Re: [PATCH v2 03/18] scsi: core: allow libata to complete successful commands via EH

[Hannes Reinecke <hare@suse.de>]

On 1/16/23 13:43, Niklas Cassel wrote:
> On Fri, Jan 13, 2023 at 08:57:37AM +0100, Hannes Reinecke wrote:
>> On 1/12/23 15:03, Niklas Cassel wrote:
>>> In SCSI, we get the sense data as part of the completion, for ATA
>>> however, we need to fetch the sense data as an extra step. For an
>>> aborted ATA command the sense data is fetched via libata's
>>> ->eh_strategy_handler().
>>>
>>> For Command Duration Limits policy 0xD:
>>> The device shall complete the command without error with the additional
>>> sense code set to DATA CURRENTLY UNAVAILABLE.
>>>
>>> In order to handle this policy in libata, we intend to send a successful
>>> command via SCSI EH, and let libata's ->eh_strategy_handler() fetch the
>>> sense data for the good command. This is similar to how we handle an
>>> aborted ATA command, just that we need to read the Successful NCQ
>>> Commands log instead of the NCQ Command Error log.
>>>
>>> When we get a SATA completion with successful commands, ATA_SENSE will
>>> be set, indicating that some commands in the completion have sense data.
>>>
>>> The sense_valid bitmask in the Sense Data for Successful NCQ Commands
>>> log will inform exactly which commands that had sense data, which might
>>> be a subset of all the commands that was completed in the same
>>> completion. (Yet all will have ATA_SENSE set, since the status is per
>>> completion.)
>>>
>>> The successful commands that have e.g. a "DATA CURRENTLY UNAVAILABLE"
>>> sense data will have a SCSI ML byte set, so scsi_eh_flush_done_q() will
>>> not set the scmd->result to DID_TIME_OUT for these commands. However,
>>> the successful commands that did not have sense data, must not get their
>>> result marked as DID_TIME_OUT by SCSI EH.
>>>
>>> Add a new flag SCMD_EH_SUCCESS_CMD, which tells SCSI EH to not mark a
>>> command as DID_TIME_OUT, even if it has scmd->result == SAM_STAT_GOOD.
>>>
>>> This will be used by libata in a follow-up patch.
>>>
>>> Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
>>> ---
>>>    drivers/scsi/scsi_error.c | 3 ++-
>>>    include/scsi/scsi_cmnd.h  | 5 +++++
>>>    2 files changed, 7 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/scsi/scsi_error.c b/drivers/scsi/scsi_error.c
>>> index 2aa2c2aee6e7..51aa5c1e31b5 100644
>>> --- a/drivers/scsi/scsi_error.c
>>> +++ b/drivers/scsi/scsi_error.c
>>> @@ -2165,7 +2165,8 @@ void scsi_eh_flush_done_q(struct list_head *done_q)
>>>    			 * scsi_eh_get_sense), scmd->result is already
>>>    			 * set, do not set DID_TIME_OUT.
>>>    			 */
>>> -			if (!scmd->result)
>>> +			if (!scmd->result &&
>>> +			    !(scmd->flags & SCMD_EH_SUCCESS_CMD))
>>>    				scmd->result |= (DID_TIME_OUT << 16);
>>>    			SCSI_LOG_ERROR_RECOVERY(3,
>>>    				scmd_printk(KERN_INFO, scmd,
>> Wouldn't it be better to use '!scmd->result && !scsi_sense_valid(scmd)'
>> instead of a new flag?
>> After all, if we have a valid sense code we _have_ been able to communicate
>> with the device. And as we did so it's questionable whether it should count
>> as a command time out ...
> 
> Hello Hannes,
> 
> Thanks a lot for helping out reviewing this series!
> 
> Unfortunately, your suggestion won't work.
> 
> 
> Let me explain:
> 
> When you get a FIS, the ACT register will have a bit set for each
> command that finished, however, all the commands will share a single
> STATUS value (since there is just a shared STATUS field in the FIS).
> 
> So let's say that tags 0-3 got finished (i.e. bits 0-3 are set in the
> ACT field) and the STATUS field has the "Sense Data Available" bit set.
> 
> This just tells us that at least one of tags 0-3 has sense data.
> 
> 
> In order to know which of these tags that actually has sense data,
> we need to read the "Sense Data for Successful NCQ Commands log",
> which contains a sense_valid bitmask (which contains one bit for
> each of the 32 tags).
> 
> So reading the "Sense Data for Successful NCQ Commands log" might
> tell us that just tag 0-1 have sense data.
> 
> So, libata calls ata_qc_schedule_eh() on tags 0-3, wait until SCSI calls
> libata .eh_strategy_handler(). libata .eh_strategy_handler() will read the
> "Sense Data for Successful NCQ Commands log", which will see that there is
> sense data for tags 0-1, and will add sense data for those commands, and
> call scsi_check_sense() for tags 0-1.
> 
> ata_eh_finish() will finally be called, to determine what to do with the
> commands that belonged to EH.
> 
> The code looks like this:
> if (qc->flags & ATA_QCFLAG_SENSE_VALID ||
>      qc->flags & ATA_QCFLAG_EH_SUCCESS_CMD) {
> 	ata_eh_qc_complete(qc);
> }
> 
> So it will call complete for all 4 tags, regardless is they had sense data
> or not.
> 
> 
> scsi_eh_flush_done_q() will soon be called, and since ata_eh_qc_complete()
> sets scmd->retries == scmd->allowed, none of the four commands will be retired.
> 
> if (!scmd->result &&
>      !(scmd->flags & SCMD_EH_SUCCESS_CMD))
> 	scmd->result |= (DID_TIME_OUT << 16);
> 
> The 2 commands with sense data will not get DID_TIMEOUT,
> because scmd->result has the SCSI ML byte set
> (which is set by scsi_check_sense()).
> 
> The 2 commands without sense data will have scmd->result == 0,
> so they will get DID_TIME_OUT set if we don't have the extra
> !(scmd->flags & SCMD_EH_SUCCESS_CMD)) condition.
> 
> 
> SCSI could add an additional check for:
> 
> if (!scmd->result && !scsi_sense_valid(scmd) &&
>      !(scmd->flags & SCMD_EH_SUCCESS_CMD))
> 	scmd->result |= (DID_TIME_OUT << 16);
> 
> so that a command with does have sense data, but scsi_check_sense()
> did not set any SCSI ML byte, does not get DID_TIME_OUT set.
> 
> However, for CDL policy 0xD, which this patch cares about,
> we would still need the "&& !(scmd->flags & SCMD_EH_SUCCESS_CMD))",
> so at least from a CDL perspective, I don't see how any benefit of
> also adding a check for "&& !scsi_sense_valid(scmd)".
> 

Right, I see.
Thanks for the explanation.

You can add:

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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