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

[Chengming Zhou <zhouchengming@bytedance.com>]

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!