Re: [patch] kwaitd, 2.5.31-A1

From: Ingo Molnar (mingo@elte.hu)
Date: Tue Aug 13 2002 - 12:37:59 EST


On Tue, 13 Aug 2002, Andrew Morton wrote:

> > the reaping of the thread is thus not done by the parent (or init), but by
> > per-CPU [kwaitd] kernel threads. The exiting thread queues itself always
> > to the CPU-local kwaitd queue, to maintain locality of reference and cheap
> > switching to kwaitd.
>
> I'd be inclined to agree with hch on that. We have an awful lot of
> kernel threads nowadays, and keventd would fill the bill.

i agree. What i did was i looked at the kernel-source as-is, and there was
no mechanism to do this properly so i took a different approach. Could
anyone send me per-CPU keventd patches? The thread-exit path is pretty
timing-critical, the current global keventd spinlock does not work.

> > +void reap_thread(task_t *p)
> > +{
> > + kwait_t *kwait = kwait_threads + smp_processor_id();
> > +
>
> get_cpu() here?

this function is called under a spinlock currently, so there's no point in
doing that.

> > + if (p->parent == kwait->task) {
> > + printk("hm, %s(%d) reaped already.\n", p->comm, p->pid);
> > + return;
> > + }
> > + REMOVE_LINKS(p);
>
> This is called under read_lock(&tasklist_lock), but modifies the
> task list.

hm, i thought it was only under the write lock. Oh, i see, a rare strace
codepath does it. Fixed - thanks.

> > ...
> > +
> > + schedule();
> > + current->state = TASK_RUNNING;
>
> schedule() always returns in state TASK_RUNNING? [...]

right - fixed :)

> And a question: is it not possible to make the exitting task just go and
> reap itself? If it's a matter of freeing the stack pages we could just
> add the page to a per-cpu deferred freeing list and reap it in page
> reclaim.

well, yes - but i dislike deferred freeing lists, it's always a problem
where to free the RAM for real. It didnt really work for bhs, it didnt
really work for other items either. And we have this nice lazy-TLB
mechanism for kernel threads so it's really a non-issue to use them.

        Ingo

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