Re: [PATCH 0/2] blk-mq: switch from preempt_disable/enable to get/put_cpu

[Jens Axboe]

On 2014-11-09 20:16, Jens Axboe wrote:
> On 2014-11-07 15:03, Paolo Bonzini wrote:
> > blk-mq is using preempt_disable/enable in order to ensure that the
> > queue runners are placed on the right CPU.  This does not work with
> > the RT patches, because __blk_mq_run_hw_queue takes a non-raw
> > spinlock with the preemption-disabled region.  If there is contention
> > on the lock, this violates the rules for preemption-disabled regions.
> >
> > While this could be fixed easily within the RT patches just by doing
> > migrate_disable/enable (note: this was not tested :)), we can do
> > better.  The first patch concentrates the preempt_disable/enable in
> > a single place in blk_mq_run_hw_queue, and also avoids useless calls
> > when the caller wants to start __blk_mq_run_hw_queue asynchronously.
> > (There is already a call to __blk_mq_run_hw_queue that does not disable
> > preemption in blk-flush.c; not coincidentially, it passes async=true).
> >
> > Once this is done, it is trivial to use get/put_cpu instead of
> > preempt_disable/smp_processor_id/preempt_enable, which is what the
> > second patch does.  The RT patches then can change this to use
> > get_cpu_light.
> >
> > With these changes (and the additional switch to get_cpu_light),
> > virtio-blk can be used again with RT kernels.
>
> These both look good. I'll double check and run through some testing,
> then apply for 3.19 if it all passes.

Looks fine to me, I've applied it now. Thanks!

--
Jens Axboe

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