[dm-devel] [PATCH 0/3] offload bios to a thread

[Mikulas Patocka]

On Thu, 30 Jun 2016, Mike Snitzer wrote:

> [cc'ing linux-block and drbd folks]
> 
> On Tue, Jun 28 2016 at  8:16pm -0400,
> Mikulas Patocka <mpatocka at 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

I found a problem with the patches when using loop device - we must not 
offload bios to the rescue thread if they are allocated from fs_bio_set. 
I'll send a second version of the patches with this change. You can 
incorporate that change to your git tree.

> 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.

The behavior depends on the timer only in a situation when the deadlock 
actually happens - the timer doesn't hurt performance on normal use. So, 
it's better to have timed delay in bio processing than a deadlock :)

The timer part can be dropped entirely if someone shows that offloading 
bios on schedule doesn't hurt performance in any way. Does anyone have a 
large collection of block layer performance tests that could be tried to 
detect if the regression happens?

Mikulas