RE: [PATCH] megaraid_sas: move command counter to correct place

[Sumit Saxena <sumit.saxena@broadcom.com>]

>-----Original Message-----
>From: Martin K. Petersen [mailto:martin.petersen@oracle.com]
>Sent: Monday, August 07, 2017 11:07 PM
>To: Tomas Henzl
>Cc: linux-scsi@vger.kernel.org; sumit.saxena@broadcom.com;
>kashyap.desai@broadcom.com
>Subject: Re: [PATCH] megaraid_sas: move command counter to correct place
>
>
>Tomas,
>
>> the eh reset function returns success when fw_outstanding equals zero,
>> that means that the counter shouldn't be decremented when the driver
>> still owns the command
>
>Applied to 4.13/scsi-fixes. Thank you!

Just realized that this patch may cause performance regression.
With this patch below scenario may occur-

-Consider outstanding IOs reaches to controller's Queue depth.
-Driver frees up command and complete it back to SML.
-Since host_busy is decremented, SML can issue one new  IO to driver.
-By the time, if "fw_outstanding" is not decremented by driver, then
driver will return newly submitted IO back to SML with return status
SCSI_MLQUEUE_HOST_BUSY because of below code
  in megaraid_sas driver's IO submission path-

    if (atomic_inc_return(&instance->fw_outstanding) >
            instance->host->can_queue) {
        atomic_dec(&instance->fw_outstanding);
        return SCSI_MLQUEUE_HOST_BUSY;
    }

This situation will be more evident when RAID1 fastpath IOs are running as
in that case driver will be issuing two IOs to firmware for single IO
issued from SML.
Above situation can be avoided, if this patch is removed.

Regarding Tomas' concern, there should not be any problem as driver calls
"synchronize_irq" before checking "fw_outstanding". Once fw_outstanding is
decremented and
driver frees up command, then only driver will be checking
"fw_outstanding" equals to zero or not so all this will always fall in a
sequence and will not
cause the problem stated by Tomas.

I am sorry for confusion and would request to revert this patch.

Thanks,
Sumit

>
>--
>Martin K. Petersen	Oracle Linux Engineering