes 1 ref; this func expects the
>>          * caller to always add the new pool as the current pool
>>          */
>> -       kref_init(&pool->kref);
>> +       ret = percpu_ref_init(&pool->ref, __zswap_pool_empty,
>> +                             PERCPU_REF_ALLOW_REINIT, GFP_KERNEL);
>> +       if (ret)
>> +               goto ref_fail;
>>         INIT_LIST_HEAD(&pool->list);
>>
>>         zswap_pool_debug("created", pool);
>>
>>         return pool;
>>
>> +ref_fail:
>> +       cpuhp_state_remove_instance(CPUHP_MM_ZSWP_POOL_PREPARE, &pool->node);
>>  error:
>>         if (pool->acomp_ctx)
>>                 free_percpu(pool->acomp_ctx);
>> @@ -435,8 +441,8 @@ static void __zswap_pool_release(struct work_struct *work)
>>
>>         synchronize_rcu();
>>
>> -       /* nobody should have been able to get a kref... */
>> -       WARN_ON(kref_get_unless_zero(&pool->kref));
> 
> Do we no longer care about this WARN? IIUC, this is to catch someone
> still holding a reference to the pool at release time, which sounds
> like a bug. I think we can simulate the similar behavior with:

Ok, I thought it has already been put to 0 when we're here, so any tryget
will fail. But keeping this WARN_ON() is also fine to me, will keep it.

Thanks.

> 
> WARN_ON(percpu_ref_tryget(&pool->ref))
> 
> no? percpu_ref_tryget() should fail when the refcnt goes back down to
> 0. Then we can do percpu_ref_exit() as well.
> 
>> +       /* nobody should have been able to get a ref... */
>> +       percpu_ref_exit(&pool->ref);
>>
>>         /* pool is now off zswap_pools list and has no references. */
>>         zswap_pool_destroy(pool);
>> @@ -444,11 +450,11 @@ static void __zswap_pool_release(struct work_struct *work)
>>
>>  static struct zswap_pool *zswap_pool_current(void);
>>
>> -static void __zswap_pool_empty(struct kref *kref)
>> +static void __zswap_pool_empty(struct percpu_ref *ref)
>>  {
>>         struct zswap_pool *pool;
>>
>> -       pool = container_of(kref, typeof(*pool), kref);
>> +       pool = container_of(ref, typeof(*pool), ref);
>>
>>         spin_lock(&zswap_pools_lock);
>>
>> @@ -467,12 +473,12 @@ static int __must_check zswap_pool_get(struct zswap_pool *pool)
>>         if (!pool)
>>                 return 0;
>>
>> -       return kref_get_unless_zero(&pool->kref);
>> +       return percpu_ref_tryget(&pool->ref);
>>  }
>>
>>  static void zswap_pool_put(struct zswap_pool *pool)
>>  {
>> -       kref_put(&pool->kref, __zswap_pool_empty);
>> +       percpu_ref_put(&pool->ref);
>>  }
>>
>>  static struct zswap_pool *__zswap_pool_current(void)
>> @@ -602,6 +608,12 @@ static int __zswap_param_set(const char *val, const struct kernel_param *kp,
>>
>>         if (!pool)
>>                 pool = zswap_pool_create(type, compressor);
>> +       else {
>> +               /* Resurrect percpu_ref to percpu mode. */
>> +               percpu_ref_resurrect(&pool->ref);
>> +               /* Drop the ref from zswap_pool_find_get(). */
>> +               zswap_pool_put(pool);
>> +       }
>>
>>         if (pool)
>>                 ret = param_set_charp(s, kp);
>> @@ -640,7 +652,7 @@ static int __zswap_param_set(const char *val, const struct kernel_param *kp,
>>          * or the new pool we failed to add
>>          */
>>         if (put_pool)
>> -               zswap_pool_put(put_pool);
>> +               percpu_ref_kill(&put_pool->ref);
>>
>>         return ret;
>>  }
>>
>> --
>> b4 0.10.1
> 
> The rest of the code looks solid to me FWIW. Number seems to indicate
> this is a good idea as well.

Re: [PATCH 2/2] mm/zswap: change zswap_pool kref to percpu_ref

[Nhat Pham]

On Mon, Feb 12, 2024 at 5:29 AM Chengming Zhou
<zhouchengming@bytedance.com> wrote:
>
> On 2024/2/12 05:21, Nhat Pham wrote:
> > On Sun, Feb 11, 2024 at 5:58 AM Chengming Zhou
> > <zhouchengming@bytedance.com> wrote:
> >>
> >> All zswap entries will take a reference of zswap_pool when
> >> zswap_store(), and drop it when free. Change it to use the
> >> percpu_ref is better for scalability performance.
> >>
> >> Testing kernel build in tmpfs with memory.max=2GB
> >> (zswap shrinker and writeback enabled with one 50GB swapfile).
> >>
> >>         mm-unstable  zswap-global-lru
> >> real    63.20        63.12
> >> user    1061.75      1062.95
> >> sys     268.74       264.44
> >>
> >> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
> >> ---
> >>  mm/zswap.c | 30 +++++++++++++++++++++---------
> >>  1 file changed, 21 insertions(+), 9 deletions(-)
> >>
> >> diff --git a/mm/zswap.c b/mm/zswap.c
> >> index 7668db8c10e3..afb31904fb08 100644
> >> --- a/mm/zswap.c
> >> +++ b/mm/zswap.c
> >> @@ -173,7 +173,7 @@ struct crypto_acomp_ctx {
> >>  struct zswap_pool {
> >>         struct zpool *zpools[ZSWAP_NR_ZPOOLS];
> >>         struct crypto_acomp_ctx __percpu *acomp_ctx;
> >> -       struct kref kref;
> >> +       struct percpu_ref ref;
> >>         struct list_head list;
> >>         struct work_struct release_work;
> >>         struct hlist_node node;
> >> @@ -303,6 +303,7 @@ static void zswap_update_total_size(void)
> >>  /*********************************
> >>  * pool functions
> >>  **********************************/
> >> +static void __zswap_pool_empty(struct percpu_ref *ref);
> >>
> >>  static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
> >>  {
> >> @@ -356,13 +357,18 @@ static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
> >>         /* being the current pool takes 1 ref; this func expects the
> >>          * caller to always add the new pool as the current pool
> >>          */
> >> -       kref_init(&pool->kref);
> >> +       ret = percpu_ref_init(&pool->ref, __zswap_pool_empty,
> >> +                             PERCPU_REF_ALLOW_REINIT, GFP_KERNEL);
> >> +       if (ret)
> >> +               goto ref_fail;
> >>         INIT_LIST_HEAD(&pool->list);
> >>
> >>         zswap_pool_debug("created", pool);
> >>
> >>         return pool;
> >>
> >> +ref_fail:
> >> +       cpuhp_state_remove_instance(CPUHP_MM_ZSWP_POOL_PREPARE, &pool->node);
> >>  error:
> >>         if (pool->acomp_ctx)
> >>                 free_percpu(pool->acomp_ctx);
> >> @@ -435,8 +441,8 @@ static void __zswap_pool_release(struct work_struct *work)
> >>
> >>         synchronize_rcu();
> >>
> >> -       /* nobody should have been able to get a kref... */
> >> -       WARN_ON(kref_get_unless_zero(&pool->kref));
> >
> > Do we no longer care about this WARN? IIUC, this is to catch someone
> > still holding a reference to the pool at release time, which sounds
> > like a bug. I think we can simulate the similar behavior with:
>
> Ok, I thought it has already been put to 0 when we're here, so any tryget
> will fail. But keeping this WARN_ON() is also fine to me, will keep it.

Yup - it should fail, if the code is not buggy. But that's a pretty big if :)

Jokes aside, we can remove it if folks think the benefit is not worth
the cost/overhead. However, I'm a bit hesitant to remove checks in
zswap, especially given how buggy it has been (some of which are
refcnt bugs as well, IIRC).

Re: [PATCH 2/2] mm/zswap: change zswap_pool kref to percpu_ref

[Yosry Ahmed]

On Sun, Feb 11, 2024 at 5:58 AM Chengming Zhou
<zhouchengming@bytedance.com> wrote:
>
> All zswap entries will take a reference of zswap_pool when
> zswap_store(), and drop it when free. Change it to use the
> percpu_ref is better for scalability performance.
>
> Testing kernel build in tmpfs with memory.max=2GB
> (zswap shrinker and writeback enabled with one 50GB swapfile).
>
>         mm-unstable  zswap-global-lru
> real    63.20        63.12
> user    1061.75      1062.95
> sys     268.74       264.44

Are these numbers from a single run or the average of multiple runs?
It just seems that the improvement is small, and percpu refcnt is
slightly less intuitive (and uses a bit more memory), so let's make
sure there is a real performance gain first.

It would also be useful to mention how many threads/CPUs are being used here.

Re: [PATCH 1/2] mm/zswap: global lru and shrinker shared by all zswap_pools

[Yosry Ahmed]

On Sun, Feb 11, 2024 at 01:57:04PM +0000, Chengming Zhou wrote:
> Dynamic zswap_pool creation may create/reuse to have multiple
> zswap_pools in a list, only the first will be current used.
> 
> Each zswap_pool has its own lru and shrinker, which is not
> necessary and has its problem:
> 
> 1. When memory has pressure, all shrinker of zswap_pools will
>    try to shrink its own lru, there is no order between them.
> 
> 2. When zswap limit hit, only the last zswap_pool's shrink_work
>    will try to shrink its lru, which is inefficient.
> 
> Anyway, having a global lru and shrinker shared by all zswap_pools
> is better and efficient.

It is also a great simplification.

> 
> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
> ---
>  mm/zswap.c | 153 ++++++++++++++++++++++---------------------------------------
>  1 file changed, 55 insertions(+), 98 deletions(-)
> 
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 62fe307521c9..7668db8c10e3 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -176,14 +176,17 @@ struct zswap_pool {
>  	struct kref kref;
>  	struct list_head list;
>  	struct work_struct release_work;
> -	struct work_struct shrink_work;
>  	struct hlist_node node;
>  	char tfm_name[CRYPTO_MAX_ALG_NAME];
> +};
> +
> +struct {

static?

>  	struct list_lru list_lru;
> -	struct mem_cgroup *next_shrink;
> -	struct shrinker *shrinker;

Just curious, any reason to change the relative ordering of members
here? It produces a couple more lines of diff :)

>  	atomic_t nr_stored;
> -};
> +	struct shrinker *shrinker;
> +	struct work_struct shrink_work;
> +	struct mem_cgroup *next_shrink;
> +} zswap;
>  
>  /*
>   * struct zswap_entry
> @@ -301,9 +304,6 @@ static void zswap_update_total_size(void)
>  * pool functions
>  **********************************/
>  
> -static void zswap_alloc_shrinker(struct zswap_pool *pool);
> -static void shrink_worker(struct work_struct *w);
> -
>  static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
>  {
>  	int i;
> @@ -353,30 +353,16 @@ static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
>  	if (ret)
>  		goto error;
>  
> -	zswap_alloc_shrinker(pool);
> -	if (!pool->shrinker)
> -		goto error;
> -
> -	pr_debug("using %s compressor\n", pool->tfm_name);
> -

Why are we removing this debug print?

>  	/* being the current pool takes 1 ref; this func expects the
>  	 * caller to always add the new pool as the current pool
>  	 */
>  	kref_init(&pool->kref);
>  	INIT_LIST_HEAD(&pool->list);
> -	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);
[..]
> @@ -816,14 +777,10 @@ void zswap_folio_swapin(struct folio *folio)
>  
>  void zswap_memcg_offline_cleanup(struct mem_cgroup *memcg)
>  {
> -	struct zswap_pool *pool;
> -
> -	/* lock out zswap pools list modification */
> +	/* lock out zswap shrinker walking memcg tree */
>  	spin_lock(&zswap_pools_lock);
> -	list_for_each_entry(pool, &zswap_pools, list) {
> -		if (pool->next_shrink == memcg)
> -			pool->next_shrink = mem_cgroup_iter(NULL, pool->next_shrink, NULL);
> -	}
> +	if (zswap.next_shrink == memcg)
> +		zswap.next_shrink = mem_cgroup_iter(NULL, zswap.next_shrink, NULL);

Now that next_shrink has nothing to do with zswap pools, it feels weird
that we are using zswap_pools_lock for its synchronization. Does it make
sense to have a separate lock for it just for semantic purposes?

>  	spin_unlock(&zswap_pools_lock);
>  }
>  
[..]
> @@ -1328,7 +1284,6 @@ static unsigned long zswap_shrinker_scan(struct shrinker *shrinker,
>  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;
> @@ -1343,7 +1298,7 @@ static unsigned long zswap_shrinker_count(struct shrinker *shrinker,
>  #else
>  	/* use pool stats instead of memcg stats */
>  	nr_backing = get_zswap_pool_size(pool) >> PAGE_SHIFT;

"pool" is still being used here.

> -	nr_stored = atomic_read(&pool->nr_stored);
> +	nr_stored = atomic_read(&zswap.nr_stored);
>  #endif
>  
>  	if (!nr_stored)
[..]  
> @@ -1804,6 +1749,21 @@ static int zswap_setup(void)
>  	if (ret)
>  		goto hp_fail;
>  
> +	shrink_wq = alloc_workqueue("zswap-shrink",
> +			WQ_UNBOUND|WQ_MEM_RECLAIM, 1);
> +	if (!shrink_wq)
> +		goto hp_fail;

I think we need a new label here to call cpuhp_remove_multi_state(), but
apparently this is missing from the current code for some reason.

> +
> +	zswap.shrinker = zswap_alloc_shrinker();
> +	if (!zswap.shrinker)
> +		goto shrinker_fail;
> +	if (list_lru_init_memcg(&zswap.list_lru, zswap.shrinker))
> +		goto lru_fail;
> +	shrinker_register(zswap.shrinker);
> +
> +	INIT_WORK(&zswap.shrink_work, shrink_worker);
> +	atomic_set(&zswap.nr_stored, 0);
> +
>  	pool = __zswap_pool_create_fallback();
>  	if (pool) {
>  		pr_info("loaded using pool %s/%s\n", pool->tfm_name,
> @@ -1815,19 +1775,16 @@ static int zswap_setup(void)
>  		zswap_enabled = false;
>  	}
>  
> -	shrink_wq = alloc_workqueue("zswap-shrink",
> -			WQ_UNBOUND|WQ_MEM_RECLAIM, 1);
> -	if (!shrink_wq)
> -		goto fallback_fail;
> -
>  	if (zswap_debugfs_init())
>  		pr_warn("debugfs initialization failed\n");
>  	zswap_init_state = ZSWAP_INIT_SUCCEED;
>  	return 0;
>  
> -fallback_fail:
> -	if (pool)
> -		zswap_pool_destroy(pool);
> +lru_fail:
> +	list_lru_destroy(&zswap.list_lru);

Do we need to call list_lru_destroy() here? I know it is currently being
called if list_lru_init_memcg() fails, but I fail to understand why. It
seems like list_lru_destroy() will do nothing anyway.

> +	shrinker_free(zswap.shrinker);
> +shrinker_fail:
> +	destroy_workqueue(shrink_wq);
>  hp_fail:
>  	kmem_cache_destroy(zswap_entry_cache);
>  cache_fail:
> 
> -- 
> b4 0.10.1

Re: [PATCH 1/2] mm/zswap: global lru and shrinker shared by all zswap_pools

[Chengming Zhou]

On 2024/2/13 20:57, Yosry Ahmed wrote:
> On Sun, Feb 11, 2024 at 01:57:04PM +0000, Chengming Zhou wrote:
>> Dynamic zswap_pool creation may create/reuse to have multiple
>> zswap_pools in a list, only the first will be current used.
>>
>> Each zswap_pool has its own lru and shrinker, which is not
>> necessary and has its problem:
>>
>> 1. When memory has pressure, all shrinker of zswap_pools will
>>    try to shrink its own lru, there is no order between them.
>>
>> 2. When zswap limit hit, only the last zswap_pool's shrink_work
>>    will try to shrink its lru, which is inefficient.
>>
>> Anyway, having a global lru and shrinker shared by all zswap_pools
>> is better and efficient.
> 
> It is also a great simplification.
> 
>>
>> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
>> ---
>>  mm/zswap.c | 153 ++++++++++++++++++++++---------------------------------------
>>  1 file changed, 55 insertions(+), 98 deletions(-)
>>
>> diff --git a/mm/zswap.c b/mm/zswap.c
>> index 62fe307521c9..7668db8c10e3 100644
>> --- a/mm/zswap.c
>> +++ b/mm/zswap.c
>> @@ -176,14 +176,17 @@ struct zswap_pool {
>>  	struct kref kref;
>>  	struct list_head list;
>>  	struct work_struct release_work;
>> -	struct work_struct shrink_work;
>>  	struct hlist_node node;
>>  	char tfm_name[CRYPTO_MAX_ALG_NAME];
>> +};
>> +
>> +struct {
> 
> static?

Ah, right, will add static.

> 
>>  	struct list_lru list_lru;
>> -	struct mem_cgroup *next_shrink;
>> -	struct shrinker *shrinker;
> 
> Just curious, any reason to change the relative ordering of members
> here? It produces a couple more lines of diff :)

The list_lru and nr_stored atomic variable are used in zswap_store/load
hotpath, the other shrinker related sound like cold path. I thought it's
normal and clearer to put them according to their usages.

> 
>>  	atomic_t nr_stored;
>> -};
>> +	struct shrinker *shrinker;
>> +	struct work_struct shrink_work;
>> +	struct mem_cgroup *next_shrink;
>> +} zswap;
>>  
>>  /*
>>   * struct zswap_entry
>> @@ -301,9 +304,6 @@ static void zswap_update_total_size(void)
>>  * pool functions
>>  **********************************/
>>  
>> -static void zswap_alloc_shrinker(struct zswap_pool *pool);
>> -static void shrink_worker(struct work_struct *w);
>> -
>>  static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
>>  {
>>  	int i;
>> @@ -353,30 +353,16 @@ static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
>>  	if (ret)
>>  		goto error;
>>  
>> -	zswap_alloc_shrinker(pool);
>> -	if (!pool->shrinker)
>> -		goto error;
>> -
>> -	pr_debug("using %s compressor\n", pool->tfm_name);
>> -
> 
> Why are we removing this debug print?

Oh, I just noticed it's only necessary to print dmesg when "create" success,
the below "zswap_pool_debug()" will print its compressor too.

> 
>>  	/* being the current pool takes 1 ref; this func expects the
>>  	 * caller to always add the new pool as the current pool
>>  	 */
>>  	kref_init(&pool->kref);
>>  	INIT_LIST_HEAD(&pool->list);
>> -	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);
> [..]
>> @@ -816,14 +777,10 @@ void zswap_folio_swapin(struct folio *folio)
>>  
>>  void zswap_memcg_offline_cleanup(struct mem_cgroup *memcg)
>>  {
>> -	struct zswap_pool *pool;
>> -
>> -	/* lock out zswap pools list modification */
>> +	/* lock out zswap shrinker walking memcg tree */
>>  	spin_lock(&zswap_pools_lock);
>> -	list_for_each_entry(pool, &zswap_pools, list) {
>> -		if (pool->next_shrink == memcg)
>> -			pool->next_shrink = mem_cgroup_iter(NULL, pool->next_shrink, NULL);
>> -	}
>> +	if (zswap.next_shrink == memcg)
>> +		zswap.next_shrink = mem_cgroup_iter(NULL, zswap.next_shrink, NULL);
> 
> Now that next_shrink has nothing to do with zswap pools, it feels weird
> that we are using zswap_pools_lock for its synchronization. Does it make
> sense to have a separate lock for it just for semantic purposes?

Agree, I think so, it's clearer to have another lock.

> 
>>  	spin_unlock(&zswap_pools_lock);
>>  }
>>  
> [..]
>> @@ -1328,7 +1284,6 @@ static unsigned long zswap_shrinker_scan(struct shrinker *shrinker,
>>  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;
>> @@ -1343,7 +1298,7 @@ static unsigned long zswap_shrinker_count(struct shrinker *shrinker,
>>  #else
>>  	/* use pool stats instead of memcg stats */
>>  	nr_backing = get_zswap_pool_size(pool) >> PAGE_SHIFT;
> 
> "pool" is still being used here.

Oops, should be changed to zswap_pool_total_size here.

> 
>> -	nr_stored = atomic_read(&pool->nr_stored);
>> +	nr_stored = atomic_read(&zswap.nr_stored);
>>  #endif
>>  
>>  	if (!nr_stored)
> [..]  
>> @@ -1804,6 +1749,21 @@ static int zswap_setup(void)
>>  	if (ret)
>>  		goto hp_fail;
>>  
>> +	shrink_wq = alloc_workqueue("zswap-shrink",
>> +			WQ_UNBOUND|WQ_MEM_RECLAIM, 1);
>> +	if (!shrink_wq)
>> +		goto hp_fail;
> 
> I think we need a new label here to call cpuhp_remove_multi_state(), but
> apparently this is missing from the current code for some reason.

You are right! This should use a new label to "cpuhp_remove_multi_state()",
will fix it.

> 
>> +
>> +	zswap.shrinker = zswap_alloc_shrinker();
>> +	if (!zswap.shrinker)
>> +		goto shrinker_fail;
>> +	if (list_lru_init_memcg(&zswap.list_lru, zswap.shrinker))
>> +		goto lru_fail;
>> +	shrinker_register(zswap.shrinker);
>> +
>> +	INIT_WORK(&zswap.shrink_work, shrink_worker);
>> +	atomic_set(&zswap.nr_stored, 0);
>> +
>>  	pool = __zswap_pool_create_fallback();
>>  	if (pool) {
>>  		pr_info("loaded using pool %s/%s\n", pool->tfm_name,
>> @@ -1815,19 +1775,16 @@ static int zswap_setup(void)
>>  		zswap_enabled = false;
>>  	}
>>  
>> -	shrink_wq = alloc_workqueue("zswap-shrink",
>> -			WQ_UNBOUND|WQ_MEM_RECLAIM, 1);
>> -	if (!shrink_wq)
>> -		goto fallback_fail;
>> -
>>  	if (zswap_debugfs_init())
>>  		pr_warn("debugfs initialization failed\n");
>>  	zswap_init_state = ZSWAP_INIT_SUCCEED;
>>  	return 0;
>>  
>> -fallback_fail:
>> -	if (pool)
>> -		zswap_pool_destroy(pool);
>> +lru_fail:
>> +	list_lru_destroy(&zswap.list_lru);
> 
> Do we need to call list_lru_destroy() here? I know it is currently being
> called if list_lru_init_memcg() fails, but I fail to understand why. It
> seems like list_lru_destroy() will do nothing anyway.

Right, it's not needed to call list_lru_destroy() here, it should do nothing,
will delete it.

Thanks!

> 
>> +	shrinker_free(zswap.shrinker);
>> +shrinker_fail:
>> +	destroy_workqueue(shrink_wq);
>>  hp_fail:
>>  	kmem_cache_destroy(zswap_entry_cache);
>>  cache_fail:
>>
>> -- 
>> b4 0.10.1

Re: [PATCH 2/2] mm/zswap: change zswap_pool kref to percpu_ref

[Chengming Zhou]

On 2024/2/13 02:53, Nhat Pham wrote:
> On Mon, Feb 12, 2024 at 5:29 AM Chengming Zhou
> <zhouchengming@bytedance.com> wrote:
>>
>> On 2024/2/12 05:21, Nhat Pham wrote:
>>> On Sun, Feb 11, 2024 at 5:58 AM Chengming Zhou
>>> <zhouchengming@bytedance.com> wrote:
>>>>
>>>> All zswap entries will take a reference of zswap_pool when
>>>> zswap_store(), and drop it when free. Change it to use the
>>>> percpu_ref is better for scalability performance.
>>>>
>>>> Testing kernel build in tmpfs with memory.max=2GB
>>>> (zswap shrinker and writeback enabled with one 50GB swapfile).
>>>>
>>>>         mm-unstable  zswap-global-lru
>>>> real    63.20        63.12
>>>> user    1061.75      1062.95
>>>> sys     268.74       264.44
>>>>
>>>> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
>>>> ---
>>>>  mm/zswap.c | 30 +++++++++++++++++++++---------
>>>>  1 file changed, 21 insertions(+), 9 deletions(-)
>>>>
>>>> diff --git a/mm/zswap.c b/mm/zswap.c
>>>> index 7668db8c10e3..afb31904fb08 100644
>>>> --- a/mm/zswap.c
>>>> +++ b/mm/zswap.c
>>>> @@ -173,7 +173,7 @@ struct crypto_acomp_ctx {
>>>>  struct zswap_pool {
>>>>         struct zpool *zpools[ZSWAP_NR_ZPOOLS];
>>>>         struct crypto_acomp_ctx __percpu *acomp_ctx;
>>>> -       struct kref kref;
>>>> +       struct percpu_ref ref;
>>>>         struct list_head list;
>>>>         struct work_struct release_work;
>>>>         struct hlist_node node;
>>>> @@ -303,6 +303,7 @@ static void zswap_update_total_size(void)
>>>>  /*********************************
>>>>  * pool functions
>>>>  **********************************/
>>>> +static void __zswap_pool_empty(struct percpu_ref *ref);
>>>>
>>>>  static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
>>>>  {
>>>> @@ -356,13 +357,18 @@ static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
>>>>         /* being the current pool takes 1 ref; this func expects the
>>>>          * caller to always add the new pool as the current pool
>>>>          */
>>>> -       kref_init(&pool->kref);
>>>> +       ret = percpu_ref_init(&pool->ref, __zswap_pool_empty,
>>>> +                             PERCPU_REF_ALLOW_REINIT, GFP_KERNEL);
>>>> +       if (ret)
>>>> +               goto ref_fail;
>>>>         INIT_LIST_HEAD(&pool->list);
>>>>
>>>>         zswap_pool_debug("created", pool);
>>>>
>>>>         return pool;
>>>>
>>>> +ref_fail:
>>>> +       cpuhp_state_remove_instance(CPUHP_MM_ZSWP_POOL_PREPARE, &pool->node);
>>>>  error:
>>>>         if (pool->acomp_ctx)
>>>>                 free_percpu(pool->acomp_ctx);
>>>> @@ -435,8 +441,8 @@ static void __zswap_pool_release(struct work_struct *work)
>>>>
>>>>         synchronize_rcu();
>>>>
>>>> -       /* nobody should have been able to get a kref... */
>>>> -       WARN_ON(kref_get_unless_zero(&pool->kref));
>>>
>>> Do we no longer care about this WARN? IIUC, this is to catch someone
>>> still holding a reference to the pool at release time, which sounds
>>> like a bug. I think we can simulate the similar behavior with:
>>
>> Ok, I thought it has already been put to 0 when we're here, so any tryget
>> will fail. But keeping this WARN_ON() is also fine to me, will keep it.
> 
> Yup - it should fail, if the code is not buggy. But that's a pretty big if :)
> 
> Jokes aside, we can remove it if folks think the benefit is not worth
> the cost/overhead. However, I'm a bit hesitant to remove checks in
> zswap, especially given how buggy it has been (some of which are
> refcnt bugs as well, IIRC).

Yes, agree. It looks clearer to keep it, which should be no cost at all.

Thanks!

Re: [PATCH 2/2] mm/zswap: change zswap_pool kref to percpu_ref

[Chengming Zhou]

On 2024/2/13 06:42, Yosry Ahmed wrote:
> On Sun, Feb 11, 2024 at 5:58 AM Chengming Zhou
> <zhouchengming@bytedance.com> wrote:
>>
>> All zswap entries will take a reference of zswap_pool when
>> zswap_store(), and drop it when free. Change it to use the
>> percpu_ref is better for scalability performance.
>>
>> Testing kernel build in tmpfs with memory.max=2GB
>> (zswap shrinker and writeback enabled with one 50GB swapfile).
>>
>>         mm-unstable  zswap-global-lru
>> real    63.20        63.12
>> user    1061.75      1062.95
>> sys     268.74       264.44
> 
> Are these numbers from a single run or the average of multiple runs?

The average of 5 runs. And I just checked/compared each run result,
the improvement is stable. So yes, it should be a real performance gain.

> It just seems that the improvement is small, and percpu refcnt is
> slightly less intuitive (and uses a bit more memory), so let's make
> sure there is a real performance gain first.

Right, percpu_ref use a bit more memory which should be ok for our use case,
since we almost have only one zswap_pool to be using. The performance gain is
for zswap_store/load hotpath.

> 
> It would also be useful to mention how many threads/CPUs are being used here.

My bad, the testing uses 32 threads on a 128 CPUs x86-64 machine.

Thanks.

Re: [PATCH 1/2] mm/zswap: global lru and shrinker shared by all zswap_pools

[Yosry Ahmed]

> >> @@ -353,30 +353,16 @@ static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
> >>  	if (ret)
> >>  		goto error;
> >>  
> >> -	zswap_alloc_shrinker(pool);
> >> -	if (!pool->shrinker)
> >> -		goto error;
> >> -
> >> -	pr_debug("using %s compressor\n", pool->tfm_name);
> >> -
> > 
> > Why are we removing this debug print?

This pr_debug() was introduced when dynamic zswap pools were introduced,
and it was supposed to be printed right after the compressor is
initialized. IOW, it is supposed to be after the call to
cpuhp_state_add_instance() succeeds. The call to zswap_alloc_shrinker()
was mistakenly added above that pr_debug() call.

Anyway, I just realized you are now removing all failure cases between
than pr_debug() and the zswap_pool_debug() below, so there is no need to
keep both. You are right.

I am wondering if these debug prints are useful at all now, but that's a
question for another day :)

Re: [PATCH 2/2] mm/zswap: change zswap_pool kref to percpu_ref

[Yosry Ahmed]

On Tue, Feb 13, 2024 at 10:31:16PM +0800, Chengming Zhou wrote:
> On 2024/2/13 06:42, Yosry Ahmed wrote:
> > On Sun, Feb 11, 2024 at 5:58 AM Chengming Zhou
> > <zhouchengming@bytedance.com> wrote:
> >>
> >> All zswap entries will take a reference of zswap_pool when
> >> zswap_store(), and drop it when free. Change it to use the
> >> percpu_ref is better for scalability performance.
> >>
> >> Testing kernel build in tmpfs with memory.max=2GB
> >> (zswap shrinker and writeback enabled with one 50GB swapfile).
> >>
> >>         mm-unstable  zswap-global-lru
> >> real    63.20        63.12
> >> user    1061.75      1062.95
> >> sys     268.74       264.44
> > 
> > Are these numbers from a single run or the average of multiple runs?
> 
> The average of 5 runs. And I just checked/compared each run result,
> the improvement is stable. So yes, it should be a real performance gain.
> 
> > It just seems that the improvement is small, and percpu refcnt is
> > slightly less intuitive (and uses a bit more memory), so let's make
> > sure there is a real performance gain first.
> 
> Right, percpu_ref use a bit more memory which should be ok for our use case,
> since we almost have only one zswap_pool to be using. The performance gain is
> for zswap_store/load hotpath.
> 
> > 
> > It would also be useful to mention how many threads/CPUs are being used here.
> 
> My bad, the testing uses 32 threads on a 128 CPUs x86-64 machine.

Thanks for the clarification. Please include such details in the commit
message.