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

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Date:   Mon, 30 Jan 2017 23:58:28 +0530
From:   Kashyap Desai <kashyap.desai@...adcom.com>
To:     Jens Axboe <axboe@...nel.dk>,
        Bart Van Assche <Bart.VanAssche@...disk.com>,
        osandov@...ndov.com
Cc:     linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
        hch@...radead.org, linux-block@...r.kernel.org,
        paolo.valente@...aro.org
Subject: RE: Device or HBA level QD throttling creates randomness in sequetial workload

> -----Original Message-----
> From: Jens Axboe [mailto:axboe@...nel.dk]
> Sent: Monday, January 30, 2017 10:03 PM
> To: Bart Van Assche; osandov@...ndov.com; kashyap.desai@...adcom.com
> Cc: linux-scsi@...r.kernel.org; linux-kernel@...r.kernel.org;
> hch@...radead.org; linux-block@...r.kernel.org; paolo.valente@...aro.org
> 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