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

[Kashyap Desai]

> -----Original Message-----
> From: Jens Axboe [mailto:axboe@xxxxxxxxx]
> Sent: Monday, January 30, 2017 10:03 PM
> To: Bart Van Assche; osandov@xxxxxxxxxxx; kashyap.desai@xxxxxxxxxxxx
> Cc: linux-scsi@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> hch@xxxxxxxxxxxxx; linux-block@xxxxxxxxxxxxxxx; paolo.valente@xxxxxxxxxx
> Subject: Re: Device or HBA level QD throttling creates randomness in
> sequetial workload
>
> 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.

Thanks for feedback. I do realize that udelay() is going to be very odd
in queue_command call back.   I will keep this note. Preferred solution is
blk mq scheduler patches.
>
> --
> Jens Axboe