Re: [PATCH RFC tip/core/rcu 1/9] rcu: Add call_rcu_tasks()

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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