Re: [bug report] shared tags causes IO hang and performance drop

[Douglas Gilbert <dgilbert@interlog.com>]

On 2021-04-14 9:59 a.m., Kashyap Desai wrote:
>>> I tried both - 5.12.0-rc1 and 5.11.0-rc2+ and there is a same
> behavior.
>>> Let me also check  megaraid_sas and see if anything generic or this is
>>> a special case of scsi_debug.
>>
>> As I mentioned, it could be one generic issue wrt. SCHED_RESTART.
>> shared tags might have to restart all hctx since all share same tags.
> 
> Ming - I tried many combination on MR shared host tag driver but there is
> no single instance of IO hang.
> I will keep trying, but when I look at scsi_debug driver code I found
> below odd settings in scsi_debug driver.
> can_queue of adapter is set to 128 but queue_depth of sdev is set to 255.
> 
> If I apply below patch, scsi_debug driver's hang is also resolved. Ideally
> sdev->queue depth cannot exceed shost->can_queue.
> Not sure why cmd_per_lun is 255 in scsi_debug driver which can easily
> exceed can_queue.  I will simulate something similar in MR driver and see
> how it behaves w.r.t IO hang issue.
> 
> diff --git a/drivers/scsi/scsi_debug.c b/drivers/scsi/scsi_debug.c
> index 70165be10f00..dded762540ee 100644
> --- a/drivers/scsi/scsi_debug.c
> +++ b/drivers/scsi/scsi_debug.c
> @@ -218,7 +218,7 @@ static const char *sdebug_version_date = "20200710";
>    */
>   #define SDEBUG_CANQUEUE_WORDS  3       /* a WORD is bits in a long */
>   #define SDEBUG_CANQUEUE  (SDEBUG_CANQUEUE_WORDS * BITS_PER_LONG)

So SDEBUG_CANQUEUE is 3*64 = 192 and is a hard limit (it is used to
dimension an array). Should it be upped to 4, say? [That will slow things
down a bit if that is an issue.]

> -#define DEF_CMD_PER_LUN  255
> +#define DEF_CMD_PER_LUN  SDEBUG_CANQUEUE
> 
>   /* UA - Unit Attention; SA - Service Action; SSU - Start Stop Unit */
>   #define F_D_IN                 1       /* Data-in command (e.g. READ) */
> @@ -7558,6 +7558,7 @@ static int sdebug_driver_probe(struct device *dev)
>          sdbg_host = to_sdebug_host(dev);
> 
>          sdebug_driver_template.can_queue = sdebug_max_queue;
> +       sdebug_driver_template.cmd_per_lun = sdebug_max_queue;

I'll  push out a patch shortly.

Doug Gilbert


>          if (!sdebug_clustering)
>                  sdebug_driver_template.dma_boundary = PAGE_SIZE - 1;
>>
>>
>> Thanks,
>> Ming