Re: [PATCH 0/2] xfs: regression fixes for 3.5-rc7

Dave Chinner
2012-07-11T23:53:01Z

On Wed, Jul 11, 2012 at 05:48:22PM -0500, Ben Myers wrote:
> Hey Dave,
>
> On Thu, Jul 12, 2012 at 07:40:41AM +1000, Dave Chinner wrote:
> > These two patches need to go to Linus before 3.5 is released. The
> > first fixes the btree cursor leak properly, and the second fixes a
> > significant performance regression reported by Mel Gorman. Can you
> > review them and send them onwards to Linus?
>
> I have some test exposure on the first one, but not on the second. I'd rather
> have a known performance regression in 3.5 than start popping stacks again this
> late toward the end of the rc cycle. I suggest that #2 go in for the 3.6 merge
> window and back to 3.5 via -stable. Any other opinions?

I'd prefer not to have to go via a -stable kernel. It's a pain to
have to keep track and maintain multiple -stable kernels...

As it is, I've never once seen a popped stack through a metadata
allocation triggered path. The worst I've seen in the past few days
is this: a bmbt block allocation in the sync write path:

Depth Size Location (45 entries)
----- ---- --------
0) 5880 40 zone_statistics+0xbd/0xc0
1) 5840 272 get_page_from_freelist+0x3db/0x870
2) 5568 304 __alloc_pages_nodemask+0x192/0x840
3) 5264 80 alloc_pages_current+0xb0/0x120
4) 5184 96 xfs_buf_allocate_memory+0xfc/0x270
5) 5088 80 xfs_buf_get_map+0x15d/0x1b0
6) 5008 64 xfs_buf_read_map+0x33/0x130
7) 4944 48 xfs_buf_readahead_map+0x51/0x70
8) 4896 64 xfs_btree_reada_bufs+0xa3/0xc0
9) 4832 48 xfs_btree_readahead_sblock+0x7e/0xa0
10) 4784 16 xfs_btree_readahead+0x5f/0x80
11) 4768 96 xfs_btree_increment+0x52/0x360
12) 4672 208 xfs_btree_rshift+0x6bf/0x730
13) 4464 288 xfs_btree_delrec+0x1308/0x14d0
14) 4176 64 xfs_btree_delete+0x41/0xe0
15) 4112 112 xfs_alloc_fixup_trees+0x21b/0x580
16) 4000 192 xfs_alloc_ag_vextent_near+0xb05/0xcd0
17) 3808 32 xfs_alloc_ag_vextent+0x228/0x2a0
18) 3776 112 __xfs_alloc_vextent+0x4b7/0x910
19) 3664 80 xfs_alloc_vextent+0xa5/0xb0
20) 3584 224 xfs_bmbt_alloc_block+0xd1/0x240
21) 3360 240 xfs_btree_split+0xe2/0x7e0
22) 3120 96 xfs_btree_make_block_unfull+0xde/0x1d0
23) 3024 240 xfs_btree_insrec+0x587/0x8e0
24) 2784 112 xfs_btree_insert+0x6b/0x1d0
25) 2672 256 xfs_bmap_add_extent_delay_real+0x1fd4/0x2210
26) 2416 80 xfs_bmapi_allocate+0x290/0x350
27) 2336 368 xfs_bmapi_write+0x66e/0xa60
28) 1968 176 xfs_iomap_write_allocate+0x131/0x350
29) 1792 112 xfs_map_blocks+0x302/0x390
30) 1680 192 xfs_vm_writepage+0x19b/0x5a0
31) 1488 32 __writepage+0x1a/0x50
32) 1456 304 write_cache_pages+0x258/0x4d0
33) 1152 96 generic_writepages+0x4d/0x70
34) 1056 48 xfs_vm_writepages+0x4d/0x60
35) 1008 16 do_writepages+0x21/0x50
36) 992 64 __filemap_fdatawrite_range+0x59/0x60
37) 928 16 filemap_fdatawrite_range+0x13/0x20
38) 912 80 xfs_flush_pages+0x75/0xb0
39) 832 176 xfs_file_buffered_aio_write+0xdf/0x1e0
40) 656 128 xfs_file_aio_write+0x157/0x1d0
41) 528 272 do_sync_write+0xde/0x120
42) 256 48 vfs_write+0xa8/0x160
43) 208 80 sys_write+0x4a/0x90
44) 128 128 system_call_fastpath+0x16/0x1b

This is not quite the insanely complex, worst case double tree split
path (i.e. bmbt split, followed by a allocbt split), but it's still
very close to the worst case stack usage I've seen in above
xfs_bmapi_write() (i.e. >3.5k of stack from xfs_vm_writepage() to
the last XFS function in the stack).

In theory, the directory code could trigger such a stack, and it
calls xfs_bmapi_write() from roughly the same stack depth. It woul
dtake quite a large directory to be causing splits in the bmbt tree,
so it's a relatively rare occurance....

Also, while this is arguably a metadata allocation here, this is in
the data writeback path so perhaps the xfs_bmbt_alloc_block() call
needs to trigger allocation via a workqueue here. That would make
the worst case bmbt split usage (even for directory allocation)
switch stacks. It's rare enough that it shouldn't introduce
performance issues....

What is notable here, however, is that none of the directory or
inode allocation paths consumed more stack than this sync write
case. That indicates that my statement assumption about inode and
directory allocation does hold some water....

> I'll feel better about if after some testing, so I'll get tests running asap.
> What testing have you and Mel done?

Mel reran his performance tests that were showing regressions, and
those regressions went away, and I've rerun all my perf tests and
xfstests for the past couple of days and not seen any changes in
performance or stack usage.

> IMO we should also consider "xfs: prevent recursion in xfs_buf_iorequest", from
> Christoph.

Yes, I think so. You don't have to wait for me to propose fixes to
suggest other patches that could be considered for Linus updates... ;)

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com