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SubjectRe: [PATCH 0/4] add task handling notifier
On Tue, 08 Jan 2008 13:38:03 +0000
"Jan Beulich" <> wrote:

> >>> Andrew Morton <> 25.12.07 23:05 >>>
> >On Sun, 23 Dec 2007 12:26:21 +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:
> >>
> >> 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?
> Am I to conclude then that there's no point in addressing the issues other
> people pointed out? While I (obviously, since I submitted the patch disagree),
> I'm not certain how others feel. My main point for disagreement here is (I'm
> sorry to repeat this) that as long as certain code isn't allowed into the kernel
> I think it is not unreasonable to at least expect the kernel to provide some
> fundamental infrastructure that can be used for those (supposedly
> unacceptable) bits. All I did here was utilizing the base infrastructure I want
> added to clean up code that appeared pretty ad-hoc.

Ah. That's a brand new requirement.

The requirement which I thought we were addressing was "clean the code up
by adding a notifier chain so multiple subsystems don't need to patch in
hard-coded calls".

My contention is that the code clarity which this gains isn't worth the
runtime cost.

Now we have a new requirement: "allow out-of-tree code to hook into these
spots without needing to patch those few callsites".

I think we'd need a pretty detailed description of the pain which this
would relieve before we would take such an extraordinary step. What are
those (unidentified) add-on features doing at present? Patching calls into

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