[dm-devel] [PATCH 0/3] offload bios to a thread
Mike Snitzer
snitzer at redhat.com
Thu Jun 30 19:40:24 UTC 2016
[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
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.
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