Re: [STACK] >3k call path in xfs

[Jörn Engel]

On Wed, 9 June 2004 10:05:42 -0500, Steve Lord wrote:
> Jörn Engel wrote:
> >xfs is quite interesting.  No single function is particularly
> >stack-hungry, but the sheer depth of the call path adds up.  Nathan,
> >can you see if some bytes can be saved here and there?
> >
> >3k is not really bad yet, I just like to keep 1k of headroom for
> >surprises like an extra int foo[256] in a structure.
> 
> Are you doing some form of call chain analysis to arrive
> at this stack?

Yup.

> Actually this one is impossible, the function xfs_bmapi is
> confusing you analyser, it is used for read and write
> calls to map from file offsets to disk blocks. The path
> you chased down from xfs_bmapi is doing a realtime
> allocation, the swapext call does not do allocations,
> it is in this case looking up the contents of an acl
> for a permission check - xfs_bmapi in this case will
> not call much of anything.

Ack.  Clearly some sort of semantical checking would be nice to have,
but for the moment, we still need humans to do that job.  Thanks!

> Once it gets to schedule it is  a little out of XFS's hands what
> happens, or which stack is actually in use. I think the
> path you followed out of schedule is the cleanup of the
> audit structure of a dead process - it is the one doing
> the panicing here. An xfs call into schedule to wait for
> I/O will not be going down that path.

Yes, same problem.

> I think you have to be careful looking at these call chains.

For sure.  But since I scale extremely badly, I try to distribute this
work.  Thanks for taking a share.

There are a few more spots worth looking at, though.  I have no idea,
how often the loops below can iterate, so they may cause problems as
well.  If they are harmless as well, xfs is in a good state wrt stack
usage.  Nice.

WARNING: trivial recursion detected:
      44  xfs_attr_node_inactive
WARNING: trivial recursion detected:
      24  xfs_bmap_count_tree
WARNING: recursion detected:
      56  xfs_map_unwritten
      60  xfs_convert_page
WARNING: recursion detected:
       0  xfs_iaccess
      16  xfs_acl_iaccess
     104  xfs_attr_fetch

Jörn

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