linux-kernel - Re: 2.6.18-rc2-mm1

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Date:	Fri, 28 Jul 2006 01:34:42 -0700
From:	Andrew Morton <akpm@...l.org>
To:	"Michal Piotrowski" <michal.k.k.piotrowski@...il.com>
Cc:	matthltc@...ibm.com, linux-kernel@...r.kernel.org
Subject: Re: 2.6.18-rc2-mm1

On Fri, 28 Jul 2006 10:17:44 +0200
"Michal Piotrowski" <michal.k.k.piotrowski@...il.com> wrote:

> Matt, can you look at this?
> 
> My hunt file shows me, that this patches are causing oops.
> GOOD
> #
> #
> task-watchers-task-watchers.patch
> task-watchers-register-process-events-task-watcher.patch
> task-watchers-refactor-process-events.patch
> task-watchers-make-process-events-configurable-as.patch
> task-watchers-allow-task-watchers-to-block.patch
> task-watchers-register-audit-task-watcher.patch
> task-watchers-register-per-task-delay-accounting.patch
> task-watchers-register-profile-as-a-task-watcher.patch
> task-watchers-add-support-for-per-task-watchers.patch
> task-watchers-register-semundo-task-watcher.patch
> task-watchers-register-per-task-semundo-watcher.patch
> BAD

Thanks for working that out.

I've actually been thinking that we shouldn't proceed with those patches.

They're a nice cleanup and make the kernel code _look_ better and I really
like them because of this.  But the cost is potentially significant.  We
replace N direct calls with a walk of a notifier chain, more than N
indirect calls, demultiplexing at the other end and then a direct call. 
That's a significant amount of additional overhead to make the kernel
source look nicer.

Plus, ugly though it is, you can look at the current code and see what it's
doing.  With a notifier chain you have to grep around the tree and work out
what might be hooking into the chain, which is harder.

Finally, the consolidation into a notifier chain forces all the
fork/exit/exec hooks into an one-size-fits-all model.  What happens if one
subsystem wants to hook in before exit_mmap() and another one wants to hook
in after exit_mmap() (for example)?

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