Messages in this thread | | | Subject | Re: [PATCH RFC v2 02/24] scsi: allocate separate queue for reserved commands | From | Hannes Reinecke <> | Date | Tue, 7 Apr 2020 16:00:10 +0200 |
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On 4/7/20 1:54 PM, John Garry wrote: > On 06/04/2020 10:05, Hannes Reinecke wrote: >> On 3/11/20 7:22 AM, Christoph Hellwig wrote: >>> On Tue, Mar 10, 2020 at 09:08:56PM +0000, John Garry wrote: >>>> On 10/03/2020 18:32, Christoph Hellwig wrote: >>>>> On Wed, Mar 11, 2020 at 12:25:28AM +0800, John Garry wrote: >>>>>> From: Hannes Reinecke <hare@suse.com> >>>>>> >>>>>> Allocate a separate 'reserved_cmd_q' for sending reserved commands. >>>>> >>>>> Why? Reserved command specifically are not in any way tied to queues. >>>>> . >>>>> >>>> >>>> So the v1 series used a combination of the sdev queue and the per-host >>>> reserved_cmd_q. Back then you questioned using the sdev queue for >>>> virtio >>>> scsi, and the unconfirmed conclusion was to use a common per-host q. >>>> This is >>>> the best link I can find now: >>>> >>>> https://www.mail-archive.com/linux-scsi@vger.kernel.org/msg83177.html >>> >>> That was just a question on why virtio uses the per-device tags, which >>> didn't look like it made any sense. What I'm worried about here is >>> mixing up the concept of reserved tags in the tagset, and queues to use >>> them. Note that we already have the scsi_get_host_dev to allocate >>> a scsi_device and thus a request_queue for the host itself. That seems >>> like the better interface to use a tag for a host wide command vs >>> introducing a parallel path. >>> >> Thinking about it some more, I don't think that scsi_get_host_dev() is >> the best way of handling it. >> Problem is that it'll create a new scsi_device with <hostno:this_id:0>, >> which will then show up via eg 'lsscsi'. > > are you sure? Doesn't this function just allocate the sdev, but do > nothing with it, like probing it? > > I bludgeoned it in here for PoC: > > https://github.com/hisilicon/kernel-dev/commit/ef0ae8540811e32776f64a5b42bd76cbed17ba47 > > > And then still: > > john@ubuntu:~$ lsscsi > [0:0:0:0] disk SEAGATE ST2000NM0045 N004 /dev/sda > [0:0:1:0] disk SEAGATE ST2000NM0045 N004 /dev/sdb > [0:0:2:0] disk ATASAMSUNG HM320JI 0_01 /dev/sdc > [0:0:3:0] disk SEAGATE ST1000NM0023 0006 /dev/sdd > [0:0:4:0] enclosu HUAWEIExpander 12Gx16 128- > john@ubuntu:~$ > > Some proper plumbing would be needed, though. > >> This would be okay if 'this_id' would have been defined by the driver; >> sadly, most drivers which are affected here do set 'this_id' to -1. >> So we wouldn't have a nice target ID to allocate the device from, let >> alone the problem that we would have to emulate a complete scsi device >> with all required minimal command support etc. >> And I'm not quite sure how well that would play with the exising SCSI >> host template; the device we'll be allocating would have basically >> nothing in common with the 'normal' SCSI devices. >> >> What we could do, though, is to try it the other way round: >> Lift the request queue from scsi_get_host_dev() into the scsi host >> itself, so that scsi_get_host_dev() can use that queue, but we also >> would be able to use it without a SCSI device attached. > > wouldn't that limit 1x scsi device per host, not that I know if any more > would ever be required? But it does still seem better to use the request > queue in the scsi device. > My concern is this:
struct scsi_device *scsi_get_host_dev(struct Scsi_Host *shost) { [ .. ] starget = scsi_alloc_target(&shost->shost_gendev, 0, shost->this_id); [ .. ]
and we have typically:
drivers/scsi/hisi_sas/hisi_sas_v3_hw.c: .this_id = -1,
It's _very_ uncommon to have a negative number as the SCSI target device; in fact, it _is_ an unsigned int already.
But alright, I'll give it a go; let's see what I'll end up with.
Cheers,
Hannes -- Dr. Hannes Reinecke Teamlead Storage & Networking hare@suse.de +49 911 74053 688 SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg HRB 36809 (AG Nürnberg), Geschäftsführer: Felix Imendörffer
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