On 09/24/12 19:56, Dave Chinner wrote:
On Mon, Sep 24, 2012 at 01:09:04PM -0500, Mark Tinguely wrote:
From bpm@xxxxxxx Mon Sep 24 12:11:59 2012
Date: Mon, 24 Sep 2012 12:11:59 -0500
From: Ben Myers<bpm@xxxxxxx>
To:<tinguely@xxxxxxx>
Subject: Re: [PATCH 0/3] xfs: allocation worker causes freelist buffer lock
hang
Cc:<xfs@xxxxxxxxxxx>
Hi Mark,
On Wed, Sep 19, 2012 at 11:31:33AM -0500, tinguely@xxxxxxx wrote:
...
I traced the callers of xfs_alloc_vextent(), noting transaction creation,
commits and cancels; I noted loops in the callers and which were marked
as metadata/userdata. I can send this document to reviewers.
I'd like to see the doc of xfs_alloc_vextent callers and which of them loop.
Can you post your doc to the list?
Regards,
Ben
Here are the Linux 3.6.x callers of xfs_alloc_vextent() stopping at the
transaction commit/cancel level.
XFS_BMAPI_METADATA *not* being set implies user data.
Userdata being set is the community designated indication that an allocate
worker is needed to prevent the overflow of the kernel stack.
Calling xfs_alloc_vextent() several times with the same transaction can cause
a dead lock if a new allocation worker can not be allocated. I noted where the
loops occur. xfs_alloc_vextent() can call itself, those calls must be in the
same allocation worker.
As a bonus, consolidating the loops into one worker actually gives a slight
performance advantage.
Can you quantify it?
I was comparing the bonnie and iozone benchmarks outputs. I will see if
someone can enlighten me on how to quantify those numbers.
Sorry this is wider than 80 characters wide.
---
xfs_bmap_btalloc(xfs_bmalloca_t)
! xfs_bmap_alloc(xfs_bmalloca_t)
! ! xfs_bmapi_allocate(xfs_bmalloca_t, ...)
! ! ! xfs_bmapi_write(xfs_trans_t ...) loops over above
! ! ! ! xfs_attr_rmtval_set(xfs_da_args_t) loops over bmapi_write (xfs_attr_set_int) **XFS_BMAPI_METADATA**
! ! ! ! xfs_da_grow_inode_int(xfs_da_args_t) loops over bmapi_write **XFS_BMAPI_METADATA**
! ! ! ! xfs_qm_dqalloc(xfs_trans_t ...) does a xfs_bmap_finish/cancel **XFS_BMAPI_METADATA**
! ! ! ! xfs_iomap_write_direct(...) alloc trans, xfs_trans_commit/cancel
! ! ! ! xfs_iomap_write_allocate(...) alloc trans, xfs_trans_commit/cancel safe loop
! ! ! ! xfs_iomap_write_unwritten(..) alloc trans, xfs_trans_commit/cancel safe loop
! ! ! ! xfs_growfs_rt_alloc(..) alloc trans, xfs_trans_commit/cancel safe loop
! ! ! ! xfs_symlink(...) allocates trans does a xfs_trans_commit/cancel
! ! ! ! xfs_alloc_file_space(...) alloc trans, xfs_trans_commit/cancel safe loop
So the only data path callers though here are
xfs_iomap_write_direct(), xfs_iomap_write_allocate() and
xfs_iomap_write_unwritten() and xfs_alloc_file_space(). Everything
else is metadata, so won't use
Correct. Only these use the worker. All the other paths directly call
the __xfs_alloc_vextent() routine. I saved the xfs_bmalloca and
fs_alloc_arg when allocating a buffer. I am quite confident that this is
the only path that causes the problem.
xfs_bmap_extents_to_btree(xfs_trans_t ...)<- set userdata to 0 (patch 3)
! xfs_bmap_add_attrfork_extents(xfs_trans_t ...)
! ! xfs_bmap_add_attrfork(...) allocates trans does a xfs_trans_commit/cancel
! xfs_bmap_add_extent_delay_real(fs_bmalloca)
! ! xfs_bmapi_allocate(xfs_bmalloca_t, ...)
! ! !<see above>
! xfs_bmap_add_extent_unwritten_real(xfs_trans_t ...)
! ! xfs_bmapi_convert_unwritten(xfs_bmalloca ...)
! ! ! xfs_bmapi_write(xfs_trans ...) calls xfs_bmapi_convert_unwritten in loop XFS_BMAPI_METADATA
! ! ! ! ...<see above>
.....
! xfs_bmap_add_extent_hole_real(xfs_bmalloca ...)
! ! xfs_bmapi_allocate(xfs_bmalloca_t, ...)
! ! ! ...<see above>
So it's bmbt modification that looks to be the problem here, right?
xfs_bmap_local_to_extents(xfs_trans_t ...)<- set userdata to 0 (patch 3)
! xfs_bmap_add_attrfork_local(xfs_trans_t ..)
! ! xfs_bmap_add_attrfork(...) trans alloc, bmap_finish trans_commit/cancel
! xfs_bmapi_write(xfs_trans_t ...) XFS_BMAPI_METADATA
! ! ...<see above>
Same here.
That's all I can see as problematic - maybe I read the output wrong
and there's others?
i.e. all other xfs_alloc_vextent() callers are either in metadata
context (so don't use the workqueue) or commit the transaction
directly after xfs_bmapi_write returns so will unlock the AGF buffer
before calling into xfs_bmapi_write a second time.
If these are the only loops, then patch 3 is all that is
necessary to avoid the problem of blocking on workqueue resource
while we are already on the workqueue, right? i.e. bmbt allocation
is a metadata allocation, even though the context is a data
allocation, and ensuring it is metadata means that the current
transaction won't get blocked waiting for workqueue resources...
What am I missing?
Patch 3 is a clean up. userdata is used by the allocation routines and
its value should be correct for those routines. I discovered the
uninitialized values tracing the calling list. The fact that the worker
routine was using and randomly creating a worker based on the stack
value is not a factor in the problem because those paths do not loop on
xfs_alloc_vextent().
Patch 2 moves the worker so that when the worker servicing data
allocation request gets the lock, it will hold the worker over the loop
in xfs_bmapi_write() until it can do a xfs_trans_commit/cancel. If it
does not have the lock, then that worker will wait until it can get the
lock.
Your patch hangs when limiting the available workers on test 109 on the
3 machines (2 x86_64 and a x86_32) that I tried it on. I am trying a
fourth machine to be sure.
Thanks,
--Mark.
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