Re: [Lse-tech] SCSI io_request_lock patch

[Jonathan Lahr <lahr@us.ibm.com>]

Jens Axboe [axboe@suse.de] wrote:
> On Mon, Nov 12 2001, Jonathan Lahr wrote:
> > 
> > This is a request for comments on the patch described below which 
> > implements a revised approach to reducing io_request_lock contention 
> > in 2.4.
> > 
> > This new version of the io_request_lock patch (siorl-v0) is available
> > at http://sourceforge.net/projects/lse/.  It employs the same
> > concurrent request queueing scheme as the iorlv0 patch but isolates 
> > code changes to the SCSI subsystem and engages the new locking scheme 
> > only for SCSI drivers which explicitly request it.  I took this more 
> > restricted approach after additional development based on comments from 
> > Jens and others indicated that iorlv0 impacted the IDE subsystem and
> > was unnecessarily broad in general.
> > 
> > The siorl-v0 patch allows drivers to enable concurrent queueing through 
> > the concurrent_queue field in the Scsi_Host_Template which is copied to 
> > the request queue.  It creates SCSI-specific versions of generic block 
> > i/o functions used by the SCSI subsystem and modifies them to conditionally 
> > engage the new locking scheme based on this field.  It allows control over 
> > which drivers use concurrent queueing and preserves original block i/o 
> > behavior by default.
> 
> Sorry Jonathan, but this is even more broken than the last patch. In
> different ways. In no particular order:
> 
> o You are duplicating way too much code and exporting block internals

The duplication is a reasonable starting point for SCSI-specific functions.
The block i/o design provides for exactly this type of tailoring through
function pointers installed in request_queue.

What problem you do see with exporting block internals?

> o You are breaking SCSI merge completely, why on earth are you suddenly
>   using ll_*_merge functions for SCSI?!
> o scsi_make_request need not worry about head active
> o scsi_make_request can safe the q->*_merge indirect
> o scsi_dispatch_cmd() io_request_lock removal looks racy

I will investigate the above comments further.

> At least you are not breaking anything other than SCSI this time...

Do you think the separation of SCSI from generic block i/o code and the
driver-activated control of concurrent queueing provides a path for future 
work to reduce io_request_lock contention in SCSI/FC?

-- 
Jonathan Lahr
IBM Linux Technology Center
Beaverton, Oregon
lahr@us.ibm.com
503-578-3385