linux-kernel - Re: [PATCH 03/11] block: block_bio

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Date:	Sun, 8 Jan 2012 17:49:49 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Namhyung Kim <namhyung.kim@....com>
Cc:	axboe@...nel.dk, mingo@...hat.com, rostedt@...dmis.org,
	fweisbec@...il.com, teravest@...gle.com, slavapestov@...gle.com,
	ctalbott@...gle.com, dsharp@...gle.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 03/11] block: block_bio_complete tracepoint was missing

Hello,

On Mon, Jan 09, 2012 at 10:30:06AM +0900, Namhyung Kim wrote:
> Just adding the TP unconditionally will produce duplicated (in some
> sense) reports about the completion. For example, normal request
> based IO reports whole request completion prior to its bio's, and
> further

Request and bio completions are separate events.  There's nothing
wrong with reporting them separately.  In fact, I think they should be
reported separately.

>, some of nested block IO handling routines - bounced bio and
> btrfs with compression, etc - call bio_endio() more than once. Also
> there are cases that bio fails before it's enqueued for some reason.

They are actually separate bio's being completed.  I don't think
trying to put extra semantics on TP itself is a good idea.  In
general, TP signals that such event happened with sufficient
information and it's the consumers' responsibility to make sense of
what's going on.  BIO_CLONED/BOUNCED are there.

> I have no idea about the ioblame can deal with all of such corner
> cases. However it might confuse blktrace somewhat, I guess.

Unless someone is doing memcpy() on bio's, ioblame should be okay.  It
only considers bio's which went through actual submission.

> I already posted similar patch a couple of weeks ago, but didn't
> receive a comment yet. [1] Please take a look this too :)

I'll reply there but don't think imposing such extra logic on TP is a
good idea.

> After a quick glance, the ioblame seems to carry all IO accounting
> info through the first bio in the request. If so, why don't you use
> the request structure for that?

Because there are bio based drivers which don't use requests at all.

Thanks.

--
tejun
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