Re: Proposed changes to generic blk tag for use in SCSI (1/3)

From: James Bottomley (
Date: Thu Jun 13 2002 - 16:26:44 EST

On Mon, Jun 10, 2002 at 10:46:44PM -0400, James Bottomley wrote:
> 2) The SCSI queue will stall if it gets an untagged request in the stream, so
> once tagged queueing is enabled, all commands (including SPECIALS) must be
> tagged. I altered the check in blk_queue_start_tag to permit this. said:
> Hmmm...this seems broken to me. Switching from tagged to untagged
> momentarily and then back is perfectly valid. Can the bio layer
> handle this and not the scsi layer, or are both layers unable to
> handle this sort of tag manipulation?

The layers can cope with the switch easily enough. The problem is that to
send an untagged command to a SCSI device you have to wait for the outstanding
tags to clear which is what causes the stall. The scsi mid-layer queue push
back system pushes all commands back to the BIO layer marked as REQ_SPECIAL
(because the upper layer drivers generate the commands and it has no idea what
they are supposed to be doing) if the driver cannot handle them. This means
for those drivers (like the new adaptec) which load up the device until it
returns a queue full (thus causing push back into the bio layer) we'd get
stutter in the command pipeline. The cleanest solution is to allow (but not
require) tagging of every request type.

On Mon, Jun 10, 2002 at 10:46:44PM -0400, James Bottomley wrote:
> There are several shortcomings of the prototype, most notably it doesn't have
> tag starvation detection and processing. However, I think I can re-introduce
> this as part of the error handler functions. said:
> If you are using the bio layer tag processing, then it should be
> doing this part I would think. If it isn't, then it sounds like
> either it's design is missing some key elements required to be fully
> functional or the integration between the scsi layer and the bio
> layer needs some additional work.

I thought about doing this. The problem is that the blk layer doesn't have
very good instrumentation for detecting the condition. The SCSI layer is the
one that has per command timers and all the other necessaries so it can detect
when a command should have returned and take corrective action.


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