[v8,6/6] zswap: shrinks zswap pool based on memory pressure

  Currently, we only shrink the zswap pool when the user-defined limit is
hit. This means that if we set the limit too high, cold data that are
unlikely to be used again will reside in the pool, wasting precious
memory. It is hard to predict how much zswap space will be needed ahead
of time, as this depends on the workload (specifically, on factors such
as memory access patterns and compressibility of the memory pages).

This patch implements a memcg- and NUMA-aware shrinker for zswap, that
is initiated when there is memory pressure. The shrinker does not
have any parameter that must be tuned by the user, and can be opted in
or out on a per-memcg basis.

Furthermore, to make it more robust for many workloads and prevent
overshrinking (i.e evicting warm pages that might be refaulted into
memory), we build in the following heuristics:

* Estimate the number of warm pages residing in zswap, and attempt to
  protect this region of the zswap LRU.
* Scale the number of freeable objects by an estimate of the memory
  saving factor. The better zswap compresses the data, the fewer pages
  we will evict to swap (as we will otherwise incur IO for relatively
  small memory saving).
* During reclaim, if the shrinker encounters a page that is also being
  brought into memory, the shrinker will cautiously terminate its
  shrinking action, as this is a sign that it is touching the warmer
  region of the zswap LRU.

As a proof of concept, we ran the following synthetic benchmark:
build the linux kernel in a memory-limited cgroup, and allocate some
cold data in tmpfs to see if the shrinker could write them out and
improved the overall performance. Depending on the amount of cold data
generated, we observe from 14% to 35% reduction in kernel CPU time used
in the kernel builds.

Signed-off-by: Nhat Pham <nphamcs@gmail.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
---
 Documentation/admin-guide/mm/zswap.rst |  10 ++
 include/linux/mmzone.h                 |   2 +
 include/linux/zswap.h                  |  25 +++-
 mm/Kconfig                             |  14 ++
 mm/mmzone.c                            |   1 +
 mm/swap_state.c                        |   2 +
 mm/zswap.c                             | 185 ++++++++++++++++++++++++-
 7 files changed, 233 insertions(+), 6 deletions(-)
  

On 2023/12/1 03:40, Nhat Pham wrote:
> Currently, we only shrink the zswap pool when the user-defined limit is
> hit. This means that if we set the limit too high, cold data that are
> unlikely to be used again will reside in the pool, wasting precious
> memory. It is hard to predict how much zswap space will be needed ahead
> of time, as this depends on the workload (specifically, on factors such
> as memory access patterns and compressibility of the memory pages).
> 
> This patch implements a memcg- and NUMA-aware shrinker for zswap, that
> is initiated when there is memory pressure. The shrinker does not
> have any parameter that must be tuned by the user, and can be opted in
> or out on a per-memcg basis.
> 
> Furthermore, to make it more robust for many workloads and prevent
> overshrinking (i.e evicting warm pages that might be refaulted into
> memory), we build in the following heuristics:
> 
> * Estimate the number of warm pages residing in zswap, and attempt to
>   protect this region of the zswap LRU.
> * Scale the number of freeable objects by an estimate of the memory
>   saving factor. The better zswap compresses the data, the fewer pages
>   we will evict to swap (as we will otherwise incur IO for relatively
>   small memory saving).
> * During reclaim, if the shrinker encounters a page that is also being
>   brought into memory, the shrinker will cautiously terminate its
>   shrinking action, as this is a sign that it is touching the warmer
>   region of the zswap LRU.
> 
> As a proof of concept, we ran the following synthetic benchmark:
> build the linux kernel in a memory-limited cgroup, and allocate some
> cold data in tmpfs to see if the shrinker could write them out and
> improved the overall performance. Depending on the amount of cold data
> generated, we observe from 14% to 35% reduction in kernel CPU time used
> in the kernel builds.
> 
> Signed-off-by: Nhat Pham <nphamcs@gmail.com>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> ---
>  Documentation/admin-guide/mm/zswap.rst |  10 ++
>  include/linux/mmzone.h                 |   2 +
>  include/linux/zswap.h                  |  25 +++-
>  mm/Kconfig                             |  14 ++
>  mm/mmzone.c                            |   1 +
>  mm/swap_state.c                        |   2 +
>  mm/zswap.c                             | 185 ++++++++++++++++++++++++-
>  7 files changed, 233 insertions(+), 6 deletions(-)
> 
> diff --git a/Documentation/admin-guide/mm/zswap.rst b/Documentation/admin-guide/mm/zswap.rst
> index 45b98390e938..62fc244ec702 100644
> --- a/Documentation/admin-guide/mm/zswap.rst
> +++ b/Documentation/admin-guide/mm/zswap.rst
> @@ -153,6 +153,16 @@ attribute, e. g.::
>  
>  Setting this parameter to 100 will disable the hysteresis.
>  
> +When there is a sizable amount of cold memory residing in the zswap pool, it
> +can be advantageous to proactively write these cold pages to swap and reclaim
> +the memory for other use cases. By default, the zswap shrinker is disabled.
> +User can enable it as follows:
> +
> +  echo Y > /sys/module/zswap/parameters/shrinker_enabled
> +
> +This can be enabled at the boot time if ``CONFIG_ZSWAP_SHRINKER_DEFAULT_ON`` is
> +selected.
> +
>  A debugfs interface is provided for various statistic about pool size, number
>  of pages stored, same-value filled pages and various counters for the reasons
>  pages are rejected.
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 7b1816450bfc..b23bc5390240 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -22,6 +22,7 @@
>  #include <linux/mm_types.h>
>  #include <linux/page-flags.h>
>  #include <linux/local_lock.h>
> +#include <linux/zswap.h>
>  #include <asm/page.h>
>  
>  /* Free memory management - zoned buddy allocator.  */
> @@ -641,6 +642,7 @@ struct lruvec {
>  #ifdef CONFIG_MEMCG
>  	struct pglist_data *pgdat;
>  #endif
> +	struct zswap_lruvec_state zswap_lruvec_state;
>  };
>  
>  /* Isolate for asynchronous migration */
> diff --git a/include/linux/zswap.h b/include/linux/zswap.h
> index e571e393669b..08c240e16a01 100644
> --- a/include/linux/zswap.h
> +++ b/include/linux/zswap.h
> @@ -5,20 +5,40 @@
>  #include <linux/types.h>
>  #include <linux/mm_types.h>
>  
> +struct lruvec;
> +
>  extern u64 zswap_pool_total_size;
>  extern atomic_t zswap_stored_pages;
>  
>  #ifdef CONFIG_ZSWAP
>  
> +struct zswap_lruvec_state {
> +	/*
> +	 * Number of pages in zswap that should be protected from the shrinker.
> +	 * This number is an estimate of the following counts:
> +	 *
> +	 * a) Recent page faults.
> +	 * b) Recent insertion to the zswap LRU. This includes new zswap stores,
> +	 *    as well as recent zswap LRU rotations.
> +	 *
> +	 * These pages are likely to be warm, and might incur IO if the are written
> +	 * to swap.
> +	 */
> +	atomic_long_t nr_zswap_protected;
> +};
> +
>  bool zswap_store(struct folio *folio);
>  bool zswap_load(struct folio *folio);
>  void zswap_invalidate(int type, pgoff_t offset);
>  void zswap_swapon(int type);
>  void zswap_swapoff(int type);
>  void zswap_memcg_offline_cleanup(struct mem_cgroup *memcg);
> -
> +void zswap_lruvec_state_init(struct lruvec *lruvec);
> +void zswap_page_swapin(struct page *page);
>  #else
>  
> +struct zswap_lruvec_state {};
> +
>  static inline bool zswap_store(struct folio *folio)
>  {
>  	return false;
> @@ -33,7 +53,8 @@ static inline void zswap_invalidate(int type, pgoff_t offset) {}
>  static inline void zswap_swapon(int type) {}
>  static inline void zswap_swapoff(int type) {}
>  static inline void zswap_memcg_offline_cleanup(struct mem_cgroup *memcg) {}
> -
> +static inline void zswap_lruvec_state_init(struct lruvec *lruvec) {}
> +static inline void zswap_page_swapin(struct page *page) {}
>  #endif
>  
>  #endif /* _LINUX_ZSWAP_H */
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 57cd378c73d6..ca87cdb72f11 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -61,6 +61,20 @@ config ZSWAP_EXCLUSIVE_LOADS_DEFAULT_ON
>  	  The cost is that if the page was never dirtied and needs to be
>  	  swapped out again, it will be re-compressed.
>  
> +config ZSWAP_SHRINKER_DEFAULT_ON
> +	bool "Shrink the zswap pool on memory pressure"
> +	depends on ZSWAP
> +	default n
> +	help
> +	  If selected, the zswap shrinker will be enabled, and the pages
> +	  stored in the zswap pool will become available for reclaim (i.e
> +	  written back to the backing swap device) on memory pressure.
> +
> +	  This means that zswap writeback could happen even if the pool is
> +	  not yet full, or the cgroup zswap limit has not been reached,
> +	  reducing the chance that cold pages will reside in the zswap pool
> +	  and consume memory indefinitely.
> +
>  choice
>  	prompt "Default compressor"
>  	depends on ZSWAP
> diff --git a/mm/mmzone.c b/mm/mmzone.c
> index b594d3f268fe..c01896eca736 100644
> --- a/mm/mmzone.c
> +++ b/mm/mmzone.c
> @@ -78,6 +78,7 @@ void lruvec_init(struct lruvec *lruvec)
>  
>  	memset(lruvec, 0, sizeof(struct lruvec));
>  	spin_lock_init(&lruvec->lru_lock);
> +	zswap_lruvec_state_init(lruvec);
>  
>  	for_each_lru(lru)
>  		INIT_LIST_HEAD(&lruvec->lists[lru]);
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index 6c84236382f3..c597cec606e4 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -687,6 +687,7 @@ struct page *swap_cluster_readahead(swp_entry_t entry, gfp_t gfp_mask,
>  					&page_allocated, false);
>  	if (unlikely(page_allocated))
>  		swap_readpage(page, false, NULL);
> +	zswap_page_swapin(page);
>  	return page;
>  }
>  
> @@ -862,6 +863,7 @@ static struct page *swap_vma_readahead(swp_entry_t targ_entry, gfp_t gfp_mask,
>  					&page_allocated, false);
>  	if (unlikely(page_allocated))
>  		swap_readpage(page, false, NULL);
> +	zswap_page_swapin(page);
>  	return page;
>  }
>  
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 49b79393e472..0f086ffd7b6a 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -148,6 +148,11 @@ module_param_named(exclusive_loads, zswap_exclusive_loads_enabled, bool, 0644);
>  /* Number of zpools in zswap_pool (empirically determined for scalability) */
>  #define ZSWAP_NR_ZPOOLS 32
>  
> +/* Enable/disable memory pressure-based shrinker. */
> +static bool zswap_shrinker_enabled = IS_ENABLED(
> +		CONFIG_ZSWAP_SHRINKER_DEFAULT_ON);
> +module_param_named(shrinker_enabled, zswap_shrinker_enabled, bool, 0644);
> +
>  /*********************************
>  * data structures
>  **********************************/
> @@ -177,6 +182,8 @@ struct zswap_pool {
>  	char tfm_name[CRYPTO_MAX_ALG_NAME];
>  	struct list_lru list_lru;
>  	struct mem_cgroup *next_shrink;
> +	struct shrinker *shrinker;
> +	atomic_t nr_stored;
>  };
>  
>  /*
> @@ -275,17 +282,26 @@ static bool zswap_can_accept(void)
>  			DIV_ROUND_UP(zswap_pool_total_size, PAGE_SIZE);
>  }
>  
> +static u64 get_zswap_pool_size(struct zswap_pool *pool)
> +{
> +	u64 pool_size = 0;
> +	int i;
> +
> +	for (i = 0; i < ZSWAP_NR_ZPOOLS; i++)
> +		pool_size += zpool_get_total_size(pool->zpools[i]);
> +
> +	return pool_size;
> +}
> +
>  static void zswap_update_total_size(void)
>  {
>  	struct zswap_pool *pool;
>  	u64 total = 0;
> -	int i;
>  
>  	rcu_read_lock();
>  
>  	list_for_each_entry_rcu(pool, &zswap_pools, list)
> -		for (i = 0; i < ZSWAP_NR_ZPOOLS; i++)
> -			total += zpool_get_total_size(pool->zpools[i]);
> +		total += get_zswap_pool_size(pool);
>  
>  	rcu_read_unlock();
>  
> @@ -344,13 +360,34 @@ static void zswap_entry_cache_free(struct zswap_entry *entry)
>  	kmem_cache_free(zswap_entry_cache, entry);
>  }
>  
> +/*********************************
> +* zswap lruvec functions
> +**********************************/
> +void zswap_lruvec_state_init(struct lruvec *lruvec)
> +{
> +	atomic_long_set(&lruvec->zswap_lruvec_state.nr_zswap_protected, 0);
> +}
> +
> +void zswap_page_swapin(struct page *page)
> +{
> +	struct lruvec *lruvec;
> +
> +	if (page) {
> +		lruvec = folio_lruvec(page_folio(page));
> +		atomic_long_inc(&lruvec->zswap_lruvec_state.nr_zswap_protected);
> +	}
> +}
> +
>  /*********************************
>  * lru functions
>  **********************************/
>  static void zswap_lru_add(struct list_lru *list_lru, struct zswap_entry *entry)
>  {
> +	atomic_long_t *nr_zswap_protected;
> +	unsigned long lru_size, old, new;
>  	int nid = entry_to_nid(entry);
>  	struct mem_cgroup *memcg;
> +	struct lruvec *lruvec;
>  
>  	/*
>  	 * Note that it is safe to use rcu_read_lock() here, even in the face of
> @@ -368,6 +405,19 @@ static void zswap_lru_add(struct list_lru *list_lru, struct zswap_entry *entry)
>  	memcg = mem_cgroup_from_entry(entry);
>  	/* will always succeed */
>  	list_lru_add(list_lru, &entry->lru, nid, memcg);
> +
> +	/* Update the protection area */
> +	lru_size = list_lru_count_one(list_lru, nid, memcg);
> +	lruvec = mem_cgroup_lruvec(memcg, NODE_DATA(nid));
> +	nr_zswap_protected = &lruvec->zswap_lruvec_state.nr_zswap_protected;
> +	old = atomic_long_inc_return(nr_zswap_protected);
> +	/*
> +	 * Decay to avoid overflow and adapt to changing workloads.
> +	 * This is based on LRU reclaim cost decaying heuristics.
> +	 */
> +	do {
> +		new = old > lru_size / 4 ? old / 2 : old;
> +	} while (!atomic_long_try_cmpxchg(nr_zswap_protected, &old, new));
>  	rcu_read_unlock();
>  }
>  
> @@ -389,6 +439,7 @@ static void zswap_lru_putback(struct list_lru *list_lru,
>  	int nid = entry_to_nid(entry);
>  	spinlock_t *lock = &list_lru->node[nid].lock;
>  	struct mem_cgroup *memcg;
> +	struct lruvec *lruvec;
>  
>  	rcu_read_lock();
>  	memcg = mem_cgroup_from_entry(entry);
> @@ -396,6 +447,10 @@ static void zswap_lru_putback(struct list_lru *list_lru,
>  	/* we cannot use list_lru_add here, because it increments node's lru count */
>  	list_lru_putback(list_lru, &entry->lru, nid, memcg);
>  	spin_unlock(lock);
> +
> +	lruvec = mem_cgroup_lruvec(memcg, NODE_DATA(entry_to_nid(entry)));
> +	/* increment the protection area to account for the LRU rotation. */
> +	atomic_long_inc(&lruvec->zswap_lruvec_state.nr_zswap_protected);
>  	rcu_read_unlock();
>  }
>  
> @@ -485,6 +540,7 @@ static void zswap_free_entry(struct zswap_entry *entry)
>  	else {
>  		zswap_lru_del(&entry->pool->list_lru, entry);
>  		zpool_free(zswap_find_zpool(entry), entry->handle);
> +		atomic_dec(&entry->pool->nr_stored);
>  		zswap_pool_put(entry->pool);
>  	}
>  	zswap_entry_cache_free(entry);
> @@ -526,6 +582,102 @@ static struct zswap_entry *zswap_entry_find_get(struct rb_root *root,
>  	return entry;
>  }
>  
> +/*********************************
> +* shrinker functions
> +**********************************/
> +static enum lru_status shrink_memcg_cb(struct list_head *item, struct list_lru_one *l,
> +				       spinlock_t *lock, void *arg);
> +
> +static unsigned long zswap_shrinker_scan(struct shrinker *shrinker,
> +		struct shrink_control *sc)
> +{
> +	struct lruvec *lruvec = mem_cgroup_lruvec(sc->memcg, NODE_DATA(sc->nid));
> +	unsigned long shrink_ret, nr_protected, lru_size;
> +	struct zswap_pool *pool = shrinker->private_data;
> +	bool encountered_page_in_swapcache = false;
> +
> +	nr_protected =
> +		atomic_long_read(&lruvec->zswap_lruvec_state.nr_zswap_protected);
> +	lru_size = list_lru_shrink_count(&pool->list_lru, sc);
> +
> +	/*
> +	 * Abort if the shrinker is disabled or if we are shrinking into the
> +	 * protected region.
> +	 *
> +	 * This short-circuiting is necessary because if we have too many multiple
> +	 * concurrent reclaimers getting the freeable zswap object counts at the
> +	 * same time (before any of them made reasonable progress), the total
> +	 * number of reclaimed objects might be more than the number of unprotected
> +	 * objects (i.e the reclaimers will reclaim into the protected area of the
> +	 * zswap LRU).
> +	 */
> +	if (!zswap_shrinker_enabled || nr_protected >= lru_size - sc->nr_to_scan) {
> +		sc->nr_scanned = 0;
> +		return SHRINK_STOP;
> +	}
> +
> +	shrink_ret = list_lru_shrink_walk(&pool->list_lru, sc, &shrink_memcg_cb,
> +		&encountered_page_in_swapcache);
> +
> +	if (encountered_page_in_swapcache)
> +		return SHRINK_STOP;
> +
> +	return shrink_ret ? shrink_ret : SHRINK_STOP;
> +}
> +
> +static unsigned long zswap_shrinker_count(struct shrinker *shrinker,
> +		struct shrink_control *sc)
> +{
> +	struct zswap_pool *pool = shrinker->private_data;
> +	struct mem_cgroup *memcg = sc->memcg;
> +	struct lruvec *lruvec = mem_cgroup_lruvec(memcg, NODE_DATA(sc->nid));
> +	unsigned long nr_backing, nr_stored, nr_freeable, nr_protected;
> +
> +#ifdef CONFIG_MEMCG_KMEM
> +	cgroup_rstat_flush(memcg->css.cgroup);
> +	nr_backing = memcg_page_state(memcg, MEMCG_ZSWAP_B) >> PAGE_SHIFT;
> +	nr_stored = memcg_page_state(memcg, MEMCG_ZSWAPPED);
> +#else
> +	/* use pool stats instead of memcg stats */
> +	nr_backing = get_zswap_pool_size(pool) >> PAGE_SHIFT;
> +	nr_stored = atomic_read(&pool->nr_stored);
> +#endif
> +
> +	if (!zswap_shrinker_enabled || !nr_stored)
When I tested with this series, with !zswap_shrinker_enabled in the default case,
I found the performance is much worse than that without this patch.

Testcase: memory.max=2G, zswap enabled, kernel build -j32 in a tmpfs directory.

The reason seems the above cgroup_rstat_flush(), caused much rstat lock contention
to the zswap_store() path. And if I put the "zswap_shrinker_enabled" check above
the cgroup_rstat_flush(), the performance become much better.

Maybe we can put the "zswap_shrinker_enabled" check above cgroup_rstat_flush()?

Thanks!

> +		return 0;
> +
> +	nr_protected =
> +		atomic_long_read(&lruvec->zswap_lruvec_state.nr_zswap_protected);
> +	nr_freeable = list_lru_shrink_count(&pool->list_lru, sc);
> +	/*
> +	 * Subtract the lru size by an estimate of the number of pages
> +	 * that should be protected.
> +	 */
> +	nr_freeable = nr_freeable > nr_protected ? nr_freeable - nr_protected : 0;
> +
> +	/*
> +	 * Scale the number of freeable pages by the memory saving factor.
> +	 * This ensures that the better zswap compresses memory, the fewer
> +	 * pages we will evict to swap (as it will otherwise incur IO for
> +	 * relatively small memory saving).
> +	 */
> +	return mult_frac(nr_freeable, nr_backing, nr_stored);
> +}
> +
> +static void zswap_alloc_shrinker(struct zswap_pool *pool)
> +{
> +	pool->shrinker =
> +		shrinker_alloc(SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE, "mm-zswap");
> +	if (!pool->shrinker)
> +		return;
> +
> +	pool->shrinker->private_data = pool;
> +	pool->shrinker->scan_objects = zswap_shrinker_scan;
> +	pool->shrinker->count_objects = zswap_shrinker_count;
> +	pool->shrinker->batch = 0;
> +	pool->shrinker->seeks = DEFAULT_SEEKS;
> +}
> +
>  /*********************************
>  * per-cpu code
>  **********************************/
> @@ -721,6 +873,7 @@ static enum lru_status shrink_memcg_cb(struct list_head *item, struct list_lru_o
>  				       spinlock_t *lock, void *arg)
>  {
>  	struct zswap_entry *entry = container_of(item, struct zswap_entry, lru);
> +	bool *encountered_page_in_swapcache = (bool *)arg;
>  	struct zswap_tree *tree;
>  	pgoff_t swpoffset;
>  	enum lru_status ret = LRU_REMOVED_RETRY;
> @@ -756,6 +909,17 @@ static enum lru_status shrink_memcg_cb(struct list_head *item, struct list_lru_o
>  		zswap_reject_reclaim_fail++;
>  		zswap_lru_putback(&entry->pool->list_lru, entry);
>  		ret = LRU_RETRY;
> +
> +		/*
> +		 * Encountering a page already in swap cache is a sign that we are shrinking
> +		 * into the warmer region. We should terminate shrinking (if we're in the dynamic
> +		 * shrinker context).
> +		 */
> +		if (writeback_result == -EEXIST && encountered_page_in_swapcache) {
> +			ret = LRU_SKIP;
> +			*encountered_page_in_swapcache = true;
> +		}
> +
>  		goto put_unlock;
>  	}
>  	zswap_written_back_pages++;
> @@ -913,6 +1077,11 @@ static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
>  				       &pool->node);
>  	if (ret)
>  		goto error;
> +
> +	zswap_alloc_shrinker(pool);
> +	if (!pool->shrinker)
> +		goto error;
> +
>  	pr_debug("using %s compressor\n", pool->tfm_name);
>  
>  	/* being the current pool takes 1 ref; this func expects the
> @@ -920,13 +1089,19 @@ static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
>  	 */
>  	kref_init(&pool->kref);
>  	INIT_LIST_HEAD(&pool->list);
> -	list_lru_init_memcg(&pool->list_lru, NULL);
> +	if (list_lru_init_memcg(&pool->list_lru, pool->shrinker))
> +		goto lru_fail;
> +	shrinker_register(pool->shrinker);
>  	INIT_WORK(&pool->shrink_work, shrink_worker);
> +	atomic_set(&pool->nr_stored, 0);
>  
>  	zswap_pool_debug("created", pool);
>  
>  	return pool;
>  
> +lru_fail:
> +	list_lru_destroy(&pool->list_lru);
> +	shrinker_free(pool->shrinker);
>  error:
>  	if (pool->acomp_ctx)
>  		free_percpu(pool->acomp_ctx);
> @@ -984,6 +1159,7 @@ static void zswap_pool_destroy(struct zswap_pool *pool)
>  
>  	zswap_pool_debug("destroying", pool);
>  
> +	shrinker_free(pool->shrinker);
>  	cpuhp_state_remove_instance(CPUHP_MM_ZSWP_POOL_PREPARE, &pool->node);
>  	free_percpu(pool->acomp_ctx);
>  	list_lru_destroy(&pool->list_lru);
> @@ -1540,6 +1716,7 @@ bool zswap_store(struct folio *folio)
>  	if (entry->length) {
>  		INIT_LIST_HEAD(&entry->lru);
>  		zswap_lru_add(&entry->pool->list_lru, entry);
> +		atomic_inc(&entry->pool->nr_stored);
>  	}
>  	spin_unlock(&tree->lock);
>
  

[..]
> > @@ -526,6 +582,102 @@ static struct zswap_entry *zswap_entry_find_get(struct rb_root *root,
> >       return entry;
> >  }
> >
> > +/*********************************
> > +* shrinker functions
> > +**********************************/
> > +static enum lru_status shrink_memcg_cb(struct list_head *item, struct list_lru_one *l,
> > +                                    spinlock_t *lock, void *arg);
> > +
> > +static unsigned long zswap_shrinker_scan(struct shrinker *shrinker,
> > +             struct shrink_control *sc)
> > +{
> > +     struct lruvec *lruvec = mem_cgroup_lruvec(sc->memcg, NODE_DATA(sc->nid));
> > +     unsigned long shrink_ret, nr_protected, lru_size;
> > +     struct zswap_pool *pool = shrinker->private_data;
> > +     bool encountered_page_in_swapcache = false;
> > +
> > +     nr_protected =
> > +             atomic_long_read(&lruvec->zswap_lruvec_state.nr_zswap_protected);
> > +     lru_size = list_lru_shrink_count(&pool->list_lru, sc);
> > +
> > +     /*
> > +      * Abort if the shrinker is disabled or if we are shrinking into the
> > +      * protected region.
> > +      *
> > +      * This short-circuiting is necessary because if we have too many multiple
> > +      * concurrent reclaimers getting the freeable zswap object counts at the
> > +      * same time (before any of them made reasonable progress), the total
> > +      * number of reclaimed objects might be more than the number of unprotected
> > +      * objects (i.e the reclaimers will reclaim into the protected area of the
> > +      * zswap LRU).
> > +      */
> > +     if (!zswap_shrinker_enabled || nr_protected >= lru_size - sc->nr_to_scan) {
> > +             sc->nr_scanned = 0;
> > +             return SHRINK_STOP;
> > +     }
> > +
> > +     shrink_ret = list_lru_shrink_walk(&pool->list_lru, sc, &shrink_memcg_cb,
> > +             &encountered_page_in_swapcache);
> > +
> > +     if (encountered_page_in_swapcache)
> > +             return SHRINK_STOP;
> > +
> > +     return shrink_ret ? shrink_ret : SHRINK_STOP;
> > +}
> > +
> > +static unsigned long zswap_shrinker_count(struct shrinker *shrinker,
> > +             struct shrink_control *sc)
> > +{
> > +     struct zswap_pool *pool = shrinker->private_data;
> > +     struct mem_cgroup *memcg = sc->memcg;
> > +     struct lruvec *lruvec = mem_cgroup_lruvec(memcg, NODE_DATA(sc->nid));
> > +     unsigned long nr_backing, nr_stored, nr_freeable, nr_protected;
> > +
> > +#ifdef CONFIG_MEMCG_KMEM
> > +     cgroup_rstat_flush(memcg->css.cgroup);
> > +     nr_backing = memcg_page_state(memcg, MEMCG_ZSWAP_B) >> PAGE_SHIFT;
> > +     nr_stored = memcg_page_state(memcg, MEMCG_ZSWAPPED);
> > +#else
> > +     /* use pool stats instead of memcg stats */
> > +     nr_backing = get_zswap_pool_size(pool) >> PAGE_SHIFT;
> > +     nr_stored = atomic_read(&pool->nr_stored);
> > +#endif
> > +
> > +     if (!zswap_shrinker_enabled || !nr_stored)
> When I tested with this series, with !zswap_shrinker_enabled in the default case,
> I found the performance is much worse than that without this patch.
>
> Testcase: memory.max=2G, zswap enabled, kernel build -j32 in a tmpfs directory.
>
> The reason seems the above cgroup_rstat_flush(), caused much rstat lock contention
> to the zswap_store() path. And if I put the "zswap_shrinker_enabled" check above
> the cgroup_rstat_flush(), the performance become much better.
>
> Maybe we can put the "zswap_shrinker_enabled" check above cgroup_rstat_flush()?

Yes, we should do nothing if !zswap_shrinker_enabled. We should also
use mem_cgroup_flush_stats() here like other places unless accuracy is
crucial, which I doubt given that reclaim uses
mem_cgroup_flush_stats().

mem_cgroup_flush_stats() has some thresholding to make sure we don't
do flushes unnecessarily, and I have a pending series in mm-unstable
that makes that thresholding per-memcg. Keep in mind that adding a
call to mem_cgroup_flush_stats() will cause a conflict in mm-unstable,
because the series there adds a memcg argument to
mem_cgroup_flush_stats(). That should be easily amenable though, I can
post a fixlet for my series to add the memcg argument there on top of
users if needed.

>
> Thanks!
>
> > +             return 0;
> > +
> > +     nr_protected =
> > +             atomic_long_read(&lruvec->zswap_lruvec_state.nr_zswap_protected);
> > +     nr_freeable = list_lru_shrink_count(&pool->list_lru, sc);
> > +     /*
> > +      * Subtract the lru size by an estimate of the number of pages
> > +      * that should be protected.
> > +      */
> > +     nr_freeable = nr_freeable > nr_protected ? nr_freeable - nr_protected : 0;
> > +
> > +     /*
> > +      * Scale the number of freeable pages by the memory saving factor.
> > +      * This ensures that the better zswap compresses memory, the fewer
> > +      * pages we will evict to swap (as it will otherwise incur IO for
> > +      * relatively small memory saving).
> > +      */
> > +     return mult_frac(nr_freeable, nr_backing, nr_stored);
> > +}
> > +
> > +static void zswap_alloc_shrinker(struct zswap_pool *pool)
> > +{
> > +     pool->shrinker =
> > +             shrinker_alloc(SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE, "mm-zswap");
> > +     if (!pool->shrinker)
> > +             return;
> > +
> > +     pool->shrinker->private_data = pool;
> > +     pool->shrinker->scan_objects = zswap_shrinker_scan;
> > +     pool->shrinker->count_objects = zswap_shrinker_count;
> > +     pool->shrinker->batch = 0;
> > +     pool->shrinker->seeks = DEFAULT_SEEKS;
> > +}
> > +
> >  /*********************************
> >  * per-cpu code
> >  **********************************/
[..]
  

On 2023/12/6 13:59, Yosry Ahmed wrote:
> [..]
>>> @@ -526,6 +582,102 @@ static struct zswap_entry *zswap_entry_find_get(struct rb_root *root,
>>>       return entry;
>>>  }
>>>
>>> +/*********************************
>>> +* shrinker functions
>>> +**********************************/
>>> +static enum lru_status shrink_memcg_cb(struct list_head *item, struct list_lru_one *l,
>>> +                                    spinlock_t *lock, void *arg);
>>> +
>>> +static unsigned long zswap_shrinker_scan(struct shrinker *shrinker,
>>> +             struct shrink_control *sc)
>>> +{
>>> +     struct lruvec *lruvec = mem_cgroup_lruvec(sc->memcg, NODE_DATA(sc->nid));
>>> +     unsigned long shrink_ret, nr_protected, lru_size;
>>> +     struct zswap_pool *pool = shrinker->private_data;
>>> +     bool encountered_page_in_swapcache = false;
>>> +
>>> +     nr_protected =
>>> +             atomic_long_read(&lruvec->zswap_lruvec_state.nr_zswap_protected);
>>> +     lru_size = list_lru_shrink_count(&pool->list_lru, sc);
>>> +
>>> +     /*
>>> +      * Abort if the shrinker is disabled or if we are shrinking into the
>>> +      * protected region.
>>> +      *
>>> +      * This short-circuiting is necessary because if we have too many multiple
>>> +      * concurrent reclaimers getting the freeable zswap object counts at the
>>> +      * same time (before any of them made reasonable progress), the total
>>> +      * number of reclaimed objects might be more than the number of unprotected
>>> +      * objects (i.e the reclaimers will reclaim into the protected area of the
>>> +      * zswap LRU).
>>> +      */
>>> +     if (!zswap_shrinker_enabled || nr_protected >= lru_size - sc->nr_to_scan) {
>>> +             sc->nr_scanned = 0;
>>> +             return SHRINK_STOP;
>>> +     }
>>> +
>>> +     shrink_ret = list_lru_shrink_walk(&pool->list_lru, sc, &shrink_memcg_cb,
>>> +             &encountered_page_in_swapcache);
>>> +
>>> +     if (encountered_page_in_swapcache)
>>> +             return SHRINK_STOP;
>>> +
>>> +     return shrink_ret ? shrink_ret : SHRINK_STOP;
>>> +}
>>> +
>>> +static unsigned long zswap_shrinker_count(struct shrinker *shrinker,
>>> +             struct shrink_control *sc)
>>> +{
>>> +     struct zswap_pool *pool = shrinker->private_data;
>>> +     struct mem_cgroup *memcg = sc->memcg;
>>> +     struct lruvec *lruvec = mem_cgroup_lruvec(memcg, NODE_DATA(sc->nid));
>>> +     unsigned long nr_backing, nr_stored, nr_freeable, nr_protected;
>>> +
>>> +#ifdef CONFIG_MEMCG_KMEM
>>> +     cgroup_rstat_flush(memcg->css.cgroup);
>>> +     nr_backing = memcg_page_state(memcg, MEMCG_ZSWAP_B) >> PAGE_SHIFT;
>>> +     nr_stored = memcg_page_state(memcg, MEMCG_ZSWAPPED);
>>> +#else
>>> +     /* use pool stats instead of memcg stats */
>>> +     nr_backing = get_zswap_pool_size(pool) >> PAGE_SHIFT;
>>> +     nr_stored = atomic_read(&pool->nr_stored);
>>> +#endif
>>> +
>>> +     if (!zswap_shrinker_enabled || !nr_stored)
>> When I tested with this series, with !zswap_shrinker_enabled in the default case,
>> I found the performance is much worse than that without this patch.
>>
>> Testcase: memory.max=2G, zswap enabled, kernel build -j32 in a tmpfs directory.
>>
>> The reason seems the above cgroup_rstat_flush(), caused much rstat lock contention
>> to the zswap_store() path. And if I put the "zswap_shrinker_enabled" check above
>> the cgroup_rstat_flush(), the performance become much better.
>>
>> Maybe we can put the "zswap_shrinker_enabled" check above cgroup_rstat_flush()?
> 
> Yes, we should do nothing if !zswap_shrinker_enabled. We should also
> use mem_cgroup_flush_stats() here like other places unless accuracy is
> crucial, which I doubt given that reclaim uses
> mem_cgroup_flush_stats().
> 

Yes. After changing to use mem_cgroup_flush_stats() here, the performance
become much better.

> mem_cgroup_flush_stats() has some thresholding to make sure we don't
> do flushes unnecessarily, and I have a pending series in mm-unstable
> that makes that thresholding per-memcg. Keep in mind that adding a
> call to mem_cgroup_flush_stats() will cause a conflict in mm-unstable,

My test branch is linux-next 20231205, and it's all good after changing
to use mem_cgroup_flush_stats(memcg).

> because the series there adds a memcg argument to
> mem_cgroup_flush_stats(). That should be easily amenable though, I can
> post a fixlet for my series to add the memcg argument there on top of
> users if needed.
> 

It's great. Thanks!
  

On Tue, Dec 5, 2023 at 10:43 PM Chengming Zhou <chengming.zhou@linux.dev> wrote:
>
> On 2023/12/6 13:59, Yosry Ahmed wrote:
> > [..]
> >>> @@ -526,6 +582,102 @@ static struct zswap_entry *zswap_entry_find_get(struct rb_root *root,
> >>>       return entry;
> >>>  }
> >>>
> >>> +/*********************************
> >>> +* shrinker functions
> >>> +**********************************/
> >>> +static enum lru_status shrink_memcg_cb(struct list_head *item, struct list_lru_one *l,
> >>> +                                    spinlock_t *lock, void *arg);
> >>> +
> >>> +static unsigned long zswap_shrinker_scan(struct shrinker *shrinker,
> >>> +             struct shrink_control *sc)
> >>> +{
> >>> +     struct lruvec *lruvec = mem_cgroup_lruvec(sc->memcg, NODE_DATA(sc->nid));
> >>> +     unsigned long shrink_ret, nr_protected, lru_size;
> >>> +     struct zswap_pool *pool = shrinker->private_data;
> >>> +     bool encountered_page_in_swapcache = false;
> >>> +
> >>> +     nr_protected =
> >>> +             atomic_long_read(&lruvec->zswap_lruvec_state.nr_zswap_protected);
> >>> +     lru_size = list_lru_shrink_count(&pool->list_lru, sc);
> >>> +
> >>> +     /*
> >>> +      * Abort if the shrinker is disabled or if we are shrinking into the
> >>> +      * protected region.
> >>> +      *
> >>> +      * This short-circuiting is necessary because if we have too many multiple
> >>> +      * concurrent reclaimers getting the freeable zswap object counts at the
> >>> +      * same time (before any of them made reasonable progress), the total
> >>> +      * number of reclaimed objects might be more than the number of unprotected
> >>> +      * objects (i.e the reclaimers will reclaim into the protected area of the
> >>> +      * zswap LRU).
> >>> +      */
> >>> +     if (!zswap_shrinker_enabled || nr_protected >= lru_size - sc->nr_to_scan) {
> >>> +             sc->nr_scanned = 0;
> >>> +             return SHRINK_STOP;
> >>> +     }
> >>> +
> >>> +     shrink_ret = list_lru_shrink_walk(&pool->list_lru, sc, &shrink_memcg_cb,
> >>> +             &encountered_page_in_swapcache);
> >>> +
> >>> +     if (encountered_page_in_swapcache)
> >>> +             return SHRINK_STOP;
> >>> +
> >>> +     return shrink_ret ? shrink_ret : SHRINK_STOP;
> >>> +}
> >>> +
> >>> +static unsigned long zswap_shrinker_count(struct shrinker *shrinker,
> >>> +             struct shrink_control *sc)
> >>> +{
> >>> +     struct zswap_pool *pool = shrinker->private_data;
> >>> +     struct mem_cgroup *memcg = sc->memcg;
> >>> +     struct lruvec *lruvec = mem_cgroup_lruvec(memcg, NODE_DATA(sc->nid));
> >>> +     unsigned long nr_backing, nr_stored, nr_freeable, nr_protected;
> >>> +
> >>> +#ifdef CONFIG_MEMCG_KMEM
> >>> +     cgroup_rstat_flush(memcg->css.cgroup);
> >>> +     nr_backing = memcg_page_state(memcg, MEMCG_ZSWAP_B) >> PAGE_SHIFT;
> >>> +     nr_stored = memcg_page_state(memcg, MEMCG_ZSWAPPED);
> >>> +#else
> >>> +     /* use pool stats instead of memcg stats */
> >>> +     nr_backing = get_zswap_pool_size(pool) >> PAGE_SHIFT;
> >>> +     nr_stored = atomic_read(&pool->nr_stored);
> >>> +#endif
> >>> +
> >>> +     if (!zswap_shrinker_enabled || !nr_stored)
> >> When I tested with this series, with !zswap_shrinker_enabled in the default case,
> >> I found the performance is much worse than that without this patch.
> >>
> >> Testcase: memory.max=2G, zswap enabled, kernel build -j32 in a tmpfs directory.
> >>
> >> The reason seems the above cgroup_rstat_flush(), caused much rstat lock contention
> >> to the zswap_store() path. And if I put the "zswap_shrinker_enabled" check above
> >> the cgroup_rstat_flush(), the performance become much better.
> >>
> >> Maybe we can put the "zswap_shrinker_enabled" check above cgroup_rstat_flush()?
> >
> > Yes, we should do nothing if !zswap_shrinker_enabled. We should also
> > use mem_cgroup_flush_stats() here like other places unless accuracy is
> > crucial, which I doubt given that reclaim uses
> > mem_cgroup_flush_stats().
> >
>
> Yes. After changing to use mem_cgroup_flush_stats() here, the performance
> become much better.
>
> > mem_cgroup_flush_stats() has some thresholding to make sure we don't
> > do flushes unnecessarily, and I have a pending series in mm-unstable
> > that makes that thresholding per-memcg. Keep in mind that adding a
> > call to mem_cgroup_flush_stats() will cause a conflict in mm-unstable,
>
> My test branch is linux-next 20231205, and it's all good after changing
> to use mem_cgroup_flush_stats(memcg).

Thanks for reporting back. We should still move the
zswap_shrinker_enabled check ahead, no need to even call
mem_cgroup_flush_stats() if we will do nothing anyway.

>
> > because the series there adds a memcg argument to
> > mem_cgroup_flush_stats(). That should be easily amenable though, I can
> > post a fixlet for my series to add the memcg argument there on top of
> > users if needed.
> >
>
> It's great. Thanks!
>
  

On 2023/12/6 15:36, Yosry Ahmed wrote:
> On Tue, Dec 5, 2023 at 10:43 PM Chengming Zhou <chengming.zhou@linux.dev> wrote:
>>
>> On 2023/12/6 13:59, Yosry Ahmed wrote:
>>> [..]
>>>>> @@ -526,6 +582,102 @@ static struct zswap_entry *zswap_entry_find_get(struct rb_root *root,
>>>>>       return entry;
>>>>>  }
>>>>>
>>>>> +/*********************************
>>>>> +* shrinker functions
>>>>> +**********************************/
>>>>> +static enum lru_status shrink_memcg_cb(struct list_head *item, struct list_lru_one *l,
>>>>> +                                    spinlock_t *lock, void *arg);
>>>>> +
>>>>> +static unsigned long zswap_shrinker_scan(struct shrinker *shrinker,
>>>>> +             struct shrink_control *sc)
>>>>> +{
>>>>> +     struct lruvec *lruvec = mem_cgroup_lruvec(sc->memcg, NODE_DATA(sc->nid));
>>>>> +     unsigned long shrink_ret, nr_protected, lru_size;
>>>>> +     struct zswap_pool *pool = shrinker->private_data;
>>>>> +     bool encountered_page_in_swapcache = false;
>>>>> +
>>>>> +     nr_protected =
>>>>> +             atomic_long_read(&lruvec->zswap_lruvec_state.nr_zswap_protected);
>>>>> +     lru_size = list_lru_shrink_count(&pool->list_lru, sc);
>>>>> +
>>>>> +     /*
>>>>> +      * Abort if the shrinker is disabled or if we are shrinking into the
>>>>> +      * protected region.
>>>>> +      *
>>>>> +      * This short-circuiting is necessary because if we have too many multiple
>>>>> +      * concurrent reclaimers getting the freeable zswap object counts at the
>>>>> +      * same time (before any of them made reasonable progress), the total
>>>>> +      * number of reclaimed objects might be more than the number of unprotected
>>>>> +      * objects (i.e the reclaimers will reclaim into the protected area of the
>>>>> +      * zswap LRU).
>>>>> +      */
>>>>> +     if (!zswap_shrinker_enabled || nr_protected >= lru_size - sc->nr_to_scan) {
>>>>> +             sc->nr_scanned = 0;
>>>>> +             return SHRINK_STOP;
>>>>> +     }
>>>>> +
>>>>> +     shrink_ret = list_lru_shrink_walk(&pool->list_lru, sc, &shrink_memcg_cb,
>>>>> +             &encountered_page_in_swapcache);
>>>>> +
>>>>> +     if (encountered_page_in_swapcache)
>>>>> +             return SHRINK_STOP;
>>>>> +
>>>>> +     return shrink_ret ? shrink_ret : SHRINK_STOP;
>>>>> +}
>>>>> +
>>>>> +static unsigned long zswap_shrinker_count(struct shrinker *shrinker,
>>>>> +             struct shrink_control *sc)
>>>>> +{
>>>>> +     struct zswap_pool *pool = shrinker->private_data;
>>>>> +     struct mem_cgroup *memcg = sc->memcg;
>>>>> +     struct lruvec *lruvec = mem_cgroup_lruvec(memcg, NODE_DATA(sc->nid));
>>>>> +     unsigned long nr_backing, nr_stored, nr_freeable, nr_protected;
>>>>> +
>>>>> +#ifdef CONFIG_MEMCG_KMEM
>>>>> +     cgroup_rstat_flush(memcg->css.cgroup);
>>>>> +     nr_backing = memcg_page_state(memcg, MEMCG_ZSWAP_B) >> PAGE_SHIFT;
>>>>> +     nr_stored = memcg_page_state(memcg, MEMCG_ZSWAPPED);
>>>>> +#else
>>>>> +     /* use pool stats instead of memcg stats */
>>>>> +     nr_backing = get_zswap_pool_size(pool) >> PAGE_SHIFT;
>>>>> +     nr_stored = atomic_read(&pool->nr_stored);
>>>>> +#endif
>>>>> +
>>>>> +     if (!zswap_shrinker_enabled || !nr_stored)
>>>> When I tested with this series, with !zswap_shrinker_enabled in the default case,
>>>> I found the performance is much worse than that without this patch.
>>>>
>>>> Testcase: memory.max=2G, zswap enabled, kernel build -j32 in a tmpfs directory.
>>>>
>>>> The reason seems the above cgroup_rstat_flush(), caused much rstat lock contention
>>>> to the zswap_store() path. And if I put the "zswap_shrinker_enabled" check above
>>>> the cgroup_rstat_flush(), the performance become much better.
>>>>
>>>> Maybe we can put the "zswap_shrinker_enabled" check above cgroup_rstat_flush()?
>>>
>>> Yes, we should do nothing if !zswap_shrinker_enabled. We should also
>>> use mem_cgroup_flush_stats() here like other places unless accuracy is
>>> crucial, which I doubt given that reclaim uses
>>> mem_cgroup_flush_stats().
>>>
>>
>> Yes. After changing to use mem_cgroup_flush_stats() here, the performance
>> become much better.
>>
>>> mem_cgroup_flush_stats() has some thresholding to make sure we don't
>>> do flushes unnecessarily, and I have a pending series in mm-unstable
>>> that makes that thresholding per-memcg. Keep in mind that adding a
>>> call to mem_cgroup_flush_stats() will cause a conflict in mm-unstable,
>>
>> My test branch is linux-next 20231205, and it's all good after changing
>> to use mem_cgroup_flush_stats(memcg).
> 
> Thanks for reporting back. We should still move the
> zswap_shrinker_enabled check ahead, no need to even call
> mem_cgroup_flush_stats() if we will do nothing anyway.
> 

Yes, agree!
  

On Tue, Dec 5, 2023 at 10:00 PM Yosry Ahmed <yosryahmed@google.com> wrote:
>
> [..]
> > > @@ -526,6 +582,102 @@ static struct zswap_entry *zswap_entry_find_get(struct rb_root *root,
> > >       return entry;
> > >  }
> > >
> > > +/*********************************
> > > +* shrinker functions
> > > +**********************************/
> > > +static enum lru_status shrink_memcg_cb(struct list_head *item, struct list_lru_one *l,
> > > +                                    spinlock_t *lock, void *arg);
> > > +
> > > +static unsigned long zswap_shrinker_scan(struct shrinker *shrinker,
> > > +             struct shrink_control *sc)
> > > +{
> > > +     struct lruvec *lruvec = mem_cgroup_lruvec(sc->memcg, NODE_DATA(sc->nid));
> > > +     unsigned long shrink_ret, nr_protected, lru_size;
> > > +     struct zswap_pool *pool = shrinker->private_data;
> > > +     bool encountered_page_in_swapcache = false;
> > > +
> > > +     nr_protected =
> > > +             atomic_long_read(&lruvec->zswap_lruvec_state.nr_zswap_protected);
> > > +     lru_size = list_lru_shrink_count(&pool->list_lru, sc);
> > > +
> > > +     /*
> > > +      * Abort if the shrinker is disabled or if we are shrinking into the
> > > +      * protected region.
> > > +      *
> > > +      * This short-circuiting is necessary because if we have too many multiple
> > > +      * concurrent reclaimers getting the freeable zswap object counts at the
> > > +      * same time (before any of them made reasonable progress), the total
> > > +      * number of reclaimed objects might be more than the number of unprotected
> > > +      * objects (i.e the reclaimers will reclaim into the protected area of the
> > > +      * zswap LRU).
> > > +      */
> > > +     if (!zswap_shrinker_enabled || nr_protected >= lru_size - sc->nr_to_scan) {
> > > +             sc->nr_scanned = 0;
> > > +             return SHRINK_STOP;
> > > +     }
> > > +
> > > +     shrink_ret = list_lru_shrink_walk(&pool->list_lru, sc, &shrink_memcg_cb,
> > > +             &encountered_page_in_swapcache);
> > > +
> > > +     if (encountered_page_in_swapcache)
> > > +             return SHRINK_STOP;
> > > +
> > > +     return shrink_ret ? shrink_ret : SHRINK_STOP;
> > > +}
> > > +
> > > +static unsigned long zswap_shrinker_count(struct shrinker *shrinker,
> > > +             struct shrink_control *sc)
> > > +{
> > > +     struct zswap_pool *pool = shrinker->private_data;
> > > +     struct mem_cgroup *memcg = sc->memcg;
> > > +     struct lruvec *lruvec = mem_cgroup_lruvec(memcg, NODE_DATA(sc->nid));
> > > +     unsigned long nr_backing, nr_stored, nr_freeable, nr_protected;
> > > +
> > > +#ifdef CONFIG_MEMCG_KMEM
> > > +     cgroup_rstat_flush(memcg->css.cgroup);
> > > +     nr_backing = memcg_page_state(memcg, MEMCG_ZSWAP_B) >> PAGE_SHIFT;
> > > +     nr_stored = memcg_page_state(memcg, MEMCG_ZSWAPPED);
> > > +#else
> > > +     /* use pool stats instead of memcg stats */
> > > +     nr_backing = get_zswap_pool_size(pool) >> PAGE_SHIFT;
> > > +     nr_stored = atomic_read(&pool->nr_stored);
> > > +#endif
> > > +
> > > +     if (!zswap_shrinker_enabled || !nr_stored)
> > When I tested with this series, with !zswap_shrinker_enabled in the default case,
> > I found the performance is much worse than that without this patch.
> >
> > Testcase: memory.max=2G, zswap enabled, kernel build -j32 in a tmpfs directory.
> >
> > The reason seems the above cgroup_rstat_flush(), caused much rstat lock contention
> > to the zswap_store() path. And if I put the "zswap_shrinker_enabled" check above
> > the cgroup_rstat_flush(), the performance become much better.
> >
> > Maybe we can put the "zswap_shrinker_enabled" check above cgroup_rstat_flush()?
>
> Yes, we should do nothing if !zswap_shrinker_enabled. We should also
> use mem_cgroup_flush_stats() here like other places unless accuracy is
> crucial, which I doubt given that reclaim uses
> mem_cgroup_flush_stats().

Ah, good points on both suggestions. We should not do extra work for
non-user. And, this is a best-effort approximation of the memory
saving factor, so as long as it is not *too* far off I think it's
acceptable.

>
> mem_cgroup_flush_stats() has some thresholding to make sure we don't
> do flushes unnecessarily, and I have a pending series in mm-unstable
> that makes that thresholding per-memcg. Keep in mind that adding a
> call to mem_cgroup_flush_stats() will cause a conflict in mm-unstable,
> because the series there adds a memcg argument to
> mem_cgroup_flush_stats(). That should be easily amenable though, I can
> post a fixlet for my series to add the memcg argument there on top of
> users if needed.

Hmm so how should we proceed from here? How about this:

a) I can send a fixlet to move the enablement check above the stats
flushing + use mem_cgroup_flush_stats
b) Then maybe, you can send a fixlet to update this new callsite?

Does that sound reasonable?

>
> >
> > Thanks!
> >
> > > +             return 0;
> > > +
> > > +     nr_protected =
> > > +             atomic_long_read(&lruvec->zswap_lruvec_state.nr_zswap_protected);
> > > +     nr_freeable = list_lru_shrink_count(&pool->list_lru, sc);
> > > +     /*
> > > +      * Subtract the lru size by an estimate of the number of pages
> > > +      * that should be protected.
> > > +      */
> > > +     nr_freeable = nr_freeable > nr_protected ? nr_freeable - nr_protected : 0;
> > > +
> > > +     /*
> > > +      * Scale the number of freeable pages by the memory saving factor.
> > > +      * This ensures that the better zswap compresses memory, the fewer
> > > +      * pages we will evict to swap (as it will otherwise incur IO for
> > > +      * relatively small memory saving).
> > > +      */
> > > +     return mult_frac(nr_freeable, nr_backing, nr_stored);
> > > +}
> > > +
> > > +static void zswap_alloc_shrinker(struct zswap_pool *pool)
> > > +{
> > > +     pool->shrinker =
> > > +             shrinker_alloc(SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE, "mm-zswap");
> > > +     if (!pool->shrinker)
> > > +             return;
> > > +
> > > +     pool->shrinker->private_data = pool;
> > > +     pool->shrinker->scan_objects = zswap_shrinker_scan;
> > > +     pool->shrinker->count_objects = zswap_shrinker_count;
> > > +     pool->shrinker->batch = 0;
> > > +     pool->shrinker->seeks = DEFAULT_SEEKS;
> > > +}
> > > +
> > >  /*********************************
> > >  * per-cpu code
> > >  **********************************/
> [..]
  

[...]
>
> Hmm so how should we proceed from here? How about this:
>
> a) I can send a fixlet to move the enablement check above the stats
> flushing + use mem_cgroup_flush_stats
> b) Then maybe, you can send a fixlet to update this new callsite?
>
> Does that sound reasonable?

I just sent out the fixlet. Yosry and Chengming, let me know if that
looks good. Thank you both for detecting this issue and proposing the
fix!
  

On Wed, Dec 6, 2023 at 11:47 AM Nhat Pham <nphamcs@gmail.com> wrote:
>
> [...]
> >
> > Hmm so how should we proceed from here? How about this:
> >
> > a) I can send a fixlet to move the enablement check above the stats
> > flushing + use mem_cgroup_flush_stats
> > b) Then maybe, you can send a fixlet to update this new callsite?
> >
> > Does that sound reasonable?
>
> I just sent out the fixlet. Yosry and Chengming, let me know if that
> looks good. Thank you both for detecting this issue and proposing the
> fix!

The fixlet looks good, and Andrew already took care of (b) before I
could send a followup fixlet out :)
  

On 2023/12/7 03:47, Nhat Pham wrote:
> [...]
>>
>> Hmm so how should we proceed from here? How about this:
>>
>> a) I can send a fixlet to move the enablement check above the stats
>> flushing + use mem_cgroup_flush_stats
>> b) Then maybe, you can send a fixlet to update this new callsite?
>>
>> Does that sound reasonable?
> 
> I just sent out the fixlet. Yosry and Chengming, let me know if that
> looks good. Thank you both for detecting this issue and proposing the
> fix!

Yeah, also looks good to me. Thanks!