Re: [PATCH] scsi: core: Log SCSI command age with errors

[Martin Wilck <mwilck@suse.de>]

On Mon, 2019-09-23 at 11:31 +0530,  Milan P. Gandhi wrote:
> Couple of users had requested to print the SCSI command age along 
> with command failure errors. This is a small change, but allows 
> users to get more important information about the command that was 
> failed, it would help the users in debugging the command failures:
> 
> Signed-off-by: Milan P. Gandhi <mgandhi@redhat.com>
> ---
> diff --git a/drivers/scsi/scsi_logging.c
> b/drivers/scsi/scsi_logging.c
> index ecc5918e372a..ca2182bc53c6 100644
> --- a/drivers/scsi/scsi_logging.c
> +++ b/drivers/scsi/scsi_logging.c
> @@ -437,6 +437,7 @@ void scsi_print_result(const struct scsi_cmnd
> *cmd, const char *msg,
>  	const char *mlret_string = scsi_mlreturn_string(disposition);
>  	const char *hb_string = scsi_hostbyte_string(cmd->result);
>  	const char *db_string = scsi_driverbyte_string(cmd->result);
> +	unsigned long cmd_age = (jiffies - cmd->jiffies_at_alloc) / HZ;

This comes down to pretty coarse time resolution, does it not? More
often than not, the time difference shown will be 0. I'd recommend at
least millisecond resolution.

>  
>  	logbuf = scsi_log_reserve_buffer(&logbuf_len);
>  	if (!logbuf)
> @@ -478,10 +479,15 @@ void scsi_print_result(const struct scsi_cmnd
> *cmd, const char *msg,
>  
>  	if (db_string)
>  		off += scnprintf(logbuf + off, logbuf_len - off,
> -				 "driverbyte=%s", db_string);
> +				 "driverbyte=%s ", db_string);
>  	else
>  		off += scnprintf(logbuf + off, logbuf_len - off,
> -				 "driverbyte=0x%02x", driver_byte(cmd-
> >result));
> +				 "driverbyte=0x%02x ",
> +				 driver_byte(cmd->result));
> +
> +	off += scnprintf(logbuf + off, logbuf_len - off,
> +			 "cmd-age=%lus", cmd_age);

This is certainly helpful in some situations. Yet I am unsure if it
should *always* be printed. I wouldn't say it's as important as the 
other stuff scsi_print_result() provides. After all, by activating
MLQUEUE+MLCOMPLETE, the time on-wire can be extracted with better
accuracy from currently available logs. 

So perhaps make this depend on a module parameter?

Also, we should carefully ponder if we want to put this on the same
line as the driver byte, as users may have created scripts for parsing
this output.

Regards,
Martin