Re: [PATCH v4 4/5] scsi_debug: add new defer type for mq poll

[Hannes Reinecke <hare@suse.de>]

On 2/15/21 8:40 AM, Kashyap Desai wrote:
> From: Douglas Gilbert <dgilbert@interlog.com>
> 
> Add a new sdeb_defer_type enumeration: SDEB_DEFER_POLL for requests
> that have REQ_HIPRI set in cmd_flags field. It is expected that
> these requests will be polled via the mq_poll entry point which
> is driven by calls to blk_poll() in the block layer. Therefore
> timer events are not 'wired up' in the normal fashion.
> 
> There are still cases with short delays (e.g. < 10 microseconds)
> where by the time the command response processing occurs, the delay
> is already exceeded in which case the code calls scsi_done()
> directly. In such cases there is no window for mq_poll() to be
> called.
> 
> Add 'mq_polls' counter that increments on each scsi_done() called
> via the mq_poll entry point. Can be used to show (with 'cat
> /proc/scsi/scsi_debug/<host_id>') that blk_poll() is causing
> completions rather than some other mechanism.
> 
> This patch is improvement over previous patch
> "scsi_debug: iouring iopoll support"
> 
> Changes since version 3
>    - Fix IO hang issue. Do not return from schedule_resp. Use new defer
>      type for mq poll to queue the REQ_HIPRI IOs.
> 
> Changes since version 2 [sent 20210206 to linux-scsi list]
>    - the sdebug_blk_mq_poll() callback didn't cope with the
>      uncommon case where sqcp->sd_dp is NULL. Fix.
> 
> Changes since version 1 [sent 20210201 to linux-scsi list]
>    - harden SDEB_DEFER_POLL which broke under testing
>    - add mq_polls counter for debug output
> 
> Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
> Tested-by: Kashyap Desai <kashyap.desai@broadcom.com>
> 
> Reviewed-by: Hannes Reinecke <hare@suse.de>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
hare@suse.de                              +49 911 74053 688
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