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Date: Wed, 16 Aug 2017 18:42:11 +0200
From: Jan Kara <jack@...e.cz>
To: Wang Shilong <wangshilong1991@...il.com>
Cc: linux-ext4@...r.kernel.org, tytso@....edu, wshilong@....com,
adilger@...ger.ca, sihara@....com, lixi@....com
Subject: Re: [PATCH v4] ext4: reduce lock contention in __ext4_new_inode
On Tue 08-08-17 13:05:17, Wang Shilong wrote:
> From: Wang Shilong <wshilong@....com>
>
> While running number of creating file threads concurrently,
> we found heavy lock contention on group spinlock:
>
> FUNC TOTAL_TIME(us) COUNT AVG(us)
> ext4_create 1707443399 1440000 1185.72
> _raw_spin_lock 1317641501 180899929 7.28
> jbd2__journal_start 287821030 1453950 197.96
> jbd2_journal_get_write_access 33441470 73077185 0.46
> ext4_add_nondir 29435963 1440000 20.44
> ext4_add_entry 26015166 1440049 18.07
> ext4_dx_add_entry 25729337 1432814 17.96
> ext4_mark_inode_dirty 12302433 5774407 2.13
>
> most of cpu time blames to _raw_spin_lock, here is some testing
> numbers with/without patch.
>
> Test environment:
> Server : SuperMicro Sever (2 x E5-2690 v3@...0GHz, 128GB 2133MHz
> DDR4 Memory, 8GbFC)
> Storage : 2 x RAID1 (DDN SFA7700X, 4 x Toshiba PX02SMU020 200GB
> Read Intensive SSD)
>
> format command:
> mkfs.ext4 -J size=4096
>
> test command:
> mpirun -np 48 mdtest -n 30000 -d /ext4/mdtest.out -F -C \
> -r -i 1 -v -p 10 -u #first run to load inode
>
> mpirun -np 48 mdtest -n 30000 -d /ext4/mdtest.out -F -C \
> -r -i 5 -v -p 10 -u
>
> Kernel version: 4.13.0-rc3
>
> Test 1,440,000 files with 48 directories by 48 processes:
>
> Without patch:
>
> File Creation File removal
> 79,033 289,569 ops/per second
> 81,463 285,359
> 79,875 288,475
> 79,917 284,624
> 79,420 290,91
>
> with patch:
> File Creation File removal
> 691,528 296,574 ops/per second
> 691,946 297,106
> 692,030 296,238
> 691,005 299,249
> 692,871 300,664
>
> Creation performance is improved more than 8X with large
> journal size. The main problem here is we test bitmap
> and do some check and journal operations which could be
> slept, then we test and set with lock hold, this could
> be racy, and make 'inode' steal by other process.
>
> However, after first try, we could confirm handle has
> been started and inode bitmap journaled too, then
> we could find and set bit with lock hold directly, this
> will mostly gurateee success with second try.
>
> This patch dosen't change logic if it comes to
> no journal mode, luckily this is not normal
> use cases i believe.
>
> Tested-by: Shuichi Ihara <sihara@....com>
> Signed-off-by: Wang Shilong <wshilong@....com>
The results look great and the code looks correct however I dislike the
somewhat complex codeflow with your hold_lock variable. So how about
cleaning up the code as follows:
Create function like
unsigned long find_inode_bit(struct super_block *sb, ext4_group_t group,
struct buffer_head *bitmap, unsigned long start_ino)
{
unsigned long ino;
next:
ino = ext4_find_next_zero_bit(...);
if (ino >= EXT4_INODES_PER_GROUP(sb))
return 0;
if (group == 0 && (ino+1) < EXT4_FIRST_INO(sb)) {
...
return 0;
}
if ((EXT4_SB(sb)->s_journal == NULL) &&
recently_deleted(sb, group, ino)) {
start_ino = ino + 1;
if (start_ino < EXT4_INODES_PER_GROUP(sb))
goto next;
}
return ino;
}
Then you can use this function from __ext4_new_inode() when looking for
free ino and also in case test_and_set_bit() fails you could just do:
ext4_lock_group(sb, group);
ret2 = ext4_test_and_set_bit(ino, inode_bitmap_bh->b_data);
if (ret2) {
/* Someone already took the bit. Repeat the search with lock held.*/
ino = find_inode_bit(sb, group, inode_bitmap_bh, ino);
if (ino) {
ret2 = ext4_test_and_set_bit(ino, inode_bitmap_bh->b_data);
WARN_ON_ONCE(!ret2);
}
}
ext4_unlock_group(sb, group);
And that's it, no strange bool variables and conditional locking. And as a
bonus it also works for nojournal mode in the same way.
Honza
> ---
> v3->v4: codes cleanup and avoid sleep.
> ---
> fs/ext4/ialloc.c | 30 +++++++++++++++++++++++++++++-
> 1 file changed, 29 insertions(+), 1 deletion(-)
>
> diff --git a/fs/ext4/ialloc.c b/fs/ext4/ialloc.c
> index 507bfb3..23380f39 100644
> --- a/fs/ext4/ialloc.c
> +++ b/fs/ext4/ialloc.c
> @@ -761,6 +761,7 @@ struct inode *__ext4_new_inode(handle_t *handle, struct inode *dir,
> ext4_group_t flex_group;
> struct ext4_group_info *grp;
> int encrypt = 0;
> + bool hold_lock;
>
> /* Cannot create files in a deleted directory */
> if (!dir || !dir->i_nlink)
> @@ -917,17 +918,40 @@ struct inode *__ext4_new_inode(handle_t *handle, struct inode *dir,
> continue;
> }
>
> + hold_lock = false;
> repeat_in_this_group:
> + /* if @hold_lock is ture, that means, journal
> + * is properly setup and inode bitmap buffer has
> + * been journaled already, we can directly hold
> + * lock and set bit if found, this will mostly
> + * gurantee forward progress for each thread.
> + */
> + if (hold_lock)
> + ext4_lock_group(sb, group);
> +
> ino = ext4_find_next_zero_bit((unsigned long *)
> inode_bitmap_bh->b_data,
> EXT4_INODES_PER_GROUP(sb), ino);
> - if (ino >= EXT4_INODES_PER_GROUP(sb))
> + if (ino >= EXT4_INODES_PER_GROUP(sb)) {
> + if (hold_lock)
> + ext4_unlock_group(sb, group);
> goto next_group;
> + }
> if (group == 0 && (ino+1) < EXT4_FIRST_INO(sb)) {
> + if (hold_lock)
> + ext4_unlock_group(sb, group);
> ext4_error(sb, "reserved inode found cleared - "
> "inode=%lu", ino + 1);
> continue;
> }
> +
> + if (hold_lock) {
> + ext4_set_bit(ino, inode_bitmap_bh->b_data);
> + ext4_unlock_group(sb, group);
> + ino++;
> + goto got;
> + }
> +
> if ((EXT4_SB(sb)->s_journal == NULL) &&
> recently_deleted(sb, group, ino)) {
> ino++;
> @@ -950,6 +974,10 @@ struct inode *__ext4_new_inode(handle_t *handle, struct inode *dir,
> ext4_std_error(sb, err);
> goto out;
> }
> +
> + if (EXT4_SB(sb)->s_journal)
> + hold_lock = true;
> +
> ext4_lock_group(sb, group);
> ret2 = ext4_test_and_set_bit(ino, inode_bitmap_bh->b_data);
> ext4_unlock_group(sb, group);
> --
> 2.9.3
>
--
Jan Kara <jack@...e.com>
SUSE Labs, CR
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