linux-kernel - Re: [PATCH v3 11/11] scsi: scsi_debug: Drop sdebug

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Date:   Mon, 3 Apr 2023 01:26:35 -0400
From:   Douglas Gilbert <dgilbert@...erlog.com>
To:     John Garry <john.g.garry@...cle.com>, jejb@...ux.ibm.com,
        martin.petersen@...cle.com
Cc:     linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
        bvanassche@....org
Subject: Re: [PATCH v3 11/11] scsi: scsi_debug: Drop sdebug_queue

On 2023-03-27 03:43, John Garry wrote:
> It's easy to get scsi_debug to error on throughput testing when we have
> multiple shosts:
> 
> $ lsscsi
> [7:0:0:0]       disk    Linux   scsi_debug      0191
> [0:0:0:0]       disk    Linux   scsi_debug      0191
> 
> $ fio --filename=/dev/sda --filename=/dev/sdb --direct=1 --rw=read --bs=4k
> --iodepth=256 --runtime=60 --numjobs=40 --time_based --name=jpg
> --eta-newline=1 --readonly --ioengine=io_uring --hipri --exitall_on_error
> jpg: (g=0): rw=read, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T) 4096B-4096B, ioengine=io_uring, iodepth=256
> ...
> fio-3.28
> Starting 40 processes
> [   27.521809] hrtimer: interrupt took 33067 ns
> [   27.904660] sd 7:0:0:0: [sdb] tag#171 FAILED Result: hostbyte=DID_ABORT driverbyte=DRIVER_OK cmd_age=0s
> [   27.904660] sd 0:0:0:0: [sda] tag#58 FAILED Result: hostbyte=DID_ABORT driverbyte=DRIVER_OK cmd_age=0s
> fio: io_u error [   27.904667] sd 0:0:0:0: [sda] tag#58 CDB: Read(10) 28 00 00 00 27 00 00 01 18 00
> on file /dev/sda[   27.904670] sd 0:0:0:0: [sda] tag#62 FAILED Result: hostbyte=DID_ABORT driverbyte=DRIVER_OK cmd_age=0s
> 
> The issue is related to how the driver manages submit queues and tags. A
> single array of submit queues - sdebug_q_arr - with its own set of tags is
> shared among all shosts. As such, for occasions when we have more than one
> shost it is possible to overload the submit queues and run out of tags.
> 
> The struct sdebug_queue is to manage tags and hold the associated
> queued command entry pointer (for that tag).
> 
> Since the tagset iters are now used for functions like
> sdebug_blk_mq_poll(), there is no need to manage these queues. Indeed,
> blk-mq already provides what we need for managing tags and queues.
> 
> Drop sdebug_queue and all its usage in the driver.
> 
> Signed-off-by: John Garry <john.g.garry@...cle.com>
Acked-by: Douglas Gilbert <dgilbert@...erlog.com>

Thanks.