Re: [PATCH 0/3] offload bios to a thread

[Mike Snitzer <snitzer@redhat.com>]

[cc'ing linux-block and drbd folks]

On Tue, Jun 28 2016 at  8:16pm -0400,
Mikulas Patocka <mpatocka@redhat.com> wrote:

> Hi
> 
> Here I'm sending three patches to fix the deadlocks in snapshot and 
> snapshot-merge.
> 
> The first patch fixes the deadlock, the following 2 patches introduce a 
> timer, so that bios are not offloaded immediatelly, they are offloaded 
> after a specified timeout, because immediate offloading can change order 
> of bios and it could theoretically produce regressions. I don't know if 
> these regressions really exist or not.
> 
> If there is some way to push the patches upstream, try it.

Some fix must happen before the more recent upstream kernels can be
reliably used in stacked bio-based workloads (in production).  We simply
cannot ignore this issue any more.

drbd is also hitting the same generic_make_request (current->bio_list)
problem, see:
https://www.redhat.com/archives/dm-devel/2016-June/msg00326.html

Mikulas, I've taken your 3 proposed patches patches and refactored them
some to split out intermediate patches that hopefully make review
easier.  Nothing other than variable names and some other style stuff
was changed -- headers were tweaked some to help with clarity.

Please see the 5 topmost "block: ..." patches here:
http://git.kernel.org/cgit/linux/kernel/git/snitzer/linux.git/log/?h=wip

It should be noted that Jens had a quick look at this set and wanted to
throw up a little when he saw the (ab)use of a timer to defer punting to
the workqueue.  I explained that without the timer, always punting to
the workqueue, we could hurt performance by reordering IO or crippling
onstack plugging.  He said he'd try to think of a cleaner way forward.

Lars, please feel free to see if this set addresses the similar deadlock
you saw/fixed with drbd.  We need to converge on an acceptable fix for
this problem -- preferably sooner rather than later!

Conversely, Mikulas: if you can easily reproduce the dm-snapshot
deadlock please try Lars' fix to see if it is workable for our DM needs.

Thanks,
Mike

p.s. I'm on holiday until next Wednesday (7/6).. so may be slow to
     respond until then.