Re: [Lse-tech] SCSI io_request_lock patch

[Jonathan Lahr]

Jens Axboe [[e-mail]] wrote:
> On Tue, Nov 13 2001, Jonathan Lahr wrote:
> > Jens Axboe [[e-mail]] 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.
> 
> Yes I know, I wrote most of said code :-)

And this approach makes good use of it.

> > What problem you do see with exporting block internals?
> 
> It's absolutely worthless. Look, it ties in with the points I made
> below. You are exporting the merge functions for instance, and setting
> them in the queue. This will cause scsi_merge not to use it's own
> functions, broken.

As in the baseline, initialize_merge_fn overwrites these pointers:
     q->back_merge_fn = scsi_back_merge_fn_;
     q->front_merge_fn = scsi_front_merge_fn_;
     q->merge_requests_fn = scsi_merge_requests_fn_;

> > 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?
> 
> Not really, but I do think it could be a viable 2.4 alternative. For 2.5
> we still want to do this the right way.

I'll try to stay apprised of the 2.5 work as it progresses.

-- 
Jonathan Lahr
IBM Linux Technology Center
Beaverton, Oregon
[e-mail]
503-578-3385