linux-ext4 - Re: why unlikely(rsv) in ext3_clear

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Message-ID: <alpine.DEB.1.10.0810271918390.19731@gandalf.stny.rr.com>
Date:	Mon, 27 Oct 2008 19:32:11 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mike Snitzer <snitzer@...il.com>
cc:	linux-ext4@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: why unlikely(rsv) in ext3_clear_inode()?


On Mon, 27 Oct 2008, Mike Snitzer wrote:

> Please see: e6022603b9aa7d61d20b392e69edcdbbc1789969
> 
> Having a look at the LKML archives this was raised back in 2006:
> http://lkml.org/lkml/2006/6/23/337
> 
> I'm not interested in whether unlikely() actually helps here.
> 
> I'm still missing _why_ rsv is mostly NULL at this callsite, as Andrew
> asserted here:
> http://lkml.org/lkml/2006/6/23/400
> 
> And then Steve here: http://lkml.org/lkml/2006/6/24/76
> Where he said:
> "The problem is that in these cases the pointer is NULL several thousands
> of times for every time it is not NULL (if ever).  The non-NULL case is
> where an error occurred or something very special.  So I don't see how
> the if here is a problem?"
> 
> I'm missing which error or what "something very special" is the
> unlikely() reason for having rsv be NULL.
> 
> Looking at the code; ext3_clear_inode() is _the_ place where the
> i_block_alloc_info is cleaned up.  In my testing the rsv is _never_
> NULL if the file was open for writing.  Are we saying that reads are
> much more common than writes?  May be a reasonable assumption but
> saying as much is very different than what Steve seemed to be eluding
> to...
> 
> Anyway, I'd appreciate some clarification here.

Attached is a patch that I used for counting.

Here's my results:
# cat /debug/tracing/ftrace_null 
45
# cat /debug/tracing/ftrace_nonnull 
7

Ah, seems that there is cases where it is nonnull more often. Anyway, it 
obviously is not a fast path (total of 52). Even if it was all null, it is 
not big enough to call for the confusion.

I'd suggest removing the if conditional, and just calling kfree.

-- Steve

View attachment "trace-ext3-null.patch" of type "TEXT/x-diff" (3750 bytes)