Re: block: flush queued bios when the process blocks

[Kent Overstreet <kmo@daterainc.com>]

On Tue, May 27, 2014 at 01:33:06PM -0400, Mike Snitzer wrote:
> On Tue, May 27 2014 at 12:26pm -0400,
> Mikulas Patocka <mpatocka@redhat.com> wrote:
> 
> > On Tue, 27 May 2014, Jens Axboe wrote:
> > 
> > > On 2014-05-27 09:23, Mikulas Patocka wrote:
> > > 
> > > > The patch adds bio list flushing to the scheduler just besides plug
> > > > flushsing.
> > > 
> > > ... which is exactly why I'm commenting. It'd be great to avoid yet one more
> > > scheduler hook for this sort of thing.
> > > 
> > > -- 
> > > Jens Axboe
> > 
> > One could create something like schedule notifier chain, but I'm not sure 
> > if it is worth the complexity because of just two users. If more users 
> > come in the future, it could be generalized.
> 
> It could be that Jens is suggesting updating blk_needs_flush_plug() and
> blk_schedule_flush_plug() to be bio_list aware too (rather than train
> sched_submit_work() from this new bio_list)?
> 
> Somewhat awkward, but _could_ be made to work.

No...

I started on this ages and ages ago, but the patches were nowhere near ready to
go out.

What I'd do is rework the existing blk_plug to work in terms of bios, not
requests. This is a fair amount of work, and then make_request_fn() needs to be
able to take multiple bios at a time, but IIRC there were other simplifications
that fell out of this and this also means bio based drivers can take advantage
of plugging/batching.

Once this is done, you just... replace the bio list in generic_make_request()
with a plug. Boom, done.

(oh, and to avoid blowing the stack the scheduler/plug callback or whatever
needs to check the current stack usage and punt off to a per request_queue
workqueue, but that's easy enough).