linux-kernel - Re: [PATCH] block: flush queued bios when the process blocks

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Message-ID: <alpine.LRH.2.02.1405271526570.3223@file01.intranet.prod.int.rdu2.redhat.com>
Date:	Tue, 27 May 2014 15:56:00 -0400 (EDT)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	Jens Axboe <axboe@...nel.dk>
cc:	Kent Overstreet <kmo@...erainc.com>, linux-kernel@...r.kernel.org,
	dm-devel@...hat.com, "Alasdair G. Kergon" <agk@...hat.com>,
	Mike Snitzer <msnitzer@...hat.com>
Subject: Re: [PATCH] block: flush queued bios when the process blocks



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.

Mikulas
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