[Bug 69201] New: qla2xxx: Low-latency storage triggers lock contention

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https://bugzilla.kernel.org/show_bug.cgi?id=69201

            Bug ID: 69201
           Summary: qla2xxx: Low-latency storage triggers lock contention
           Product: SCSI Drivers
           Version: 2.5
    Kernel Version: 3.12.7
          Hardware: All
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: QLOGIC QLA2XXX
          Assignee: scsi_drivers-qla2xxx@kernel-bugs.osdl.org
          Reporter: bvanassche@acm.org
        Regression: No

Running a fio test on an initiator system with an 8 Gb/s QLogic FC adapter
revealed a bottleneck in the qla2xxx initiator driver - lock contention on 
ha->hardware_lock.

The test that revealed this is as follows:
- On a target system with 4 CPU threads (Intel i5), an 8 Gb/s QLogic FC HBA and
kernel 3.12.7, download the SCST trunk r5194, build it in release mode, load
the brd kernel module and configure SCST such that it exports /dev/ram[0123]
via the vdisk_blockio driver. Set the vdisk_blockio parameter threads_num to 2.
Export these four RAM disks as LUNs 0..3.
- On an initiator system with 12 CPU threads (Intel Core i7 with hyperthreading
enabled), an 8 Gb/s QLogic HBA and kernel 3.12.7, run the following fio job
(where /dev/sd[cdef] corresponds to the SCST LUNs):

fio --bs=4K --ioengine=libaio --rw=randrw --buffered=0 --numjobs=12 \
    --iodepth=16 --iodepth_batch=8 --iodepth_batch_complete=8   \
    --thread --loops=$((2**31)) --runtime=60 --group_reporting    \
    --gtod_reduce=1 --invalidate=1                \
    $(for d in /dev/sd[cdef]; do echo --name=$d --filename=$d; done)

- While this fio job is running, run the following commands:

perf record -ag sleep 10
perf report –stdio >perf-report-fc.txt

The perf report shows that quite some time is spent in the spin_lock_irqsave()
call invoked from qla24xx_dif_start_scsi(). Does this mean that this test
revealed lock contention on ha->hardware_lock ?

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