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

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Date:	Mon, 27 Oct 2008 20:14:18 -0400
From:	"Mike Snitzer" <snitzer@...il.com>
To:	"Steven Rostedt" <rostedt@...dmis.org>
Cc:	linux-ext4@...r.kernel.org,
	"Andrew Morton" <akpm@...ux-foundation.org>
Subject: Re: why unlikely(rsv) in ext3_clear_inode()?

On Mon, Oct 27, 2008 at 7:32 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
>
> 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.

What was your workload that resulted in this break-down?  AFAIK you'd
have 100% in ftrace_nonnull if you simply opened new files and wrote
to them.

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

Yes, probably.

thanks,
Mike
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