Re: [PATCH 09/10] xfs: on-stack delayed write buffer lists

Dave Chinner
2012-04-21T00:43:00Z

On Fri, Apr 20, 2012 at 01:19:46PM -0500, Mark Tinguely wrote:
> On 03/27/12 11:44, Christoph Hellwig wrote:
> >Queue delwri buffers on a local on-stack list instead of a per-buftarg one,
> >and write back the buffers per-process instead of by waking up xfsbufd.
> >
> >This is now easily doable given that we have very few places left that write
> >delwri buffers:
> >
> > - log recovery:
> > Only done at mount time, and already forcing out the buffers
> > synchronously using xfs_flush_buftarg
> >
> > - quotacheck:
> > Same story.
> >
> > - dquot reclaim:
> > Writes out dirty dquots on the LRU under memory pressure. We might
> > want to look into doing more of this via xfsaild, but it's already
> > more optimal than the synchronous inode reclaim that writes each
> > buffer synchronously.
> >
> > - xfsaild:
> > This is the main beneficiary of the change. By keeping a local list
> > of buffers to write we reduce latency of writing out buffers, and
> > more importably we can remove all the delwri list promotions which
> > were hitting the buffer cache hard under sustained metadata loads.
> >
> >The implementation is very straight forward - xfs_buf_delwri_queue now gets
> >a new list_head pointer that it adds the delwri buffers to, and all callers
> >need to eventually submit the list using xfs_buf_delwi_submit or
> >xfs_buf_delwi_submit_nowait. Buffers that already are on a delwri list are
> >skipped in xfs_buf_delwri_queue, assuming they already are on another delwri
> >list. The biggest change to pass down the buffer list was done to the AIL
> >pushing. Now that we operate on buffers the trylock, push and pushbuf log
> >item methods are merged into a single push routine, which tries to lock the
> >item, and if possible add the buffer that needs writeback to the buffer list.
> >This leads to much simpler code than the previous split but requires the
> >individual IOP_PUSH instances to unlock and reacquire the AIL around calls
> >to blocking routines.
> >
> >Given that xfsailds now also handles writing out buffers the conditions for
> >log forcing and the sleep times needed some small changes. The most
> >important one is that we consider an AIL busy as long we still have buffers
> >to push, and the other one is that we do increment the pushed LSN for
> >buffers that are under flushing at this moment, but still count them towards
> >the stuck items for restart purposes. Without this we could hammer on stuck
> >items without ever forcing the log and not make progress under heavy random
> >delete workloads on fast flash storage devices.
> >
> >Signed-off-by: Christoph Hellwig<hch@lst.de>
>
> Test 106 runs to completion with patch 06.
>
> Patch 07 and 08 do not compile without patch 09.
>
> Starting with patch 09, I get the following hang on every test 106:

FYI, test 106 is not in the auto group, which means it typically
isn't run on regression test runs by anyone who isn't modifying
quota code. That'll be why nobody else is seeing this.

As it is, I don't understand why it isnt in the auto group. The
commit that removed it:

62f8947 Test case for repair dir2 freetab botch.

was completely unrelated to quota stuff - it added test 110, but
remove 106-108 from the auto group as well. Perhaps that was an
oversight? I note that 108 has been brought back into the auto
group, but not 106/107....

>
> ID: 27992 TASK: ffff8808310d00c0 CPU: 2 COMMAND: "mount"
> #0 [ffff880834237938] __schedule at ffffffff81417200
> #1 [ffff880834237a80] schedule at ffffffff81417574
> #2 [ffff880834237a90] schedule_timeout at ffffffff81415805
> #3 [ffff880834237b30] wait_for_common at ffffffff81416a67
> #4 [ffff880834237bc0] wait_for_completion at ffffffff81416bd8
> #5 [ffff880834237bd0] xfs_buf_iowait at ffffffffa04fc5a5 [xfs]
> #6 [ffff880834237c00] xfs_buf_delwri_submit at ffffffffa04fe4b9 [xfs]
> #7 [ffff880834237c40] xfs_qm_quotacheck at ffffffffa055cb2d [xfs]
> #8 [ffff880834237cc0] xfs_qm_mount_quotas at ffffffffa055cdf0 [xfs]
> #9 [ffff880834237cf0] xfs_mountfs at ffffffffa054c041 [xfs]
> #10 [ffff880834237d40] xfs_fs_fill_super at ffffffffa050ca80 [xfs]
> #11 [ffff880834237d70] mount_bdev at ffffffff81150c5c
> #12 [ffff880834237de0] xfs_fs_mount at ffffffffa050ac00 [xfs]
> #13 [ffff880834237df0] mount_fs at ffffffff811505f8
> #14 [ffff880834237e40] vfs_kern_mount at ffffffff8116c070
> #15 [ffff880834237e80] do_kern_mount at ffffffff8116c16e
> #16 [ffff880834237ec0] do_mount at ffffffff8116d6f0
> #17 [ffff880834237f20] sys_mount at ffffffff8116d7f3
> #18 [ffff880834237f80] system_call_fastpath at ffffffff814203b9

And event trace is going to be the only way to find out why it is
still waiting.....

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com