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Message-ID: <Zc0eJ84FeR9yQ99T@google.com>
Date: Wed, 14 Feb 2024 20:10:15 +0000
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Chengming Zhou <zhouchengming@...edance.com>
Cc: Johannes Weiner <hannes@...xchg.org>, Andrew Morton <akpm@...ux-foundation.org>,
Nhat Pham <nphamcs@...il.com>, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] mm/zswap: change zswap_pool kref to percpu_ref
On Wed, Feb 14, 2024 at 08:54:38AM +0000, Chengming Zhou 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.
>
> Although 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.
>
> Testing kernel build (32 threads) in tmpfs with memory.max=2GB.
> (zswap shrinker and writeback enabled with one 50GB swapfile,
> on a 128 CPUs x86-64 machine, below is the average of 5 runs)
>
> 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@...edance.com>
> ---
> mm/zswap.c | 31 ++++++++++++++++++++++---------
> 1 file changed, 22 insertions(+), 9 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index dbff67d7e1c7..f6470d30d337 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;
> @@ -304,6 +304,7 @@ 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)
> {
> @@ -357,13 +358,18 @@ 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);
> @@ -436,8 +442,9 @@ 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... */
> + WARN_ON(percpu_ref_tryget(&pool->ref));
Just curious, was there any value from using kref_get_unless_zero() over
kref_read() here? If not, I think percpu_ref_is_zero() is more
intuitive. This also seems like it fits more as a debug check.
> + percpu_ref_exit(&pool->ref);
>
> /* pool is now off zswap_pools list and has no references. */
> zswap_pool_destroy(pool);
> @@ -445,11 +452,11 @@ 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);
>
> @@ -468,12 +475,12 @@ 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)
> @@ -603,6 +610,12 @@ 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);
I think this is not very clear. The previous code relied on the ref from
zswap_pool_find_get() to replace the initial ref that we had dropped
before. This is not needed with percpu_ref_resurrect() because it
already restores the initial ref dropped by percpu_ref_kill().
Perhaps something like:
/*
* Restore the initial ref dropped by percpu_ref_kill()
* when the pool was decommissioned and switch it again
* to percpu mode.
/
, or am I overthinking this?
> + /* Drop the ref from zswap_pool_find_get(). */
> + zswap_pool_put(pool);
> + }
>
> if (pool)
> ret = param_set_charp(s, kp);
> @@ -641,7 +654,7 @@ 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
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