Re: [PATCH] blk-mq: don't queue passthrough request into scheduler

[Christoph Hellwig <hch@infradead.org>]

On Fri, May 12, 2023 at 09:08:54AM -0600, Jens Axboe wrote:
> On 5/12/23 9:03?AM, Ming Lei wrote:
> > Passthrough(pt) request shouldn't be queued to scheduler, especially some
> > schedulers(such as bfq) supposes that req->bio is always available and
> > blk-cgroup can be retrieved via bio.
> > 
> > Sometimes pt request could be part of error handling, so it is better to always
> > queue it into hctx->dispatch directly.
> > 
> > Fix this issue by queuing pt request from plug list to hctx->dispatch
> > directly.
> 
> Why not just add the check to the BFQ insertion? That would be a lot
> more trivial and would not be poluting the core with this stuff.

Because we really need to keep the passthrough code separate.  The
fact that a passthrough request can leak into common code in various
places is really a bit of a problem.  We have most of these nicely
separate with two exceptions:

 - the plug list
 - the requeue list

The higher level and the more obvious we special case the passthrough
request there, the better for debuggability and maintainability.