Re: [PATCH] ext4: make __ext4_get_inode_loc plug

[Zhangjs Jinshui]

> 在 2019年6月19日，19:08，Jan Kara <jack@xxxxxxx> 写道：
> 
> On Mon 17-06-19 23:57:12, jinshui zhang wrote:
>> From: zhangjs <zachary@xxxxxxxxxxxxxxxx>
>> 
>> If the task is unplugged when called, the inode_readahead_blks may not be merged, 
>> these will cause small pieces of io, It should be plugged.
>> 
>> Signed-off-by: zhangjs <zachary@xxxxxxxxxxxxxxxx>
> 
> Out of curiosity, on which path do you see __ext4_get_inode_loc() being
> called without IO already plugged?
> 
> Otherwise the patch looks good to me. You can add:
> 
> Reviewed-by: Jan Kara <jack@xxxxxxx>
> 
> 								Honza
> 
>> ---
>> fs/ext4/inode.c | 6 ++++++
>> 1 file changed, 6 insertions(+)
>> 
>> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
>> index c7f77c6..8fe046b 100644
>> --- a/fs/ext4/inode.c
>> +++ b/fs/ext4/inode.c
>> @@ -4570,6 +4570,7 @@ static int __ext4_get_inode_loc(struct inode *inode,
>> 	struct buffer_head	*bh;
>> 	struct super_block	*sb = inode->i_sb;
>> 	ext4_fsblk_t		block;
>> +	struct blk_plug		plug;
>> 	int			inodes_per_block, inode_offset;
>> 
>> 	iloc->bh = NULL;
>> @@ -4654,6 +4655,8 @@ static int __ext4_get_inode_loc(struct inode *inode,
>> 		}
>> 
>> make_io:
>> +		blk_start_plug(&plug);
>> +
>> 		/*
>> 		 * If we need to do any I/O, try to pre-readahead extra
>> 		 * blocks from the inode table.
>> @@ -4688,6 +4691,9 @@ static int __ext4_get_inode_loc(struct inode *inode,
>> 		get_bh(bh);
>> 		bh->b_end_io = end_buffer_read_sync;
>> 		submit_bh(REQ_OP_READ, REQ_META | REQ_PRIO, bh);
>> +
>> +		blk_finish_plug(&plug);
>> +
>> 		wait_on_buffer(bh);
>> 		if (!buffer_uptodate(bh)) {
>> 			EXT4_ERROR_INODE_BLOCK(inode, block,
>> -- 
>> 1.8.3.1
>> 
> -- 
> Jan Kara <jack@xxxxxxxx>
> SUSE Labs, CR

You can blktrace

  8,80  31       11     0.296373038 2885275  Q  RA 8279571464 + 8 [xxxx]
  8,80  31       12     0.296374017 2885275  G  RA 8279571464 + 8 [xxxx]
  8,80  31       13     0.296375468 2885275  I  RA 8279571464 + 8 [xxxx]
  8,80  31       14     0.296382099  3886  D  RA 8279571464 + 8 [kworker/31:1H]
  8,80  31       15     0.296391907 2885275  Q  RA 8279571472 + 8 [xxxx]
  8,80  31       16     0.296392275 2885275  G  RA 8279571472 + 8 [xxxx]
  8,80  31       17     0.296393305 2885275  I  RA 8279571472 + 8 [xxxx]
  8,80  31       18     0.296395844  3886  D  RA 8279571472 + 8 [kworker/31:1H]
  8,80  31       19     0.296399685 2885275  Q  RA 8279571480 + 8 [xxxx]
  8,80  31       20     0.296400025 2885275  G  RA 8279571480 + 8 [xxxx]
  8,80  31       21     0.296401232 2885275  I  RA 8279571480 + 8 [xxxx]
  8,80  31       22     0.296403422  3886  D  RA 8279571480 + 8 [kworker/31:1H]
  8,80  31       23     0.296407375 2885275  Q  RA 8279571488 + 8 [xxxx]
  8,80  31       24     0.296407721 2885275  G  RA 8279571488 + 8 [xxxx]
  8,80  31       25     0.296408904 2885275  I  RA 8279571488 + 8 [xxxx]
  8,80  31       26     0.296411127  3886  D  RA 8279571488 + 8 [kworker/31:1H]
  8,80  31       27     0.296414779 2885275  Q  RA 8279571496 + 8 [xxxx]
  8,80  31       28     0.296415119 2885275  G  RA 8279571496 + 8 [xxxx]
  8,80  31       29     0.296415744 2885275  I  RA 8279571496 + 8 [xxxx]
  8,80  31       30     0.296417779  3886  D  RA 8279571496 + 8 [kworker/31:1H]

these RA io were caused by ext4_inode_readahead_blks, there are all not merged becourse of the unplugged state.
the backtrace shows below, was traced by systemtap ioblock.request filtered by "opf & 1 << 19"

 0xffffffff8136fb20 : generic_make_request+0x0/0x2f0 [kernel]
 0xffffffff8136fe7e : submit_bio+0x6e/0x130 [kernel]
 0xffffffff812971e6 : submit_bh_wbc+0x156/0x190 [kernel]
 0xffffffff81297bca : ll_rw_block+0x6a/0xb0 [kernel]
 0xffffffff81297cc0 : __breadahead+0x40/0x70 [kernel]
 0xffffffffa0392c9a : __ext4_get_inode_loc+0x37a/0x3d0 [ext4]
 0xffffffffa0396a6c : ext4_iget+0x8c/0xc00 [ext4]
 0xffffffffa03ad98a : ext4_lookup+0xca/0x1d0 [ext4]
 0xffffffff8126b814 : path_openat+0xcb4/0x1250 [kernel]
 0xffffffff8126dc41 : do_filp_open+0x91/0x100 [kernel]
 0xffffffff8125ad86 : do_sys_open+0x126/0x210 [kernel]
 0xffffffff81003864 : do_syscall_64+0x74/0x1a0 [kernel]
 0xffffffff81800081 : entry_SYSCALL_64_after_hwframe+0x3d/0xa2 [kernel]

I have patched it on online servers, It can improved the performance.