Re: [PATCH 6/7] mpt2sas: store scsi io tracker data in the scsi command / request

From: Jens Axboe
Date: Tue Apr 07 2015 - 12:13:51 EST

On 04/05/2015 10:03 AM, Christoph Hellwig wrote:
On Fri, Apr 03, 2015 at 09:58:22AM -0600, Jens Axboe wrote:
+struct scsiio_tracker *
+mpt2sas_get_st_from_smid(struct MPT2SAS_ADAPTER *ioc, u16 smid)
+ if (shost_use_blk_mq(ioc->shost)) {
+ struct scsi_cmnd *scmd;
+ scmd = scsi_mq_find_tag(ioc->shost, smid - 1);
+ if (!scmd)
+ return NULL;
+ return scsi_mq_scmd_to_pdu(scmd);
+ } else
+ return &ioc->scsi_lookup[smid - 1];

The mq case will also work for the !mq case when you call
scsi_host_find_tag and scsi_cmd_priv. In general all the mq-specific
codepathes you add should become the default and only one, even if this
requires a lit bit of additional core work.

For the core code, I definitely agree. But for this case, in scsi-mq mode, we know that tag == smid - 1. That's not the case if we are not using scsi-mq.

In general, it'd be great if we could "convert" drivers and not have to support both scsi-mq and legacy mode. Then I could just rip the old code.

@@ -1724,6 +1739,18 @@ mpt2sas_base_get_smid_scsiio(struct MPT2SAS_ADAPTER *ioc, u8 cb_idx,
struct scsiio_tracker *request;
u16 smid;

+ if (shost_use_blk_mq(ioc->shost)) {
+ /*
+ * If we don't have a SCSI command associated with this smid,
+ * bump it to high-prio
+ */
+ if (!scmd)
+ return mpt2sas_base_get_smid_hpr(ioc, cb_idx);

Seems like _ctl_do_mpt_command should be changed to just
call mpt2sas_base_get_smid_hpr unconditionally instead of adding this
hack Preferably as a standalone preparatory patch.

Sounds reasonable, I'll do that.

unsigned long flags;
int i;
- struct chain_tracker *chain_req, *next;
+ if (shost_use_blk_mq(ioc->shost) && smid < ioc->hi_priority_smid) {
+ struct scsiio_tracker *st;
+ st = mpt2sas_get_st_from_smid(ioc, smid);
+ if (!st)
+ return;
+ st->direct_io = 0;
+ if (!list_empty(&st->chain_list)) {
+ spin_lock_irqsave(&ioc->scsi_lookup_lock, flags);
+ _dechain_st(ioc, st);
+ spin_unlock_irqrestore(&ioc->scsi_lookup_lock, flags);
+ }

This whole chain list thing looks bonkers to me. We always allocated
a fixed multiple of the queue depth in ->chain_lookup, but then do this
required list manipulation at least once per I/O submission and completion.

It is completely crazy, and very suboptimal. The only thing that "saves" it is that we only need to do it multiple times for larger IOs, where a larger per-IO hit can be accepted. But yes, it really should just die a horrible death.

Seems like we should instead add an array of (cpu address, dma address)
tuples to the scsiio_tracker and avoid all the chain_lookup / chain_list
lookups entirely.


+ if (shost_use_blk_mq(ioc->shost)) {
+ scmd = scsi_mq_find_tag(ioc->shost, i);
+ if (scsi_mq_scmd_started(scmd))
+ pending++;

Ok, I guess we should move the request_started check into the _find_tag
helpers, as tags that aren't started aren't something that driver
should ever lookup.

I'll move it in there.

+static bool
+_scmd_match(struct scsi_cmnd *scmd, u16 handle, u32 lun)
+ struct MPT2SAS_DEVICE *priv_data;
+ if (scmd == NULL || scmd->device == NULL ||
+ scmd->device->hostdata == NULL)
+ return false;

If the queue is started this can't ever happen.

+ if (lun != scmd->device->lun)
+ return false;

If you pass in a specific scsi_device and thus request_queue this
can't happen.

+static u16
+_ctl_find_smid(struct MPT2SAS_ADAPTER *ioc, u16 handle, u32 lun)
+ if (shost_use_blk_mq(ioc->shost))
+ return _ctl_find_smid_mq(ioc, handle, lun);
+ else
+ return _ctl_find_smid_legacy(ioc, handle, lun);

The caller of this looks entirely broken. It's a driver specific API
to submit task management commands, duplicating the mid level code,
and it doesn't even allow which task to target. I think we should
just return a error when invoking MPI2_FUNCTION_SCSI_TASK_MGMT instead
of digging us an even deeper grave here. If someone complains we'll
have to find a way to redirect it to the generic EH ioctls.

Sounds fine to me, will make my life a lot easier and we can kill this horrible lookup mess.

Jens Axboe

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