On Sun, Feb 11, 2024 at 5:58 AM Chengming Zhou
<[email protected]> 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 <[email protected]>
> ---
> 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:
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.
On 2024/2/12 05:21, Nhat Pham wrote:
> On Sun, Feb 11, 2024 at 5:58 AM Chengming Zhou
> <[email protected]> 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 <[email protected]>
>> ---
>> 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.
On Mon, Feb 12, 2024 at 5:29 AM Chengming Zhou
<[email protected]> wrote:
>
> On 2024/2/12 05:21, Nhat Pham wrote:
> > On Sun, Feb 11, 2024 at 5:58 AM Chengming Zhou
> > <[email protected]> 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 <[email protected]>
> >> ---
> >> 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).
On 2024/2/13 02:53, Nhat Pham wrote:
> On Mon, Feb 12, 2024 at 5:29 AM Chengming Zhou
> <[email protected]> wrote:
>>
>> On 2024/2/12 05:21, Nhat Pham wrote:
>>> On Sun, Feb 11, 2024 at 5:58 AM Chengming Zhou
>>> <[email protected]> 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 <[email protected]>
>>>> ---
>>>> 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!