On Tue, Jul 29, 2014 at 09:50:55AM +0200, Peter Zijlstra wrote:
> On Mon, Jul 28, 2014 at 03:56:12PM -0700, Paul E. McKenney wrote:
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index bc1638b33449..a0d2f3a03566 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -2762,6 +2762,7 @@ need_resched:
> > } else {
> > deactivate_task(rq, prev, DEQUEUE_SLEEP);
> > prev->on_rq = 0;
> > + rcu_note_voluntary_context_switch(prev);
> >
> > /*
> > * If a worker went to sleep, notify and ask workqueue
> > @@ -2828,6 +2829,7 @@ asmlinkage __visible void __sched schedule(void)
> > struct task_struct *tsk = current;
> >
> > sched_submit_work(tsk);
> > + rcu_note_voluntary_context_switch(tsk);
> > __schedule();
> > }
>
> Yeah, not entirely happy with that, you add two calls into one of the
> hotest paths of the kernel.
I did look into leveraging counters, but cannot remember why I decided
that this was a bad idea. I guess it is time to recheck...
The ->nvcsw field in the task_struct structure looks promising:
o Looks like it does in fact get incremented in __schedule() via
the switch_count pointer.
o Looks like it is unconditionally compiled in.
o There are no memory barriers, but a synchronize_sched()
should take care of that, given that this counter is
incremented with interrupts disabled.
So I should be able to snapshot the task_struct structure's ->nvcsw
field and avoid the added code in the fastpaths.
Seem plausible, or am I confused about the role of ->nvcsw?
> And I'm still not entirely sure why, your 0/x babbled something about
> trampolines, but I'm not sure I understand how those lead to this.
Steven Rostedt sent an email recently giving more detail. And of course
now I am having trouble finding it. Maybe he will take pity on us and
send along a pointer to it. ;-)
Thanx, Paul
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