Re: [PATCH v2 2/2] mm/zswap: change zswap_pool kref to percpu_ref

[Chengming Zhou <zhouchengming@bytedance.com>]

On 2024/2/15 04:10, Yosry Ahmed wrote:
> 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@bytedance.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.

Agree, percpu_ref_is_zero() is better for debug.

> 
>> +	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.
> 		 /
> 

Ok, will add this comment, it's clearer.

Thanks!