Messages in this thread | | | Date | Sun, 09 Nov 2014 20:16:30 -0700 | From | Jens Axboe <> | Subject | Re: [PATCH 0/2] blk-mq: switch from preempt_disable/enable to get/put_cpu |
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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.
-- Jens Axboe
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