Messages in this thread | | | Date | Tue, 29 Jul 2014 08:57:47 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH RFC tip/core/rcu 1/9] rcu: Add call_rcu_tasks() |
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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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