Re: [PATCH 1/7] block: use legacy path for flush requests for MQ with a scheduler

[Johannes Thumshirn <jthumshirn@suse.de>]

On Mon, Dec 05, 2016 at 08:07:10AM -0700, Jens Axboe wrote:
> On 12/05/2016 06:05 AM, Christoph Hellwig wrote:
> > On Fri, Dec 02, 2016 at 08:15:15PM -0700, Jens Axboe wrote:
> >> No functional changes with this patch, it's just in preparation for
> >> supporting legacy schedulers on blk-mq.
> > 
> > Ewww.  I think without refactoring to clear what 'use_mq_path'
> > means here and better naming this is a total non-started.  Even with
> > that we'll now have yet another code path to worry about.  Is there
> > any chance to instead consolidate into a single path?
> 
> It's not pretty at all. I should have prefaced this patchset with saying
> that it's an experiment in seeing what it would take to simply use the
> old IO schedulers, as a temporary measure, on blk/scsi-mq. I did clean
> it up a bit after posting:
> 
> http://git.kernel.dk/cgit/linux-block/log/?h=blk-mq-legacy-sched
> 
> but I'm not going to claim this is anywhere near merge read, nor clean.
> 
> >>  struct request *blk_get_request(struct request_queue *q, int rw, gfp_t gfp_mask)
> >>  {
> >> -	if (q->mq_ops)
> >> +	if (blk_use_mq_path(q))
> >>  		return blk_mq_alloc_request(q, rw,
> >>  			(gfp_mask & __GFP_DIRECT_RECLAIM) ?
> >>  				0 : BLK_MQ_REQ_NOWAIT);
> > 
> > So now with blk-mq and an elevator set we go into blk_old_get_request,
> > hich will simply allocate new requests.  How does this not break
> > every existing driver?
> 
> Since Johannes found that confusion, maybe I should explain how it all
> works.

To clarify the naming, how about sth. like blk_mq_use_sched() (to align
with blk_mq_sched_dispatch())?


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Johannes Thumshirn                                          Storage
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