linux-kernel - Re: [PATCH 05/10] block: remove per-queue plugging

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Date:	Wed, 9 Mar 2011 19:58:10 -0500
From:	Mike Snitzer <snitzer@...hat.com>
To:	Jens Axboe <jaxboe@...ionio.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"hch@...radead.org" <hch@...radead.org>, dm-devel@...hat.com,
	neilb@...e.de
Subject: Re: [PATCH 05/10] block: remove per-queue plugging

On Tue, Mar 08 2011 at  5:05pm -0500,
Mike Snitzer <snitzer@...hat.com> wrote:

> On Tue, Mar 08 2011 at  3:27pm -0500,
> Jens Axboe <jaxboe@...ionio.com> wrote:
> 
> > On 2011-03-08 21:21, Mike Snitzer wrote:
> > > On Tue, Mar 08 2011 at  7:16am -0500,
> > > Jens Axboe <jaxboe@...ionio.com> wrote:
> > > 
> > >> On 2011-03-03 23:13, Mike Snitzer wrote:
> > >>> I'm now hitting a lockdep issue, while running a 'for-2.6.39/stack-plug'
> > >>> kernel, when I try an fsync heavy workload to a request-based mpath
> > >>> device (the kernel ultimately goes down in flames, I've yet to look at
> > >>> the crashdump I took)
> > >>
> > >> Mike, can you re-run with the current stack-plug branch? I've fixed the
> > >> !CONFIG_BLOCK and rebase issues, and also added a change for this flush
> > >> on schedule event. It's run outside of the runqueue lock now, so
> > >> hopefully that should solve this one.
> > > 
> > > Works for me, thanks.
> > 
> > Super, thanks! Out of curiousity, did you use dm/md?
> 
> Yes, I've been using a request-based DM multipath device.

Hi Jens,

I just got to reviewing your onstack plugging DM changes (I looked at
the core block layer changes for additional context and also had a brief
look at MD).

I need to put more time to the review of all this code but one thing
that is immediately apparent is that after these changes DM only has one
onstack plug/unplug -- in drivers/md/dm-kcopyd.c:do_work()

You've removed a considerable amount of implicit plug/explicit unplug
code from DM (and obviously elsewhere but I have my DM hat on ;).

First question: is relying on higher-level (aio, fs, read-ahead)
explicit plugging/unplugging sufficient?  Seems odd to not have the
control/need to unplug the DM device upon resume (after a suspend).

(this naive question/concern stems from me needing to understand the
core block layer's onstack plugging changes better)

(but if those higher-level explicit onstack plug changes make all this
code removal possible shouldn't those commits come before changing
underlying block drivers like DM, MD, etc?)

I noticed that driver/md/dm-raid1.c:do_mirror() seems to follow the same
pattern of drivers/md/dm-kcopyd.c:do_work().. so rather than remove
dm_table_unplug_all() shouldn't it be replaced with a
blk_start_plug/blk_finish_plug?

Also, in your MD changes, you removed all calls to md_unplug() but
didn't remove md_unplug().  Seems it should be removed along with the
'plug' member of 'struct mddev_t'?  Neil?

Thanks,
Mike
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