v4.12 release has brought lots of interesting things, and -pf made those things even more interesting. So far:
VRQ is horribly broken. Investigation is ongoing, reliable reproducer is provided, go and check current results here. Meanwhile, I've reverted VRQ from -pf tree, but still keeping an eye on it. Also, likely and hopefully, MuQSS might be available in a week or so. As of now, I've picked CFS tunables from zen-kernel to make mainline scheduler suck less. And indeed, it sucks less with those tunables.
UPDATE: MuQSS is broken too. Same reproducer is able to crash a kernel. Similar bugreport has been filed for Con here.
blk-mq looks like broken too somehow. There are 2 discussions regarding suspend/resume issues. First is here, and it looks like it should be fixed by this commit, which is already in -pf. Second is here, and I'd like to check what an outcome is. For me, both suspend and resume fail periodically, and I've disabled -mq to check if it helps. Maybe, I'll have to merge BFQ for SQ back.
CONFIG_DM_MQ_DEFAULT=y is not needed for non-multipath hardware. CONFIG_SCSI_MQ_DEFAULT=y is sufficient to have -mq enabled on commodity hardware.
If you do not play with CPU affinity and suspend/resume — everything works like a charm :).
Will keep you updated once have some news on this.