linux-ext4 - [RFC] Journal credits debugging (Was: Re: Bug#615998: ...)

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Date:	Wed, 29 Jun 2011 10:10:44 +0300
From:	Amir Goldstein <amir73il@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	"Moffett, Kyle D" <Kyle.D.Moffett@...ing.com>,
	"Ted Ts'o" <tytso@....edu>, Lukas Czerner <lczerner@...hat.com>,
	Sean Ryle <seanbo@...il.com>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	Sachin Sant <sachinp@...ibm.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Subject: [RFC] Journal credits debugging (Was: Re: Bug#615998: ...)

On Wed, Jun 29, 2011 at 1:57 AM, Jan Kara <jack@...e.cz> wrote:
> On Tue 28-06-11 14:30:55, Moffett, Kyle D wrote:
>> This is really helpful to me, but it's deviated a bit from solving
>> the original bug.  Based on the last log that I generated showing that
>> the error occurs in journal_stop(), what else should I be testing?
>  I was looking at it for a while but so far I have no idea...
>

Hi Jan,

I would like to suggest an approach that may help us track down these
sort of bugs more easily.

Add a new class_id argument to ext4_handle_dirty_metadata() and collect
statistics of used credits per class_id.

There are only so many types of journaled objects:
SUPER, GDT, BB, IB, ITB, IND, EXT, DATA, XATTR, QUOT...
So it shouldn't be a problem to save the statistics per handle.

If you look at struct jbd2_journal_handle, you will find a bunch of h_cow_XXX
fields intended as COW debugging counters.
We may as well turn these fields into a generic counters array, which can
be used by either COW debugging or credits debugging code.

This should be simple enough to implement and should provide
a more detailed report when buffer credits have run out.

However, if we are going to modify all call sites of
ext4_handle_dirty_metadata(),
it would be wise to also add an object_id (or a better name) argument
that will provide
the group no. or inode no. or quota type, or any other id relevant for
classification.

We can use this information, along with where, line, handle, ino,
block_nr, buffer_credits
and create a stable trace point in __ext4_handle_dirty_metadata().

Sorry for writing emails instead of patches...
Amir.
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