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

[Ritesh Harjani <ritesh.list@xxxxxxxxx>]

On 8/14/20 6:41 AM, brookxu wrote:
> Thanks for review this patch :)
>
> Ritesh Harjani wrote on 2020/8/13 21:52:
> > 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);

Can we please also add "needed" argument in mb_debug() msg here.

With that feel free to add:
Reviewed-by: Ritesh Harjani <riteshh@xxxxxxxxxxxxx>


> > > -    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_lis
>
> The operation here uses LRU to maintain the prealloc list, so we can trim the least recently used PA
> from the tail of the list. According to the principle of locality, this may reduce the impact on the
> program.
>
> > 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?

Sorry, I had got confused here with something else. This looks fine to 
me now :)