linux-kernel - Re: [PATCH -v3 6/8] fsnotify: add group priorities

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Date:	Mon, 01 Dec 2008 16:37:41 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Eric Paris <eparis@...hat.com>
Cc:	linux-kernel@...r.kernel.org, malware-list@...ts.printk.net,
	viro@...iv.linux.org.uk, akpm@...ux-foundation.org,
	alan@...rguk.ukuu.org.uk, arjan@...radead.org, hch@...radead.org
Subject: Re: [PATCH -v3 6/8] fsnotify: add group priorities

On Mon, 2008-12-01 at 10:20 -0500, Eric Paris wrote:
> 
> > I can
> > imagine for many groups and limit range a priority list might be better
> > suited.
> 
> talking about plist.h?  Since I don't allow 2 groups with the same
> priority I'd say a lot of the plist code would just be overhead (the
> prio list and the node list would be the same)
> 
> That's not a big deal since I don't really care about the add/remove
> code paths since they are all notification overhead/setup/teardown.  I
> would think that cleaner simpler code would probably be a better idea
> rather than performance for these areas especially since it looks like
> the speed critical parts of plists (list_for_each_entry) would be the
> exact same.
> 
> what I don't see is plists being protected by RCU and looking at
> plist_del it doesn't seem like it would be rcu safe.  RCU safe plists
> might be a good idea, but for now I think I should just do my own
> priority listing so I don't have to hold a lock while I walk the group
> list (that path is VERY hot)

plist.h provides a 2d structure, where you can iterate the priorities in
constant time no matter how many items of any one priority are enqueued.

Its basically a list of lists.

If as you say, you only have a hand full of items, there is no point.


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