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

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