Messages in this thread | | | Subject | Re: [PATCH RFC v2 02/24] scsi: allocate separate queue for reserved commands | From | John Garry <> | Date | Tue, 7 Apr 2020 15:35:24 +0100 |
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On 07/04/2020 15:00, Hannes Reinecke wrote: > 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. >
FWIW, the only other driver (gdth) which I see uses this API has this_id = -1 in the scsi host template.
> But alright, I'll give it a go; let's see what I'll end up with.
note: If we want a fixed scsi_device per host, calling scsi_mq_setup_tags() -> scsi_get_host_dev() will fail as shost state is not running. Maybe we need to juggle some things there to provide a generic solution.
thanks
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