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Thu, 29 Feb 2024 10:58:49 -0800 (PST) MIME-Version: 1.0 References: <20240229-zswap-xarray-v2-1-e50284dfcdb1@kernel.org> <66a86e12-e567-4705-b683-0485276007d2@bytedance.com> In-Reply-To: <66a86e12-e567-4705-b683-0485276007d2@bytedance.com> From: Chris Li Date: Thu, 29 Feb 2024 10:58:36 -0800 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v2] zswap: replace RB tree with xarray To: Chengming Zhou Cc: Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Yosry Ahmed , Nhat Pham , Johannes Weiner , "Matthew Wilcox (Oracle)" , Barry Song Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspam-User: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 900B3100013 X-Stat-Signature: oiq54pjaxn4wdjyo9dxj4fn5no71bmbk X-HE-Tag: 1709233134-243329 X-HE-Meta: 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Hi Chengming, Thanks for the review and feedback. On Thu, Feb 29, 2024 at 1:44=E2=80=AFAM Chengming Zhou wrote: > > Hi Chris, > > On 2024/2/29 16:46, Chris Li wrote: > > Very deep RB tree requires rebalance at times. That > > contributes to the zswap fault latencies. Xarray does not > > need to perform tree rebalance. Replacing RB tree to xarray > > can have some small performance gain. > > > > One small difference is that xarray insert might fail with > > ENOMEM, while RB tree insert does not allocate additional > > memory. > > > > The zswap_entry size will reduce a bit due to removing the > > RB node, which has two pointers and a color field. Xarray > > store the pointer in the xarray tree rather than the > > zswap_entry. Every entry has one pointer from the xarray > > tree. Overall, switching to xarray should save some memory, > > if the swap entries are densely packed. > > > > Notice the zswap_rb_search and zswap_rb_insert always > > followed by zswap_rb_erase. Fold the entry erase into > > zswap_xa_search_and_erase and zswap_xa_insert. That saves > > one tree lookup as well. > > > > Remove zswap_invalidate_entry due to no need to call > > zswap_rb_erase any more. Use zswap_free_entry instead. > > > > The "struct zswap_tree" has been replaced by "struct xarray". > > The tree spin lock has transferred to the xarray lock. > > > > Thanks to Chengming for providing the kernel build test number. > > > > Run the kernel build testing 5 times for each version, averages: > > (memory.max=3D2GB, zswap shrinker and writeback enabled, one 50GB swapf= ile.) > > > > mm-266f922c0b5e zswap-xarray-test > > real 63.43 63.12 > > user 1063.78 1062.59 > > sys 272.49 265.66 > > > > The sys time is about 2.5% faster. > > > > Tested-by: Chengming Zhou > > --- > > > > > > Signed-off-by: Chris Li > > --- > > Changes in v2: > > - Replace struct zswap_tree with struct xarray. > > - Remove zswap_tree spinlock, use xarray lock instead. > > - Fold zswap_rb_erase() into zswap_xa_search_and_delete() and zswap_xa_= insert(). > > - Delete zswap_invalidate_entry(), use zswap_free_entry() instead. > > - Link to v1: https://lore.kernel.org/r/20240117-zswap-xarray-v1-0-6daa= 86c08fae@kernel.org > > --- > > mm/zswap.c | 173 +++++++++++++++++++++++------------------------------= -------- > > 1 file changed, 64 insertions(+), 109 deletions(-) > > > > diff --git a/mm/zswap.c b/mm/zswap.c > > index 011e068eb355..ac9ef14d88be 100644 > > --- a/mm/zswap.c > > +++ b/mm/zswap.c > > @@ -20,7 +20,6 @@ > > #include > > #include > > #include > > -#include > > #include > > #include > > #include > > @@ -71,6 +70,8 @@ static u64 zswap_reject_compress_poor; > > static u64 zswap_reject_alloc_fail; > > /* Store failed because the entry metadata could not be allocated (rar= e) */ > > static u64 zswap_reject_kmemcache_fail; > > +/* Store failed because xarray can't insert the entry*/ > > +static u64 zswap_reject_xarray_fail; > > > > /* Shrinker work queue */ > > static struct workqueue_struct *shrink_wq; > > @@ -196,7 +197,6 @@ static struct { > > * This structure contains the metadata for tracking a single compress= ed > > * page within zswap. > > * > > - * rbnode - links the entry into red-black tree for the appropriate sw= ap type > > * swpentry - associated swap entry, the offset indexes into the red-b= lack tree > > * length - the length in bytes of the compressed page data. Needed d= uring > > * decompression. For a same value filled page length is 0, a= nd both > > @@ -208,7 +208,6 @@ static struct { > > * lru - handle to the pool's lru used to evict pages. > > */ > > struct zswap_entry { > > - struct rb_node rbnode; > > swp_entry_t swpentry; > > unsigned int length; > > struct zswap_pool *pool; > > @@ -220,12 +219,7 @@ struct zswap_entry { > > struct list_head lru; > > }; > > > > -struct zswap_tree { > > - struct rb_root rbroot; > > - spinlock_t lock; > > -}; > > - > > -static struct zswap_tree *zswap_trees[MAX_SWAPFILES]; > > +static struct xarray *zswap_trees[MAX_SWAPFILES]; > > static unsigned int nr_zswap_trees[MAX_SWAPFILES]; > > > > /* RCU-protected iteration */ > > @@ -253,10 +247,10 @@ static bool zswap_has_pool; > > * helpers and fwd declarations > > **********************************/ > > > > -static inline struct zswap_tree *swap_zswap_tree(swp_entry_t swp) > > +static inline struct xarray *swap_zswap_tree(swp_entry_t swp) > > { > > - return &zswap_trees[swp_type(swp)][swp_offset(swp) > > - >> SWAP_ADDRESS_SPACE_SHIFT]; > > + return zswap_trees[swp_type(swp)] + (swp_offset(swp) > > + >> SWAP_ADDRESS_SPACE_SHIFT); > > } > > > > #define zswap_pool_debug(msg, p) \ > > @@ -805,60 +799,38 @@ void zswap_memcg_offline_cleanup(struct mem_cgrou= p *memcg) > > } > > > > /********************************* > > -* rbtree functions > > +* xarray functions > > **********************************/ > > -static struct zswap_entry *zswap_rb_search(struct rb_root *root, pgoff= _t offset) > > +static struct zswap_entry *zswap_xa_search_and_erase(struct xarray *tr= ee, pgoff_t offset) > > { > > - struct rb_node *node =3D root->rb_node; > > - struct zswap_entry *entry; > > - pgoff_t entry_offset; > > - > > - while (node) { > > - entry =3D rb_entry(node, struct zswap_entry, rbnode); > > - entry_offset =3D swp_offset(entry->swpentry); > > - if (entry_offset > offset) > > - node =3D node->rb_left; > > - else if (entry_offset < offset) > > - node =3D node->rb_right; > > - else > > - return entry; > > - } > > - return NULL; > > + return xa_erase(tree, offset); > > } > > > > /* > > + * Expects xa_lock to be held on entry. > > * In the case that a entry with the same offset is found, a pointer t= o > > - * the existing entry is stored in dupentry and the function returns -= EEXIST > > + * the existing entry is stored in old and erased from the tree. > > + * Function return error on insert. > > */ > > -static int zswap_rb_insert(struct rb_root *root, struct zswap_entry *e= ntry, > > - struct zswap_entry **dupentry) > > +static int zswap_xa_insert(struct xarray *tree, struct zswap_entry *en= try, > > + struct zswap_entry **old) > > { > > - struct rb_node **link =3D &root->rb_node, *parent =3D NULL; > > - struct zswap_entry *myentry; > > - pgoff_t myentry_offset, entry_offset =3D swp_offset(entry->swpent= ry); > > - > > - while (*link) { > > - parent =3D *link; > > - myentry =3D rb_entry(parent, struct zswap_entry, rbnode); > > - myentry_offset =3D swp_offset(myentry->swpentry); > > - if (myentry_offset > entry_offset) > > - link =3D &(*link)->rb_left; > > - else if (myentry_offset < entry_offset) > > - link =3D &(*link)->rb_right; > > - else { > > - *dupentry =3D myentry; > > - return -EEXIST; > > - } > > - } > > - rb_link_node(&entry->rbnode, parent, link); > > - rb_insert_color(&entry->rbnode, root); > > - return 0; > > -} > > + int err; > > + struct zswap_entry *e; > > + pgoff_t offset =3D swp_offset(entry->swpentry); > > > > -static void zswap_rb_erase(struct rb_root *root, struct zswap_entry *e= ntry) > > -{ > > - rb_erase(&entry->rbnode, root); > > - RB_CLEAR_NODE(&entry->rbnode); > > + e =3D __xa_store(tree, offset, entry, GFP_KERNEL); > > + err =3D xa_err(e); > > + > > + if (err) { > > + e =3D __xa_erase(tree, offset); zswap_xa_insert will always erase the old entry, even when __xa_store fails= . > > + if (err =3D=3D -ENOMEM) > > + zswap_reject_alloc_fail++; > > + else > > + zswap_reject_xarray_fail++; > > + } > > + *old =3D e; Old pointer is set regardless of the error. > > + return err; > > } > > > > /********************************* > > @@ -872,7 +844,6 @@ static struct zswap_entry *zswap_entry_cache_alloc(= gfp_t gfp, int nid) > > entry =3D kmem_cache_alloc_node(zswap_entry_cache, gfp, nid); > > if (!entry) > > return NULL; > > - RB_CLEAR_NODE(&entry->rbnode); > > return entry; > > } > > > > @@ -914,17 +885,6 @@ static void zswap_entry_free(struct zswap_entry *e= ntry) > > zswap_update_total_size(); > > } > > > > -/* > > - * The caller hold the tree lock and search the entry from the tree, > > - * so it must be on the tree, remove it from the tree and free it. > > - */ > > -static void zswap_invalidate_entry(struct zswap_tree *tree, > > - struct zswap_entry *entry) > > -{ > > - zswap_rb_erase(&tree->rbroot, entry); > > - zswap_entry_free(entry); > > -} > > - > > /********************************* > > * compressed storage functions > > **********************************/ > > @@ -1113,7 +1073,9 @@ static void zswap_decompress(struct zswap_entry *= entry, struct page *page) > > static int zswap_writeback_entry(struct zswap_entry *entry, > > swp_entry_t swpentry) > > { > > - struct zswap_tree *tree; > > + struct xarray *tree; > > + pgoff_t offset =3D swp_offset(swpentry); > > + struct zswap_entry *e; > > struct folio *folio; > > struct mempolicy *mpol; > > bool folio_was_allocated; > > @@ -1150,19 +1112,14 @@ static int zswap_writeback_entry(struct zswap_e= ntry *entry, > > * be dereferenced. > > */ > > tree =3D swap_zswap_tree(swpentry); > > - spin_lock(&tree->lock); > > - if (zswap_rb_search(&tree->rbroot, swp_offset(swpentry)) !=3D ent= ry) { > > - spin_unlock(&tree->lock); > > + e =3D zswap_xa_search_and_erase(tree, offset); > > + if (e !=3D entry) { > > IIUC, here we should use xa_cmpxchg() instead of erasing it unconditional= ly. Good catch, I agree with your suggestion. I will spin a V3 to correct that. > > > delete_from_swap_cache(folio); > > folio_unlock(folio); > > folio_put(folio); > > return -ENOMEM; > > } > > > > - /* Safe to deref entry after the entry is verified above. */ > > - zswap_rb_erase(&tree->rbroot, entry); > > - spin_unlock(&tree->lock); > > - > > zswap_decompress(entry, &folio->page); > > > > count_vm_event(ZSWPWB); > > @@ -1471,10 +1428,11 @@ bool zswap_store(struct folio *folio) > > { > > swp_entry_t swp =3D folio->swap; > > pgoff_t offset =3D swp_offset(swp); > > - struct zswap_tree *tree =3D swap_zswap_tree(swp); > > - struct zswap_entry *entry, *dupentry; > > + struct xarray *tree =3D swap_zswap_tree(swp); > > + struct zswap_entry *entry, *old; > > struct obj_cgroup *objcg =3D NULL; > > struct mem_cgroup *memcg =3D NULL; > > + int err; > > > > VM_WARN_ON_ONCE(!folio_test_locked(folio)); > > VM_WARN_ON_ONCE(!folio_test_swapcache(folio)); > > @@ -1562,21 +1520,25 @@ bool zswap_store(struct folio *folio) > > } > > > > /* map */ > > - spin_lock(&tree->lock); > > + xa_lock(tree); > > /* > > * The folio may have been dirtied again, invalidate the > > * possibly stale entry before inserting the new entry. > > */ > > - if (zswap_rb_insert(&tree->rbroot, entry, &dupentry) =3D=3D -EEXI= ST) { > > - zswap_invalidate_entry(tree, dupentry); > > - WARN_ON(zswap_rb_insert(&tree->rbroot, entry, &dupentry))= ; > > + err =3D zswap_xa_insert(tree, entry, &old); > > + if (old) > > + zswap_entry_free(old); > > Maybe it's safer to check old after !err, since "old" variable is not ini= tialized > to NULL, and zswap_xa_insert() maybe won't overwrite "old" to NULL when e= rr return? That is the intended behavior. See the above in zswap_xa_insert(). It will always erase and return "old" even when the __xa_store() has an error. That is because by the time zswap needs to store a new entry at this swap entry. The old data is already outdated. We should just remove the old data. If __xa_store failed due to out of memory. That is the same as allocating an entry out of memory. It is fine to fail swap_store. Then the folio will just stay in the swap cache for the next time. Do you see any ill effects can be caused by deleting the old entry on xa_insert error? > > + if (err) { > > + xa_unlock(tree); > > + goto free_zpool; > > } > > + > > if (entry->length) { > > INIT_LIST_HEAD(&entry->lru); > > zswap_lru_add(&zswap.list_lru, entry); > > atomic_inc(&zswap.nr_stored); > > } > > It seems that we can put this part out of the xarray lock section, then i= t's enough to > just use xa_insert(). It is not enough protection. Consider this race: CPU1 CPU2 xa_insert() entry =3D swap_xa_search_and_erase() zswap_free_entry(entry) if (entry->length) ... CPU1 is using entry after free. Chris