Date: Fri, 16 Feb 2024 02:41:41 -0800
From: Nhat Pham <nphamcs@...il.com>
To: Chengming Zhou <zhouchengming@...edance.com>
Cc: Johannes Weiner <hannes@...xchg.org>, Yosry Ahmed <yosryahmed@...gle.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/2] mm/zswap: change zswap_pool kref to percpu_ref

On Fri, Feb 16, 2024 at 12:55 AM Chengming Zhou
<zhouchengming@...edance.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.
>
> 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

Idea is straightforward + code looks solid to me FWIW, so:
Reviewed-by: Nhat Pham <nphamcs@...il.com>

>
> Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
> ---
>  mm/zswap.c | 36 +++++++++++++++++++++++++++---------
>  1 file changed, 27 insertions(+), 9 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index d275eb523fc4..961349162997 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;
> @@ -305,6 +305,7 @@ 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)
>  {
> @@ -358,13 +359,18 @@ 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);
> @@ -437,8 +443,9 @@ 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_is_zero(&pool->ref));

Ah nice - this is actually even clearer :) For some reason I missed
it, my apologies.

> +       percpu_ref_exit(&pool->ref);
>
>         /* pool is now off zswap_pools list and has no references. */
>         zswap_pool_destroy(pool);
> @@ -446,11 +453,11 @@ 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);
>
> @@ -469,12 +476,12 @@ 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)
> @@ -604,6 +611,17 @@ static int __zswap_param_set(const char *val, const struct kernel_param *kp,
>
>         if (!pool)
>                 pool = zswap_pool_create(type, compressor);
> +       else {
> +               /*
> +                * Restore the initial ref dropped by percpu_ref_kill()
> +                * when the pool was decommissioned and switch it again
> +                * 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);
> @@ -642,7 +660,7 @@ 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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