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

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On Sat, Jun 30, 2018 at 09:30:30AM +0800, Ming Lei wrote:
> 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:
> > > 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.
> 
Yes, that fixes the problem. Thanks for the quick turnaround!

Guenter



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