Re: [PATCH v4 2/2] ext4: limit the length of per-inode prealloc list

[Ritesh Harjani <riteshh@xxxxxxxxxxxxx>]

Hi brookxu,

Few queries

On 8/7/20 2:14 PM, brookxu wrote:
> In the scenario of writing sparse files, the Per-inode prealloc list may
> be very long, resulting in high overhead for ext4_mb_use_preallocated().
> To circumvent this problem, we limit the maximum length of per-inode
> prealloc list to 512 and allow users to modify it.
>
> After patching, we observed that the sys ratio of cpu has dropped, and
> the system throughput has increased significantly. We created a process
> to write the sparse file, and the running time of the process on the
> fixed kernel was significantly reduced, as follows:
>
> Running time on unfixed kernel：
> [root@TENCENT64 ~]# time taskset 0x01 ./sparse /data1/sparce.dat
> real    0m2.051s
> user    0m0.008s
> sys     0m2.026s
>
> Running time on fixed kernel：
> [root@TENCENT64 ~]# time taskset 0x01 ./sparse /data1/sparce.dat
> real    0m0.471s
> user    0m0.004s
> sys     0m0.395s


Please use "--thread" option in your git-format-patch script. So that
your patch series comes in a threaded form. It is easier then to apply
the series.

Somehow I am unable to get my head around how this patch is working.
Could you please help explain the theory of operation by answering below
queries.

> Signed-off-by: Chunguang Xu <brookxu@xxxxxxxxxxx>
> ---
>   fs/ext4/ext4.h              |  3 ++-
>   fs/ext4/extents.c           | 10 ++++----
>   fs/ext4/file.c              |  2 +-
>   fs/ext4/indirect.c          |  2 +-
>   fs/ext4/inode.c             |  6 ++---
>   fs/ext4/ioctl.c             |  2 +-
>   fs/ext4/mballoc.c           | 57 +++++++++++++++++++++++++++++++++++++++++----
>   fs/ext4/mballoc.h           |  4 ++++
>   fs/ext4/move_extent.c       |  4 ++--
>   fs/ext4/super.c             |  2 +-
>   fs/ext4/sysfs.c             |  2 ++
>   include/trace/events/ext4.h | 14 ++++++-----
>   12 files changed, 83 insertions(+), 25 deletions(-)
>
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 42f5060..68e0ebe 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -1501,6 +1501,7 @@ struct ext4_sb_info {
>   	unsigned int s_mb_stats;
>   	unsigned int s_mb_order2_reqs;
>   	unsigned int s_mb_group_prealloc;
> +	unsigned int s_mb_max_inode_prealloc;
>   	unsigned int s_max_dir_size_kb;
>   	/* where last allocation was done - for stream allocation */
>   	unsigned long s_mb_last_group;
> @@ -2651,7 +2652,7 @@ extern int ext4_init_inode_table(struct super_block *sb,
>   extern ext4_fsblk_t ext4_mb_new_blocks(handle_t *,
>   				struct ext4_allocation_request *, int *);
>   extern int ext4_mb_reserve_blocks(struct super_block *, int);
> -extern void ext4_discard_preallocations(struct inode *);
> +extern void ext4_discard_preallocations(struct inode *, unsigned int);
>   extern int __init ext4_init_mballoc(void);
>   extern void ext4_exit_mballoc(void);
>   extern void ext4_free_blocks(handle_t *handle, struct inode *inode,
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index 221f240..a40f928 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -100,7 +100,7 @@ static int ext4_ext_trunc_restart_fn(struct inode *inode, int *dropped)
>   	 * i_mutex. So we can safely drop the i_data_sem here.
>   	 */
>   	BUG_ON(EXT4_JOURNAL(inode) == NULL);
> -	ext4_discard_preallocations(inode);
> +	ext4_discard_preallocations(inode, 0);
>   	up_write(&EXT4_I(inode)->i_data_sem);
>   	*dropped = 1;
>   	return 0;
> @@ -4272,7 +4272,7 @@ int ext4_ext_map_blocks(handle_t *handle, struct inode *inode,
>   			 * not a good idea to call discard here directly,
>   			 * but otherwise we'd need to call it every free().
>   			 */
> -			ext4_discard_preallocations(inode);
> +			ext4_discard_preallocations(inode, 0);
>   			if (flags & EXT4_GET_BLOCKS_DELALLOC_RESERVE)
>   				fb_flags = EXT4_FREE_BLOCKS_NO_QUOT_UPDATE;
>   			ext4_free_blocks(handle, inode, NULL, newblock,
> @@ -5299,7 +5299,7 @@ static int ext4_collapse_range(struct inode *inode, loff_t offset, loff_t len)
>   	}
>
>   	down_write(&EXT4_I(inode)->i_data_sem);
> -	ext4_discard_preallocations(inode);
> +	ext4_discard_preallocations(inode, 0);
>
>   	ret = ext4_es_remove_extent(inode, punch_start,
>   				    EXT_MAX_BLOCKS - punch_start);
> @@ -5313,7 +5313,7 @@ static int ext4_collapse_range(struct inode *inode, loff_t offset, loff_t len)
>   		up_write(&EXT4_I(inode)->i_data_sem);
>   		goto out_stop;
>   	}
> -	ext4_discard_preallocations(inode);
> +	ext4_discard_preallocations(inode, 0);
>
>   	ret = ext4_ext_shift_extents(inode, handle, punch_stop,
>   				     punch_stop - punch_start, SHIFT_LEFT);
> @@ -5445,7 +5445,7 @@ static int ext4_insert_range(struct inode *inode, loff_t offset, loff_t len)
>   		goto out_stop;
>
>   	down_write(&EXT4_I(inode)->i_data_sem);
> -	ext4_discard_preallocations(inode);
> +	ext4_discard_preallocations(inode, 0);
>
>   	path = ext4_find_extent(inode, offset_lblk, NULL, 0);
>   	if (IS_ERR(path)) {
> diff --git a/fs/ext4/file.c b/fs/ext4/file.c
> index 2a01e31..e3ab8ea 100644
> --- a/fs/ext4/file.c
> +++ b/fs/ext4/file.c
> @@ -148,7 +148,7 @@ static int ext4_release_file(struct inode *inode, struct file *filp)
>   		        !EXT4_I(inode)->i_reserved_data_blocks)
>   	{
>   		down_write(&EXT4_I(inode)->i_data_sem);
> -		ext4_discard_preallocations(inode);
> +		ext4_discard_preallocations(inode, 0);
>   		up_write(&EXT4_I(inode)->i_data_sem);
>   	}
>   	if (is_dx(inode) && filp->private_data)
> diff --git a/fs/ext4/indirect.c b/fs/ext4/indirect.c
> index be2b66e..ec6b930 100644
> --- a/fs/ext4/indirect.c
> +++ b/fs/ext4/indirect.c
> @@ -696,7 +696,7 @@ static int ext4_ind_trunc_restart_fn(handle_t *handle, struct inode *inode,
>   	 * i_mutex. So we can safely drop the i_data_sem here.
>   	 */
>   	BUG_ON(EXT4_JOURNAL(inode) == NULL);
> -	ext4_discard_preallocations(inode);
> +	ext4_discard_preallocations(inode, 0);
>   	up_write(&EXT4_I(inode)->i_data_sem);
>   	*dropped = 1;
>   	return 0;
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 10dd470..bb9e1cd 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -383,7 +383,7 @@ void ext4_da_update_reserve_space(struct inode *inode,
>   	 */
>   	if ((ei->i_reserved_data_blocks == 0) &&
>   	    !inode_is_open_for_write(inode))
> -		ext4_discard_preallocations(inode);
> +		ext4_discard_preallocations(inode, 0);
>   }
>
>   static int __check_block_validity(struct inode *inode, const char *func,
> @@ -4056,7 +4056,7 @@ int ext4_punch_hole(struct inode *inode, loff_t offset, loff_t length)
>   	if (stop_block > first_block) {
>
>   		down_write(&EXT4_I(inode)->i_data_sem);
> -		ext4_discard_preallocations(inode);
> +		ext4_discard_preallocations(inode, 0);
>
>   		ret = ext4_es_remove_extent(inode, first_block,
>   					    stop_block - first_block);
> @@ -4211,7 +4211,7 @@ int ext4_truncate(struct inode *inode)
>
>   	down_write(&EXT4_I(inode)->i_data_sem);
>
> -	ext4_discard_preallocations(inode);
> +	ext4_discard_preallocations(inode, 0);
>
>   	if (ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS))
>   		err = ext4_ext_truncate(handle, inode);
> diff --git a/fs/ext4/ioctl.c b/fs/ext4/ioctl.c
> index 999cf6a..a5fcc23 100644
> --- a/fs/ext4/ioctl.c
> +++ b/fs/ext4/ioctl.c
> @@ -202,7 +202,7 @@ static long swap_inode_boot_loader(struct super_block *sb,
>   	reset_inode_seed(inode);
>   	reset_inode_seed(inode_bl);
>
> -	ext4_discard_preallocations(inode);
> +	ext4_discard_preallocations(inode, 0);
>
>   	err = ext4_mark_inode_dirty(handle, inode);
>   	if (err < 0) {
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 4f21f34..28a139f 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -2736,6 +2736,7 @@ int ext4_mb_init(struct super_block *sb)
>   	sbi->s_mb_stats = MB_DEFAULT_STATS;
>   	sbi->s_mb_stream_request = MB_DEFAULT_STREAM_THRESHOLD;
>   	sbi->s_mb_order2_reqs = MB_DEFAULT_ORDER2_REQS;
> +	sbi->s_mb_max_inode_prealloc = MB_DEFAULT_MAX_INODE_PREALLOC;
>   	/*
>   	 * The default group preallocation is 512, which for 4k block
>   	 * sizes translates to 2 megabytes.  However for bigalloc file
> @@ -4103,7 +4104,7 @@ static void ext4_mb_new_preallocation(struct ext4_allocation_context *ac)
>    *
>    * FIXME!! Make sure it is valid at all the call sites
>    */
> -void ext4_discard_preallocations(struct inode *inode)
> +void ext4_discard_preallocations(struct inode *inode, unsigned int needed)
>   {
>   	struct ext4_inode_info *ei = EXT4_I(inode);
>   	struct super_block *sb = inode->i_sb;
> @@ -4121,15 +4122,18 @@ void ext4_discard_preallocations(struct inode *inode)
>
>   	mb_debug(sb, "discard preallocation for inode %lu\n",
>   		 inode->i_ino);
> -	trace_ext4_discard_preallocations(inode);
> +	trace_ext4_discard_preallocations(inode,  needed);
>
>   	INIT_LIST_HEAD(&list);
>
> +	if (needed == 0)
> +		needed = UINT_MAX;
> +
>   repeat:
>   	/* first, collect all pa's in the inode */
>   	spin_lock(&ei->i_prealloc_lock);
> -	while (!list_empty(&ei->i_prealloc_list)) {
> -		pa = list_entry(ei->i_prealloc_list.next,
> +	while (!list_empty(&ei->i_prealloc_list) && needed) {
> +		pa = list_entry(ei->i_prealloc_list.prev,
>   				struct ext4_prealloc_space, pa_inode_list);
>   		BUG_ON(pa->pa_obj_lock != &ei->i_prealloc_lock);
>   		spin_lock(&pa->pa_lock);
> @@ -4150,6 +4154,7 @@ void ext4_discard_preallocations(struct inode *inode)
>   			spin_unlock(&pa->pa_lock);
>   			list_del_rcu(&pa->pa_inode_list);
>   			list_add(&pa->u.pa_tmp_list, &list);
> +			needed--;
>   			continue;
>   		}
>
> @@ -4549,10 +4554,42 @@ static void ext4_mb_add_n_trim(struct ext4_allocation_context *ac)
>   }
>
>   /*
> + * if per-inode prealloc list is too long, trim some PA
> + */
> +static void
> +ext4_mb_trim_inode_pa(struct inode *inode)
> +{
> +	struct ext4_inode_info *ei = EXT4_I(inode);
> +	struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
> +	struct ext4_prealloc_space *pa;
> +	int count = 0, delta;
> +
> +	rcu_read_lock();
> +	list_for_each_entry_rcu(pa, &ei->i_prealloc_list, pa_inode_list) {
> +		spin_lock(&pa->pa_lock);
> +		if (pa->pa_deleted) {
> +			spin_unlock(&pa->pa_lock);
> +			continue;
> +		}
> +		count++;
> +		spin_unlock(&pa->pa_lock);
> +	}
> +	rcu_read_unlock();
> +
> +	delta = (sbi->s_mb_max_inode_prealloc >> 2) + 1;
> +	if (count > sbi->s_mb_max_inode_prealloc + delta) {
> +		count -= sbi->s_mb_max_inode_prealloc;
> +		ext4_discard_preallocations(inode, count);
> +	}
> +}
> +
> +/*
>    * release all resource we used in allocation
>    */
>   static int ext4_mb_release_context(struct ext4_allocation_context *ac)
>   {
> +	struct inode *inode = ac->ac_inode;
> +	struct ext4_inode_info *ei = EXT4_I(inode);
>   	struct ext4_sb_info *sbi = EXT4_SB(ac->ac_sb);
>   	struct ext4_prealloc_space *pa = ac->ac_pa;
>   	if (pa) {
> @@ -4578,6 +4615,17 @@ static int ext4_mb_release_context(struct ext4_allocation_context *ac)
>   				ext4_mb_add_n_trim(ac);
>   			}
>   		}
> +
> +		if (pa->pa_type == MB_INODE_PA) {
> +			/*
> +			 * treat per-inode prealloc list as a lru list, then try
> +			 * to trim the least recently used PA.
> +			 */
> +			spin_lock(pa->pa_obj_lock);
> +			list_move(&ei->i_prealloc_list, &pa->pa_inode_list);

So what you are doing here is essentially moving back 
ei->i_prealloc_list back into pa->pa_inode_list.

Then in ext4_mb_trim_inode_pa(), it will traverse over i_prealloc_list
which is empty anyways no? (considering single threaded write here).
I am definitely missing something, could you help explain how is it
getting trimmed then?
and how are you adding it back to ext4_inode_info->i_prealloc_list
which is traversed to check if we can use pe-existing preallocation
of an inode in ext4_mb_use_preallocated(ac)



> +			spin_unlock(pa->pa_obj_lock);
> +		}
> +
>   		ext4_mb_put_pa(ac, ac->ac_sb, pa);
>   	}
>   	if (ac->ac_bitmap_page)
> @@ -4587,6 +4635,7 @@ static int ext4_mb_release_context(struct ext4_allocation_context *ac)
>   	if (ac->ac_flags & EXT4_MB_HINT_GROUP_ALLOC)
>   		mutex_unlock(&ac->ac_lg->lg_mutex);
>   	ext4_mb_collect_stats(ac);
> +	ext4_mb_trim_inode_pa(inode);
>   	return 0;
>   }