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SubjectRe: [Lse-tech] SCSI io_request_lock patch
Jens Axboe [] 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 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

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