[dm-devel] [PATCH] block: flush queued bios when the process blocks

[Kent Overstreet]

On Tue, May 27, 2014 at 03:56:00PM -0400, Mikulas Patocka wrote:
> 
> 
> On Tue, 27 May 2014, Jens Axboe wrote:
> 
> > On 2014-05-27 10:26, Mikulas Patocka 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.
> > 
> > Except such a thing already exists, there are unplug callback chains. All I'm
> > asking is that you look into how feasible it would be to use something like
> > that, instead of reinventing the wheel.
> > 
> > -- 
> > Jens Axboe
> 
> Do you mean moving current->bio_list to struct blk_plug and calling 
> blk_start_plug/blk_finish_plug around generic_make_request?
> 
> It would be possible on a condition that we can redirect all bios to a 
> workqueue (i.e. eliminate bio_kmalloc and always use bio_alloc_bioset).
> 
> What are performance implications of this - does it make sense to have 
> blk_start_plug/blk_finish_plug around every call to generic_make_request? 
> - that means that all i/o requests will be added to a plug and then 
> unplugged.

We've already got blk_start_plug() calls around IO submission at higher points
in the stack. (I actually have seen it show up in profiles though, it probably
would be worth inlining and slimming down a bit).