[dm-devel] [for-4.14 RFC PATCH 1/2] dm rq: avoid deadlock if dm-mq is stacked on old .request_fn device(s)
Mike Snitzer
snitzer at redhat.com
Fri Jul 14 21:15:39 UTC 2017
On Fri, Jul 14 2017 at 1:17pm -0400,
Ewan D. Milne <emilne at redhat.com> wrote:
> On Fri, 2017-07-14 at 10:19 -0400, Mike Snitzer wrote:
> >
> > Do you see a benefit to extracting that portion of your WIP patch
> > (removing the ->complete handler entirely)?
> >
> > Or leave well enough alone and just continue to disable dm-mq's ability
> > to stack on .request_fn paths?
> >
> > Given SCSI's switch to scsi-mq by default I cannot see value in propping
> > up stacking on the old .request_fn devices.
>
> So, the dm_mod.use_blk_mq flag is global, right? I guess the question
> is whether all of the block device types used on a system under DM are
> supported under MQ. If that is the case then we would be OK.
I didn't quite understand Ewan's question so we talked in person. His
concern was whether other DM targets needed to be worried about blk-mq
vs not. I clarified that DM multipath is the only target that is
request-based and that it is fine with stacking on scsi-mq. And all the
bio-based targets obviously stack just fine on scsi-mq devices.
> The other question is whether there are negative performance
> consequences in any (corner?) cases with MQ that would result in it
> being preferable to run in non-MQ mode (e.g. tag space with lpfc, did
> we ever resolve that?) but the right approach there is to put the effort
> into the MQ path going forward, as has been the case.
Yeap.
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