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Message-ID: <900cd5da-da96-4107-b5f0-c7d975a8ba97@bytedance.com>
Date: Mon, 12 Feb 2024 21:29:53 +0800
From: Chengming Zhou <zhouchengming@...edance.com>
To: Nhat Pham <nphamcs@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Johannes Weiner <hannes@...xchg.org>, Yosry Ahmed <yosryahmed@...gle.com>,
linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm/zswap: change zswap_pool kref to percpu_ref
On 2024/2/12 05:21, Nhat Pham wrote:
> On Sun, Feb 11, 2024 at 5:58 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.
>>
>> 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@...edance.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.
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