Re: [PATCH v2 2/2] zswap: shrinks zswap pool based on memory pressure

[Yosry Ahmed]

On Tue, Sep 19, 2023 at 10:14 AM Nhat Pham <nphamcs@xxxxxxxxx> 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.

What's the use case for having per-memcg opt-in/out?

If there is memory pressure, reclaiming swap-backed pages will push
pages out of zswap anyway, regardless of this patch. With this patch,
any sort of reclaim can push pages out of zswap. Wouldn't that be
preferable to reclaiming memory that is currently resident in memory
(so arguably hotter than the pages in zswap)? Why would this decision
be different per-memcg?

>
> 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.

I don't have an opinion about the reclaim heuristics here, I will let
reclaim experts chip in.

>
> On a benchmark that we have run:

Please add more details (as much as possible) about the benchmarks used here.

>
> (without the shrinker)
> real -- mean: 153.27s, median: 153.199s
> sys -- mean: 541.652s, median: 541.903s
> user -- mean: 4384.9673999999995s, median: 4385.471s
>
> (with the shrinker)
> real -- mean: 151.4956s, median: 151.456s
> sys -- mean: 461.14639999999997s, median: 465.656s
> user -- mean: 4384.7118s, median: 4384.675s
>
> We observed a 14-15% reduction in kernel CPU time, which translated to
> over 1% reduction in real time.
>
> On another benchmark, where there was a lot more cold memory residing in
> zswap, we observed even more pronounced gains:
>
> (without the shrinker)
> real -- mean: 157.52519999999998s, median: 157.281s
> sys -- mean: 769.3082s, median: 780.545s
> user -- mean: 4378.1622s, median: 4378.286s
>
> (with the shrinker)
> real -- mean: 152.9608s, median: 152.845s
> sys -- mean: 517.4446s, median: 506.749s
> user -- mean: 4387.694s, median: 4387.935s
>
> Here, we saw around 32-35% reduction in kernel CPU time, which
> translated to 2.8% reduction in real time. These results confirm our
> hypothesis that the shrinker is more helpful the more cold memory we
> have.
>
> Suggested-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> Signed-off-by: Nhat Pham <nphamcs@xxxxxxxxx>
> ---
>  Documentation/admin-guide/mm/zswap.rst |  12 ++
>  include/linux/memcontrol.h             |   1 +
>  include/linux/mmzone.h                 |  14 ++
>  mm/memcontrol.c                        |  33 +++++
>  mm/swap_state.c                        |  31 ++++-
>  mm/zswap.c                             | 180 ++++++++++++++++++++++++-
>  6 files changed, 263 insertions(+), 8 deletions(-)
>
> diff --git a/Documentation/admin-guide/mm/zswap.rst b/Documentation/admin-guide/mm/zswap.rst
> index 45b98390e938..ae8597a67804 100644
> --- a/Documentation/admin-guide/mm/zswap.rst
> +++ b/Documentation/admin-guide/mm/zswap.rst
> @@ -153,6 +153,18 @@ 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 by first switching on the global knob:
> +
> +  echo Y > /sys/module/zswap/par meters/shrinker_enabled
> +
> +When the kernel is compiled with CONFIG_MEMCG_KMEM, user needs to further turn
> +it on for each cgroup that the shrinker should target:
> +
> +  echo 1 > /sys/fs/cgroup/<cgroup-name>/memory.zswap.shrinker.enabled
> +
>  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/memcontrol.h b/include/linux/memcontrol.h
> index 05d34b328d9d..f005ea667863 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -219,6 +219,7 @@ struct mem_cgroup {
>
>  #if defined(CONFIG_MEMCG_KMEM) && defined(CONFIG_ZSWAP)
>         unsigned long zswap_max;
> +       atomic_t zswap_shrinker_enabled;
>  #endif
>
>         unsigned long soft_limit;
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 4106fbc5b4b3..81f4c5ea3e16 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -637,6 +637,20 @@ struct lruvec {
>  #ifdef CONFIG_MEMCG
>         struct pglist_data *pgdat;
>  #endif
> +#ifdef CONFIG_ZSWAP
> +       /*
> +        * 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.
> +        */
> +       unsigned long nr_zswap_protected;
> +#endif

Would this be better abstracted in a zswap lruvec struct?

>  };
>
>  /* Isolate unmapped pages */
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 9f84b3f7b469..1a2c97cf396f 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -5352,6 +5352,8 @@ mem_cgroup_css_alloc(struct cgroup_subsys_state *parent_css)
>         WRITE_ONCE(memcg->soft_limit, PAGE_COUNTER_MAX);
>  #if defined(CONFIG_MEMCG_KMEM) && defined(CONFIG_ZSWAP)
>         memcg->zswap_max = PAGE_COUNTER_MAX;
> +       /* Disable the shrinker by default */
> +       atomic_set(&memcg->zswap_shrinker_enabled, 0);
>  #endif
>         page_counter_set_high(&memcg->swap, PAGE_COUNTER_MAX);
>         if (parent) {
> @@ -7877,6 +7879,31 @@ static ssize_t zswap_max_write(struct kernfs_open_file *of,
>         return nbytes;
>  }
>
> +static int zswap_shrinker_enabled_show(struct seq_file *m, void *v)
> +{
> +       struct mem_cgroup *memcg = mem_cgroup_from_seq(m);
> +
> +       seq_printf(m, "%d\n", atomic_read(&memcg->zswap_shrinker_enabled));
> +       return 0;
> +}
> +
> +static ssize_t zswap_shrinker_enabled_write(struct kernfs_open_file *of,
> +                              char *buf, size_t nbytes, loff_t off)
> +{
> +       struct mem_cgroup *memcg = mem_cgroup_from_css(of_css(of));
> +       int zswap_shrinker_enabled;
> +       ssize_t parse_ret = kstrtoint(strstrip(buf), 0, &zswap_shrinker_enabled);
> +
> +       if (parse_ret)
> +               return parse_ret;
> +
> +       if (zswap_shrinker_enabled < 0 || zswap_shrinker_enabled > 1)
> +               return -ERANGE;
> +
> +       atomic_set(&memcg->zswap_shrinker_enabled, zswap_shrinker_enabled);
> +       return nbytes;
> +}
> +
>  static struct cftype zswap_files[] = {
>         {
>                 .name = "zswap.current",
> @@ -7889,6 +7916,12 @@ static struct cftype zswap_files[] = {
>                 .seq_show = zswap_max_show,
>                 .write = zswap_max_write,
>         },
> +       {
> +               .name = "zswap.shrinker.enabled",
> +               .flags = CFTYPE_NOT_ON_ROOT,
> +               .seq_show = zswap_shrinker_enabled_show,
> +               .write = zswap_shrinker_enabled_write,
> +       },
>         { }     /* terminate */
>  };
>  #endif /* CONFIG_MEMCG_KMEM && CONFIG_ZSWAP */
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index 1c826737aacb..788e36a06c34 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -618,6 +618,22 @@ static unsigned long swapin_nr_pages(unsigned long offset)
>         return pages;
>  }
>
> +#ifdef CONFIG_ZSWAP
> +/*
> + * Refault is an indication that warmer pages are not resident in memory.
> + * Increase the size of zswap's protected area.
> + */
> +static void inc_nr_protected(struct page *page)
> +{
> +       struct lruvec *lruvec = folio_lruvec(page_folio(page));
> +       unsigned long flags;
> +
> +       spin_lock_irqsave(&lruvec->lru_lock, flags);
> +       lruvec->nr_zswap_protected++;
> +       spin_unlock_irqrestore(&lruvec->lru_lock, flags);
> +}
> +#endif
> +

A few questions:
- Why is this function named in such a generic way?
- Why is this function here instead of in mm/zswap.c?
- Why is this protected by the heavily contested lruvec lock instead
of being an atomic?

>  /**
>   * swap_cluster_readahead - swap in pages in hope we need them soon
>   * @entry: swap entry of this memory
> @@ -686,7 +702,12 @@ struct page *swap_cluster_readahead(swp_entry_t entry, gfp_t gfp_mask,
>         lru_add_drain();        /* Push any new pages onto the LRU now */
>  skip:
>         /* The page was likely read above, so no need for plugging here */
> -       return read_swap_cache_async(entry, gfp_mask, vma, addr, NULL);
> +       page = read_swap_cache_async(entry, gfp_mask, vma, addr, NULL);
> +#ifdef CONFIG_ZSWAP
> +       if (page)
> +               inc_nr_protected(page);
> +#endif
> +       return page;
>  }
>
>  int init_swap_address_space(unsigned int type, unsigned long nr_pages)
> @@ -853,8 +874,12 @@ static struct page *swap_vma_readahead(swp_entry_t fentry, gfp_t gfp_mask,
>         lru_add_drain();
>  skip:
>         /* The page was likely read above, so no need for plugging here */
> -       return read_swap_cache_async(fentry, gfp_mask, vma, vmf->address,
> -                                    NULL);
> +       page = read_swap_cache_async(fentry, gfp_mask, vma, vmf->address, NULL);
> +#ifdef CONFIG_ZSWAP
> +       if (page)
> +               inc_nr_protected(page);
> +#endif
> +       return page;
>  }
>
>  /**
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 1a469e5d5197..79cb18eeb8bf 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -145,6 +145,26 @@ 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
>
> +/*
> + * Global flag to enable/disable memory pressure-based shrinker for all memcgs.
> + * If CONFIG_MEMCG_KMEM is on, we can further selectively disable
> + * the shrinker for each memcg.
> + */
> +static bool zswap_shrinker_enabled;
> +module_param_named(shrinker_enabled, zswap_shrinker_enabled, bool, 0644);
> +#ifdef CONFIG_MEMCG_KMEM
> +static bool is_shrinker_enabled(struct mem_cgroup *memcg)
> +{
> +       return zswap_shrinker_enabled &&
> +               atomic_read(&memcg->zswap_shrinker_enabled);
> +}
> +#else
> +static bool is_shrinker_enabled(struct mem_cgroup *memcg)
> +{
> +       return zswap_shrinker_enabled;
> +}
> +#endif
> +
>  /*********************************
>  * data structures
>  **********************************/
> @@ -174,6 +194,8 @@ 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;
>  };
>
>  /*
> @@ -273,17 +295,26 @@ 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();
>
> @@ -318,8 +349,23 @@ static bool zswap_lru_add(struct list_lru *list_lru, struct zswap_entry *entry)
>  {
>         struct mem_cgroup *memcg = entry->objcg ?
>                 get_mem_cgroup_from_objcg(entry->objcg) : NULL;
> +       struct lruvec *lruvec = mem_cgroup_lruvec(memcg, NODE_DATA(entry->nid));
>         bool added = __list_lru_add(list_lru, &entry->lru, entry->nid, memcg);
> +       unsigned long flags, lru_size;
> +
> +       if (added) {
> +               lru_size = list_lru_count_one(list_lru, entry->nid, memcg);
> +               spin_lock_irqsave(&lruvec->lru_lock, flags);
> +               lruvec->nr_zswap_protected++;
>
> +               /*
> +                * Decay to avoid overflow and adapt to changing workloads.
> +                * This is based on LRU reclaim cost decaying heuristics.
> +                */
> +               if (lruvec->nr_zswap_protected > lru_size / 4)
> +                       lruvec->nr_zswap_protected /= 2;
> +               spin_unlock_irqrestore(&lruvec->lru_lock, flags);
> +       }
>         mem_cgroup_put(memcg);
>         return added;
>  }
> @@ -420,6 +466,7 @@ 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);
> @@ -461,6 +508,98 @@ 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 zswap_pool *pool = shrinker->private_data;
> +       unsigned long shrink_ret, nr_zswap_protected, flags,
> +               lru_size = list_lru_shrink_count(&pool->list_lru, sc);
> +       struct lruvec *lruvec = mem_cgroup_lruvec(sc->memcg, NODE_DATA(sc->nid));
> +       bool encountered_page_in_swapcache = false;
> +
> +       spin_lock_irqsave(&lruvec->lru_lock, flags);
> +       nr_zswap_protected = lruvec->nr_zswap_protected;
> +       spin_unlock_irqrestore(&lruvec->lru_lock, flags);
> +
> +       /*
> +        * Abort if the shrinker is disabled or if we are shrinking into the
> +        * protected region.
> +        */
> +       if (!is_shrinker_enabled(sc->memcg) ||
> +                       nr_zswap_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, flags;
> +
> +#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 (!is_shrinker_enabled(memcg) || !nr_stored)
> +               return 0;
> +
> +       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.
> +        */
> +       spin_lock_irqsave(&lruvec->lru_lock, flags);
> +       nr_freeable = nr_freeable > lruvec->nr_zswap_protected ?
> +               nr_freeable - lruvec->nr_zswap_protected : 0;
> +       spin_unlock_irqrestore(&lruvec->lru_lock, flags);
> +
> +       /*
> +        * 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
>  **********************************/
> @@ -656,11 +795,14 @@ 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 mem_cgroup *memcg;
>         struct zswap_tree *tree;
> +       struct lruvec *lruvec;
>         pgoff_t swpoffset;
>         enum lru_status ret = LRU_REMOVED_RETRY;
>         int writeback_result;
> +       unsigned long flags;
>
>         /*
>          * Once the lru lock is dropped, the entry might get freed. The
> @@ -696,8 +838,24 @@ static enum lru_status shrink_memcg_cb(struct list_head *item, struct list_lru_o
>                 /* we cannot use zswap_lru_add here, because it increments node's lru count */
>                 list_lru_putback(&entry->pool->list_lru, item, entry->nid, memcg);
>                 spin_unlock(lock);
> -               mem_cgroup_put(memcg);
>                 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;
> +               }
> +               lruvec = mem_cgroup_lruvec(memcg, NODE_DATA(entry->nid));
> +               spin_lock_irqsave(&lruvec->lru_lock, flags);
> +               /* Increment the protection area to account for the LRU rotation. */
> +               lruvec->nr_zswap_protected++;
> +               spin_unlock_irqrestore(&lruvec->lru_lock, flags);
> +
> +               mem_cgroup_put(memcg);
>                 goto put_unlock;
>         }
>
> @@ -828,6 +986,11 @@ 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
> @@ -836,12 +999,17 @@ static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
>         kref_init(&pool->kref);
>         INIT_LIST_HEAD(&pool->list);
>         INIT_WORK(&pool->shrink_work, shrink_worker);
> -       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);
>
>         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);
> @@ -899,6 +1067,7 @@ 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);
> @@ -1431,6 +1600,7 @@ bool zswap_store(struct folio *folio)
>         if (entry->length) {
>                 INIT_LIST_HEAD(&entry->lru);
>                 zswap_lru_add(&pool->list_lru, entry);
> +               atomic_inc(&pool->nr_stored);
>         }
>         spin_unlock(&tree->lock);
>
> --
> 2.34.1