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SubjectRe: [PATCH 0/4] add task handling notifier
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 ( 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:
> 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:
> 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:

And here's why I think the microbenchmark results improved to the point
there was a small performance improvement over mainline:

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.

-Matt Helsley

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