Subject: Re: [PATCH 0/4] add task handling notifier
From: Matthew Helsley <matthltc@us.ibm.com>
To: Christoph Hellwig <hch@infradead.org>
Cc: Jan Beulich <jbeulich@novell.com>, linux-kernel@vger.kernel.org,
       pagg@oss.sgi.com, erikj@sgi.com, pj@sgi.com
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References: <476A780C.76E4.0078.0@novell.com>
	 <20071223122621.GA19310@infradead.org>
	 <1199845447.17010.149.camel@localhost.localdomain>
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Date: Tue, 08 Jan 2008 19:27:23 -0800
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On Tue, 2008-01-08 at 18:24 -0800, Matt Helsley wrote:
> On Sun, 2007-12-23 at 12:26 +0000, Christoph Hellwig 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/.
> 
> Apologies for the late reply -- I haven't had internet access for the
> last few weeks.
> 
> > For some reason neither ever made a lot of progess (performance
> > problems?).
> 
> Yeah. Some discussion on measuring the performance of Task Watchers:
> http://thread.gmane.org/gmane.linux.lse/4698
> 
> The requirements for Task Watchers were:
> 
> Allow sleeping in most/all notifier functions in these paths:
> 	fork
> 	exec
> 	exit
> 	change [re][ug]id
> No performance overhead
> One "chain" per path ("I only care about exec().")
> Easy to use
> Scales to large numbers of CPUs
> Useful to make most in-tree code more readable. Task Watchers took
> direct calls to these pieces of code out of the fork/exec/exit paths:
> 	audit
> 	semundo
> 	cpusets
> 	mempolicy
> 	trace irqflags
> 	lockdep
> 	keys (for processes -- not for thread groups)
> 	process events connector
> Useful for loadable modules
> 
> Performance overhead in microbenchmarks was measurable at around 1% (see
> the URL above). Overhead on benchmarks like kernbench on the other hand
> were in the noise margins (which were around 1.6%) and hence I couldn't
> determine the overhead there.
> 
> I never got the loadable module part completely working due to races
> between notifier functions and the module unload path. The solution to
> the races seemed to require adding more overhead to the notifier
> function paths (SRCU-like grace periods).
> 
> I stopped pushing the patch set because I hadn't found any new
> optimizations to offset the overheads while still meeting all the
> requirements and Andrew still felt that the "make it more readable"
> argument was not sufficient to justify its inclusion.

Oops. It's been nearly two years so I've forgotten exactly where Task
Watchers v2 was when I stopped pushing it. After a bit more searching I
found a more recent posting:
http://lkml.org/lkml/2006/12/14/384

And here's why I think the microbenchmark results improved to the point
there was a small performance improvement over mainline:
http://lkml.org/lkml/2006/12/19/124

I seem to recall kernbench was still too noisy to tell.

The patch allowing modules to register Task Watchers still isn't posted
there for the reasons I've already described.

Cheers,
	-Matt Helsley

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