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Date:	Tue, 25 Dec 2007 14:05:26 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Jan Beulich <jbeulich@...ell.com>, linux-kernel@...r.kernel.org,
	pagg@....sgi.com, erikj@....com, matthltc@...ibm.com, pj@....com
Subject: Re: [PATCH 0/4] add task handling notifier

On Sun, 23 Dec 2007 12:26:21 +0000 Christoph Hellwig <hch@...radead.org> wrote:

> On Thu, Dec 20, 2007 at 01:11:24PM +0000, Jan Beulich wrote:
> > With more and more sub-systems/sub-components leaving their footprint
> > in task handling functions, it seems reasonable to add notifiers that
> > these components can use instead of having them all patch themselves
> > directly into core files.
> 
> I agree that we probably want something like this.  As do some others,
> so we already had a few a few attempts at similar things.  The first one
> is from SGI and called PAGG (http://oss.sgi.com/projects/pagg/) and also
> includes allocating per-task data for it's users.  Then also from SGI
> there has been a simplified version called pnotify that's also available
> from the website above.
> 
> Later Matt Helsley had something called "Task Watchers" which lwn has
> an article on: http://lwn.net/Articles/208117/.
> 
> For some reason neither ever made a lot of progess (performance
> problems?).
> 

I had it in -mm, sorted out all the problems but ended up not pulling the
trigger.

Problem is, it adds runtime overhead purely for the convenience of kernel
programmers, and I don't think that's a good tradeoff.

Sprinkling direct calls into a few well-known sites won't kill us, and
we've survived this long.  Why not keep doing that, and save everyone a few
cycles?


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