linux-kernel - Re: [PATCH 08/20] sysfs: Optimize just changing the sysfs file mode.

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Message-ID: <4A1513B8.90105@kernel.org>
Date:	Thu, 21 May 2009 17:41:28 +0900
From:	Tejun Heo <tj@...nel.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	linux-kernel@...r.kernel.org,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	linux-fsdevel@...r.kernel.org,
	"Eric W. Biederman" <ebiederm@...stanetworks.com>
Subject: Re: [PATCH 08/20] sysfs: Optimize just changing the sysfs file mode.

Hello,

Eric W. Biederman wrote:
>> One visible difference is lack of timestamp update.  Is there any use
>> case where sysfs file mode changing needs to be fast?\
> 
> Not really.  If the time changes we set something besides ATTR_MODE
> like ATTR_MTIME or ATTR_CTIME.  If we come in through any of the
> user space entry points ATTR_CTIME appears to be set so this optimization
> will not trigger.
> 
> I think there are cases where we only opportunistically track time
> changes, when the structure is allocated that this changes but it
> is a very small percentage of the time.
> 
> The practical effect of my changes should be that we only track timestamps
> when user space actually performs an explicit change to the file.
> 
> If someone was depending on some weird indirect side effect like that
> on one of the 5-6 files that calls sysfs_chmod let's make it explicit.
> 
> For me this isn't about making this go faster.  This is about keeping
> the sysfs data structures small when we can.
> 
> It doesn't really complicate the code and we wind up doing the obvious thing.

Well, it doesn't add a lot of complexity but also seems pointless when
there basically is no use case which would benefit from this change.
I suppose it's upto the maintainer.  Greg?

Thanks.

-- 
tejun
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