On Tue, Apr 17, 2007 at 10:26:31AM +0200, Ingo Molnar wrote:
>
> * Nick Piggin <[EMAIL PROTECTED]> wrote:
>
> > Actually I think this is something that makes sense to add, even if
> > just for debugging, but maybe also for production, depending on how
> > much it impacts things. Child runs first is an heuristic optimisation
> > that exploits a VM detail (however fundamental). But for things that
> > don't exec right after forking (and maybe some things that do), it can
> > be nicer to reduce context switches, improve cache patterns, and allow
> > children to be load balanced away before touching memory, if
> > child_runs_first is turned off.
>
> yeah, the primary intent was debug. Nick, am i confused to conclude that
> mainline in fact runs the _parent_ first, despite all the elaborate
> runqueue juggling we do there? This piece of code in wake_up_new_task()
> caught my eyes:
>
> p->prio = current->prio;
> p->normal_prio = current->normal_prio;
> list_add_tail(&p->run_list,
> ¤t->run_list);
> p->array = current->array;
> p->array->nr_active++;
> inc_nr_running(p, rq);
>
> shouldnt the list_add_tail() be list_add(), so that task pickup sees the
> child first? Maybe we still do child-runs-first in practice, due to the
> timeslice and sleep average fixups that happen if the parent preempts,
> but the above piece of code seems a quite elaborate way of doing
> activate_task(). To have the child _before_ the parent we'd need the
> add-on patch below. But ... i could be wrong, this is just a quick
> thought.
I think that it works because the list we're adding to is not the
normal runqueue list head, but the parent's list_head on that runqueue.
Which adds the child directly ahead of the parent... I think?
>
> Ingo
>
> ---
> kernel/sched.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> Index: linux/kernel/sched.c
> ===================================================================
> --- linux.orig/kernel/sched.c
> +++ linux/kernel/sched.c
> @@ -1685,7 +1685,7 @@ void fastcall wake_up_new_task(struct ta
> else {
> p->prio = current->prio;
> p->normal_prio = current->normal_prio;
> - list_add_tail(&p->run_list, ¤t->run_list);
> + list_add(&p->run_list, ¤t->run_list);
> p->array = current->array;
> p->array->nr_active++;
> inc_nr_running(p, rq);
-
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