Re: Device or HBA level QD throttling creates randomness in sequetial workload

[Jens Axboe]

On 01/30/2017 09:30 AM, Bart Van Assche wrote:
> On Mon, 2017-01-30 at 19:22 +0530, Kashyap Desai wrote:
>> -   if (atomic_inc_return(&instance->fw_outstanding) >
>> -           instance->host->can_queue) {
>> -       atomic_dec(&instance->fw_outstanding);
>> -       return SCSI_MLQUEUE_HOST_BUSY;
>> -   }
>> +   if (atomic_inc_return(&instance->fw_outstanding) > safe_can_queue) {
>> +       is_nonrot = blk_queue_nonrot(scmd->device->request_queue);
>> +       /* For rotational device wait for sometime to get fusion command
>> from pool.
>> +        * This is just to reduce proactive re-queue at mid layer which is
>> not
>> +        * sending sorted IO in SCSI.MQ mode.
>> +        */
>> +       if (!is_nonrot)
>> +           udelay(100);
>> +   }
> 
> The SCSI core does not allow to sleep inside the queuecommand() callback
> function.

udelay() is a busy loop, so it's not sleeping. That said, it's obviously
NOT a great idea. We want to fix the reordering due to requeues, not
introduce random busy delays to work around it.

-- 
Jens Axboe