Re: [PATCH 4/4] scsi: core: don't limit per-LUN queue depth for SSD

Ewan D. Milne
2019-11-20T21:36:53Z

On Wed, 2019-11-20 at 12:56 -0800, Bart Van Assche wrote:
> On 11/20/19 9:00 AM, Ewan D. Milne wrote:
> > On Wed, 2019-11-20 at 11:05 +0100, Hannes Reinecke wrote:
> > > I must admit I patently don't like this explicit dependency on
> > > blk_nonrot(). Having a conditional counter is just an open invitation to
> > > getting things wrong...
> >
> > This concerns me as well, it seems like the SCSI ML should have it's
> > own per-device attribute if we actually need to control this per-device
> > instead of on a per-host or per-driver basis. And it seems like this
> > is something that is specific to high-performance drivers, so changing
> > the way this works for all drivers seems a bit much.
> >
> > Ordinarily I'd prefer a host template attribute as Sumanesh proposed,
> > but I dislike wrapping the examination of that and the queue flag in
> > a macro that makes it not obvious how the behavior is affected.
> > (Plus Hannes just submitted submitted the patches to remove .use_cmd_list,
> > which was another piece of ML functionality used by only a few drivers.)
> >
> > Ming's patch does freeze the queue if NONROT is changed by sysfs, but
> > the flag can be changed by other kernel code, e.g. sd_revalidate_disk()
> > clears it and then calls sd_read_block_characteristics() which may set
> > it again. So it's not clear to me how reliable this is.
>
> How about changing the default behavior into ignoring sdev->queue_depth
> and only honoring sdev->queue_depth if a "quirk" flag is set or if
> overridden by setting a sysfs attribute? My understanding is that the
> goal of the queue ramp-up/ramp-down mechanism is to reduce the number of
> times a SCSI device responds "BUSY". An alternative for queue
> ramp-up/ramp-down is a delayed queue re-run. I think if scsi_queue_rq()
> returns BLK_STS_RESOURCE that the queue is only rerun after a delay.
> From blk_mq_dispatch_rq_list():
>
> [ ... ]
> else if (needs_restart && (ret == BLK_STS_RESOURCE))
> blk_mq_delay_run_hw_queue(hctx, BLK_MQ_RESOURCE_DELAY);
>
> Bart.

In general it is better not to put in changes that affect older drivers
that are not regularly tested, I think. I would prefer an opt-in for
drivers desiring higher performance.

Delaying the queue re-run vs. a ramp down might negatively affect performance.
I'm not sure how accurate the ramp is at discovering the optimum though.

-Ewan