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

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Message-ID: <20151005205943.GB25762@redhat.com>
Date:	Mon, 5 Oct 2015 16:59:44 -0400
From:	Mike Snitzer <snitzer@...hat.com>
To:	Mikulas Patocka <mpatocka@...hat.com>
Cc:	Jens Axboe <axboe@...nel.dk>, Kent Overstreet <kmo@...erainc.com>,
	"Alasdair G. Kergon" <agk@...hat.com>,
	linux-kernel@...r.kernel.org, dm-devel@...hat.com
Subject: Re: block: flush queued bios when the process blocks

On Thu, May 29 2014 at  7:52pm -0400,
Mikulas Patocka <mpatocka@...hat.com> 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
> 
> 
> You can use this patch as an example that moves current->bio_list to 
> struct plug, but I don't recommend to put it in the kernel - this patch 
> still has some issues (some lvm raid tests fail).


Mikulas,

Could it be that cond_resched() wasn't unplugging?  As was
recently raised in this thread: https://lkml.org/lkml/2015/9/18/378
Chris Mason's patch from that thread fixed this	issue... I _think_ Linus
has since committed Chris' work but I haven't kept my finger on the
pulse of that issue.

FYI, I've put rebased versions of your 2 patches in my wip branch, see:
http://git.kernel.org/cgit/linux/kernel/git/snitzer/linux.git/log/?h=wip

I tweaked the 2nd patch that adds bio_list to plug so that
generic_make_request's checks for in_generic_make_request isn't racey
(your original patch could happen to have current-plug set but
in_generic_make_request not yet set).

Anyway, I'm just dusting off your earlier work and seeing if I can make
sense of a way forward that meets Jens' approval.  Jens, if adding a
bio_list to struct blk_plug isn't what you were after either please be
more specific on what you would like to see...

Thanks,
Mike
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