Re: [PATCH 00/11] Task watchers: Introduction
From: Peter Williams
Date: Wed Jun 21 2006 - 21:09:48 EST
Matt Helsley wrote:
On Thu, 2006-06-22 at 09:04 +1000, Peter Williams wrote:
Matt Helsley wrote:
On Wed, 2006-06-21 at 21:41 +1000, Peter Williams wrote:
Peter Williams wrote:
Matt Helsley wrote:
On Wed, 2006-06-21 at 15:41 +1000, Peter Williams wrote:
On a related note, I can't see where the new task's notify field gets
initialized during fork.
It's initialized in kernel/sys.c:notify_per_task_watchers(), which calls
RAW_INIT_NOTIFIER_HEAD(&task->notify) in response to WATCH_TASK_INIT.
I think that's too late. It needs to be done at the start of
notify_watchers() before any other watchers are called for the new task.
I don't see why you think it's too late. It needs to be initialized
before it's used. Waiting until notify_per_task_watchers() is called
with WATCH_TASK_INIT does this.
I probably didn't understand the code well enough. I'm still learning
how it all hangs together :-).
On second thoughts, it would simpler just before the WATCH_TASK_INIT
call in copy_process() in fork.c. It can be done unconditionally there.
Peter
That would work. It would not simplify the control flow of the code.
The branch for WATCH_TASK_INIT in notify_per_task_watchers() is
unavoidable; we need to call the parent task's chain in that case since
we know the child task's is empty.
It is also counter to one goal of the patches -- reducing the "clutter"
in these paths. Arguably task watchers is the same kind of clutter that
existed before. However, it is a means of factoring such clutter into
fewer instances (ideally one) of the pattern.
Maybe a few comments in the code to help reviewers such as me learn how
it works more quickly would be worthwhile.
Good point. I'll keep this in mind as I consider the multi-chain
approach suggested by Andrew -- I suspect improvments in my commenting
will be even more critical there.
BTW as a former user of PAGG, I think there are ideas in the PAGG
implementation that you should look at. In particular:
1. The use of an array of function pointers (one for each hook) can cut
down on the overhead. The notifier_block only needs to contain a
pointer to the array so there's no increase in the size of that
structure. Within the array a null pointer would mean "don't bother
calling". Only one real array needs to exist even for per task as
they're all using the same functions (just separate data). It removes
the need for a switch statement in the client's function as well as
saving on unnecessary function calls.
I don't think having an explicit array of function pointers is likely
to be as fast as a switch statement (or a simple branch) generated by
the compiler.
With the array there's no need for any switch or branching. You know
exactly which function in the array to use in each hook.
It doesn't save unecessary function calls unless I modify the core
notifier block structure. Otherwise I still need to stuff a generic
function into .notifier_call and from there get the pointer to the array
to make the next call. So it adds more pointer indirection but does not
reduce the number of intermediate function calls.
There comes a point when trying to reuse existing code is less cost
effective than starting over.
As far as the multi-chain approach is concerned, I'm still leaning
towards registering a single function with a mask describing what it
wants to be notified of.
I think that will be less efficient than the function array.
2. A helper mechanism to allow a client that's being loaded as a module
to visit all existing tasks and do whatever initialization it needs to
do. Without this every client would have to implement such a mechanism
themselves (and it's not pretty).
Interesting idea. It should resemble existing macros. Something like:
register_task_watcher(&my_nb, &unnoticed);
for_each_unnoticed_task(unnoticed)
...
Something like that. It involved some tricky locking issues and was
reasonably complex (which made providing it a good option when compared
to each client implementing its own version). Rather than trying to do
this from scratch I'd advise getting a copy of the most recent PAGG
patches and using that as a model as a fair bit of effort was spent
ironing out all the problems involved. It's not as easy as it sounds.
Peter
--
Peter Williams pwil3058@xxxxxxxxxxxxxx
"Learning, n. The kind of ignorance distinguishing the studious."
-- Ambrose Bierce
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