Re: [V2, 2/2] scsi: avoid to hold host-wide counter of host_busy for scsi_mq

On Fri, Jun 29, 2018 at 09:20:54AM -0700, Guenter Roeck wrote:

Hi,

On Sun, Jun 24, 2018 at 10:03:27PM +0800, Ming Lei wrote:

quoted

It isn't necessary to check the host depth in scsi_queue_rq() any more
since it has been respected by blk-mq before calling scsi_queue_rq() via
getting driver tag.

Lots of LUNs may attach to same host, and per-host IOPS may reach millions
level, so we should avoid to this expensive atomic operations on the
hostwide counter in IO path.

This patch implemens scsi_host_busy() via blk_mq_tagset_busy_iter() for
reading the count of busy IOs for scsi_mq.

It is observed that IOPS is increased by 15% in IO test on scsi_debug
(32 LUNs, 32 submit queues, 1024 can_queue, libaio/dio) in one
dual-socket system.

This patch breaks two of my qemu test builds in -next: parisc:defconfig
and arm:versatilepb-scsi:versatile_defconfig (which is versatilepb booting
from scsi disk). The symptom is the same for both: Boot stalls after scsi
bus initialization.

arm:

sym53c8xx 0000:00:0c.0: enabling device (0100 -> 0103)
sym0: <895a> rev 0x0 at pci 0000:00:0c.0 irq 66
sym0: No NVRAM, ID 7, Fast-40, LVD, parity checking
sym0: SCSI BUS has been reset.
scsi host0: sym-2.2.3
random: fast init done
[stalls]

parisc:

sym53c8xx 0000:00:00.0: enabling SERR and PARITY (0107 -> 0147)
sym0: <895a> rev 0x0 at pci 0000:00:00.0 irq 17
sym0: PA-RISC Firmware, ID 7, Fast-40, LVD, parity checking
sym0: SCSI BUS has been reset.
scsi host0: sym-2.2.3
random: fast init done
[stalls]

Reverting the patch fixes the problem. Bisect log is attached.

Please test the patch of 'scsi: fix scsi_host_queue_ready', which has
been posted on linux-scsi list and CCed to you.

Thanks,
Ming