On Tue 19-01-16 12:59:12, Brian Foster wrote:
> From: Dave Chinner <dchinner@xxxxxxxxxx>
>
> wait_sb_inodes() currently does a walk of all inodes in the
> filesystem to find dirty one to wait on during sync. This is highly
> inefficient and wastes a lot of CPU when there are lots of clean
> cached inodes that we don't need to wait on.
>
> To avoid this "all inode" walk, we need to track inodes that are
> currently under writeback that we need to wait for. We do this by
> adding inodes to a writeback list on the sb when the mapping is
> first tagged as having pages under writeback. wait_sb_inodes() can
> then walk this list of "inodes under IO" and wait specifically just
> for the inodes that the current sync(2) needs to wait for.
>
> Define a couple helpers to add/remove an inode from the writeback
> list and call them when the overall mapping is tagged for or cleared
> from writeback. Update wait_sb_inodes() to walk only the inodes
> under writeback due to the sync.
The patch looks good. Just one comment: This grows struct inode by two
longs. Such a growth should be justified by measuring the improvements. So
can you measure some numbers showing how much the patch helped? I think it
would be interesting to see:
a) How much sync(2) speed has improved if there's not much to wait for.
b) See whether parallel heavy stat(2) load which is rotating lots of inodes
in inode cache sees some improvement when it doesn't have to contend with
sync(2) on s_inode_list_lock. I believe Dave Chinner had some loads where
the contention on s_inode_list_lock due to sync and rotation of inodes was
pretty heavy.
Thanks.
Honza
> Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
> Signed-off-by: Josef Bacik <jbacik@xxxxxx>
> Signed-off-by: Brian Foster <bfoster@xxxxxxxxxx>
> ---
> fs/fs-writeback.c | 106 +++++++++++++++++++++++++++++++++++-----------
> fs/inode.c | 2 +
> fs/super.c | 2 +
> include/linux/fs.h | 4 ++
> include/linux/writeback.h | 3 ++
> mm/page-writeback.c | 18 ++++++++
> 6 files changed, 110 insertions(+), 25 deletions(-)
>
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 6915c95..63b878b 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -943,6 +943,37 @@ void inode_io_list_del(struct inode *inode)
> }
>
> /*
> + * mark an inode as under writeback on the sb
> + */
> +void sb_mark_inode_writeback(struct inode *inode)
> +{
> + struct super_block *sb = inode->i_sb;
> + unsigned long flags;
> +
> + if (list_empty(&inode->i_wb_list)) {
> + spin_lock_irqsave(&sb->s_inode_wblist_lock, flags);
> + if (list_empty(&inode->i_wb_list))
> + list_add_tail(&inode->i_wb_list, &sb->s_inodes_wb);
> + spin_unlock_irqrestore(&sb->s_inode_wblist_lock, flags);
> + }
> +}
> +
> +/*
> + * clear an inode as under writeback on the sb
> + */
> +void sb_clear_inode_writeback(struct inode *inode)
> +{
> + struct super_block *sb = inode->i_sb;
> + unsigned long flags;
> +
> + if (!list_empty(&inode->i_wb_list)) {
> + spin_lock_irqsave(&sb->s_inode_wblist_lock, flags);
> + list_del_init(&inode->i_wb_list);
> + spin_unlock_irqrestore(&sb->s_inode_wblist_lock, flags);
> + }
> +}
> +
> +/*
> * Redirty an inode: set its when-it-was dirtied timestamp and move it to the
> * furthest end of its superblock's dirty-inode list.
> *
> @@ -2106,7 +2137,7 @@ EXPORT_SYMBOL(__mark_inode_dirty);
> */
> static void wait_sb_inodes(struct super_block *sb)
> {
> - struct inode *inode, *old_inode = NULL;
> + LIST_HEAD(sync_list);
>
> /*
> * We need to be protected against the filesystem going from
> @@ -2115,38 +2146,60 @@ static void wait_sb_inodes(struct super_block *sb)
> WARN_ON(!rwsem_is_locked(&sb->s_umount));
>
> mutex_lock(&sb->s_sync_lock);
> - spin_lock(&sb->s_inode_list_lock);
>
> /*
> - * Data integrity sync. Must wait for all pages under writeback,
> - * because there may have been pages dirtied before our sync
> - * call, but which had writeout started before we write it out.
> - * In which case, the inode may not be on the dirty list, but
> - * we still have to wait for that writeout.
> + * Splice the writeback list onto a temporary list to avoid waiting on
> + * inodes that have started writeback after this point.
> + *
> + * Use rcu_read_lock() to keep the inodes around until we have a
> + * reference. s_inode_wblist_lock protects sb->s_inodes_wb as well as
> + * the local list because inodes can be dropped from either by writeback
> + * completion.
> + */
> + rcu_read_lock();
> + spin_lock_irq(&sb->s_inode_wblist_lock);
> + list_splice_init(&sb->s_inodes_wb, &sync_list);
> +
> + /*
> + * Data integrity sync. Must wait for all pages under writeback, because
> + * there may have been pages dirtied before our sync call, but which had
> + * writeout started before we write it out. In which case, the inode
> + * may not be on the dirty list, but we still have to wait for that
> + * writeout.
> */
> - list_for_each_entry(inode, &sb->s_inodes, i_sb_list) {
> + while (!list_empty(&sync_list)) {
> + struct inode *inode = list_first_entry(&sync_list, struct inode,
> + i_wb_list);
> struct address_space *mapping = inode->i_mapping;
>
> + /*
> + * Move each inode back to the wb list before we drop the lock
> + * to preserve consistency between i_wb_list and the mapping
> + * writeback tag. Writeback completion is responsible to remove
> + * the inode from either list once the writeback tag is cleared.
> + */
> + list_move_tail(&inode->i_wb_list, &sb->s_inodes_wb);
> +
> + /*
> + * The mapping can appear untagged while still on-list since we
> + * do not have the mapping lock. Skip it here, wb completion
> + * will remove it.
> + */
> + if (!mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK))
> + continue;
> +
> + spin_unlock_irq(&sb->s_inode_wblist_lock);
> +
> spin_lock(&inode->i_lock);
> - if ((inode->i_state & (I_FREEING|I_WILL_FREE|I_NEW)) ||
> - (mapping->nrpages == 0)) {
> + if (inode->i_state & (I_FREEING|I_WILL_FREE|I_NEW)) {
> spin_unlock(&inode->i_lock);
> +
> + spin_lock_irq(&sb->s_inode_wblist_lock);
> continue;
> }
> __iget(inode);
> spin_unlock(&inode->i_lock);
> - spin_unlock(&sb->s_inode_list_lock);
> -
> - /*
> - * We hold a reference to 'inode' so it couldn't have been
> - * removed from s_inodes list while we dropped the
> - * s_inode_list_lock. We cannot iput the inode now as we can
> - * be holding the last reference and we cannot iput it under
> - * s_inode_list_lock. So we keep the reference and iput it
> - * later.
> - */
> - iput(old_inode);
> - old_inode = inode;
> + rcu_read_unlock();
>
> /*
> * We keep the error status of individual mapping so that
> @@ -2157,10 +2210,13 @@ static void wait_sb_inodes(struct super_block *sb)
>
> cond_resched();
>
> - spin_lock(&sb->s_inode_list_lock);
> + iput(inode);
> +
> + rcu_read_lock();
> + spin_lock_irq(&sb->s_inode_wblist_lock);
> }
> - spin_unlock(&sb->s_inode_list_lock);
> - iput(old_inode);
> + spin_unlock_irq(&sb->s_inode_wblist_lock);
> + rcu_read_unlock();
> mutex_unlock(&sb->s_sync_lock);
> }
>
> diff --git a/fs/inode.c b/fs/inode.c
> index e491e54..f5a7eb9 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -358,6 +358,7 @@ void inode_init_once(struct inode *inode)
> INIT_HLIST_NODE(&inode->i_hash);
> INIT_LIST_HEAD(&inode->i_devices);
> INIT_LIST_HEAD(&inode->i_io_list);
> + INIT_LIST_HEAD(&inode->i_wb_list);
> INIT_LIST_HEAD(&inode->i_lru);
> address_space_init_once(&inode->i_data);
> i_size_ordered_init(inode);
> @@ -500,6 +501,7 @@ void clear_inode(struct inode *inode)
> BUG_ON(!list_empty(&inode->i_data.private_list));
> BUG_ON(!(inode->i_state & I_FREEING));
> BUG_ON(inode->i_state & I_CLEAR);
> + BUG_ON(!list_empty(&inode->i_wb_list));
> /* don't need i_lock here, no concurrent mods to i_state */
> inode->i_state = I_FREEING | I_CLEAR;
> }
> diff --git a/fs/super.c b/fs/super.c
> index 1182af8..60dd44a 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -206,6 +206,8 @@ static struct super_block *alloc_super(struct file_system_type *type, int flags)
> mutex_init(&s->s_sync_lock);
> INIT_LIST_HEAD(&s->s_inodes);
> spin_lock_init(&s->s_inode_list_lock);
> + INIT_LIST_HEAD(&s->s_inodes_wb);
> + spin_lock_init(&s->s_inode_wblist_lock);
>
> if (list_lru_init_memcg(&s->s_dentry_lru))
> goto fail;
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index eb73d74..ac2797d 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -651,6 +651,7 @@ struct inode {
> #endif
> struct list_head i_lru; /* inode LRU list */
> struct list_head i_sb_list;
> + struct list_head i_wb_list; /* backing dev writeback list */
> union {
> struct hlist_head i_dentry;
> struct rcu_head i_rcu;
> @@ -1377,6 +1378,9 @@ struct super_block {
> /* s_inode_list_lock protects s_inodes */
> spinlock_t s_inode_list_lock ____cacheline_aligned_in_smp;
> struct list_head s_inodes; /* all inodes */
> +
> + spinlock_t s_inode_wblist_lock;
> + struct list_head s_inodes_wb; /* writeback inodes */
> };
>
> extern struct timespec current_fs_time(struct super_block *sb);
> diff --git a/include/linux/writeback.h b/include/linux/writeback.h
> index b333c94..90a380c 100644
> --- a/include/linux/writeback.h
> +++ b/include/linux/writeback.h
> @@ -379,4 +379,7 @@ void tag_pages_for_writeback(struct address_space *mapping,
>
> void account_page_redirty(struct page *page);
>
> +void sb_mark_inode_writeback(struct inode *inode);
> +void sb_clear_inode_writeback(struct inode *inode);
> +
> #endif /* WRITEBACK_H */
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 6fe7d15..a8b718137 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -2745,6 +2745,11 @@ int test_clear_page_writeback(struct page *page)
> __wb_writeout_inc(wb);
> }
> }
> +
> + if (mapping->host && !mapping_tagged(mapping,
> + PAGECACHE_TAG_WRITEBACK))
> + sb_clear_inode_writeback(mapping->host);
> +
> spin_unlock_irqrestore(&mapping->tree_lock, flags);
> } else {
> ret = TestClearPageWriteback(page);
> @@ -2773,11 +2778,24 @@ int __test_set_page_writeback(struct page *page, bool keep_write)
> spin_lock_irqsave(&mapping->tree_lock, flags);
> ret = TestSetPageWriteback(page);
> if (!ret) {
> + bool on_wblist;
> +
> + on_wblist = mapping_tagged(mapping,
> + PAGECACHE_TAG_WRITEBACK);
> +
> radix_tree_tag_set(&mapping->page_tree,
> page_index(page),
> PAGECACHE_TAG_WRITEBACK);
> if (bdi_cap_account_writeback(bdi))
> __inc_wb_stat(inode_to_wb(inode), WB_WRITEBACK);
> +
> + /*
> + * We can come through here when swapping anonymous
> + * pages, so we don't necessarily have an inode to track
> + * for sync.
> + */
> + if (mapping->host && !on_wblist)
> + sb_mark_inode_writeback(mapping->host);
> }
> if (!PageDirty(page))
> radix_tree_tag_clear(&mapping->page_tree,
> --
> 2.4.3
>
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
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