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Message-ID: <ZctnRnNMOwQNn_3j@google.com>
Date: Tue, 13 Feb 2024 12:57:42 +0000
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Chengming Zhou <zhouchengming@...edance.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Johannes Weiner <hannes@...xchg.org>,
Nhat Pham <nphamcs@...il.com>, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm/zswap: global lru and shrinker shared by all zswap_pools
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@...edance.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
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