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

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Message-ID: <1485793840.2712.1.camel@sandisk.com>
Date:   Mon, 30 Jan 2017 16:30:59 +0000
From:   Bart Van Assche <Bart.VanAssche@...disk.com>
To:     "osandov@...ndov.com" <osandov@...ndov.com>,
        "kashyap.desai@...adcom.com" <kashyap.desai@...adcom.com>,
        "axboe@...nel.dk" <axboe@...nel.dk>
CC:     "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "hch@...radead.org" <hch@...radead.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "paolo.valente@...aro.org" <paolo.valente@...aro.org>
Subject: Re: Device or HBA level QD throttling creates randomness in
 sequetial workload

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.

Bart.