On Mon, Jun 18, 2012 at 12:25:37PM -0600, Andreas Dilger wrote:

On 2012-06-18, at 6:08 AM, Christoph Hellwig wrote:

quoted

May saw the release of Linux 3.4, including a decent sized XFS update.
Remarkable XFS features in Linux 3.4 include moving over all metadata
updates to use transactions, the addition of a work queue for the
low-level allocator code to avoid stack overflows due to extreme stack
use in the Linux VM/VFS call chain,

This is essentially a workaround for too-small stacks in the kernel,
which we've had to do at times as well, by doing work in a separate
thread (with a new stack) and waiting for the results?  This is a
generic problem that any reasonably-complex filesystem will have when
running under memory pressure on a complex storage stack (e.g. LVM +
iSCSI), but causes unnecessary context switching.

I've seen no performance issues from the context switching.  The
overhead of them is so small to be unmeasurable most cases, because
a typical allocation already requires context switches for contended
locks and metadata IO....

Any thoughts on a better way to handle this, or will there continue
to be a 4kB stack limit

We were blowing 8k stacks on x86-64 with alarming ease. Even the
flusher threads were overflowing.

and hack around this with repeated kmalloc
on callpaths for any struct over a few tens of bytes, implementing
memory pools all over the place, and "forking" over to other threads
to continue the stack consumption for another 4kB to work around
the small stack limit?

I mentioned that we needed to consider 16k stacks at last years
Kernel Summit and the response was along the lines of "you've got to
be kidding - fix your broken filesystem". That's the perception you
have to change, and i don't feel like having a 4k stacks battle
again...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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