Re: [PATCH,RFC] perf: panic due to inclied cpu context task_ctxvalue

[Paul E. McKenney]

On Wed, Mar 30, 2011 at 06:37:30PM +0200, Oleg Nesterov wrote:
> On 03/30, Peter Zijlstra wrote:
> >
> > --- linux-2.6.orig/kernel/perf_event.c
> > +++ linux-2.6/kernel/perf_event.c
> > @@ -125,9 +125,25 @@ enum event_type_t {
> >   * perf_sched_events : >0 events exist
> >   * perf_cgroup_events: >0 per-cpu cgroup events exist on this cpu
> >   */
> > -atomic_t perf_sched_events __read_mostly;
> > +atomic_t perf_sched_events_in __read_mostly;
> > +atomic_t perf_sched_events_out __read_mostly;
> >  static DEFINE_PER_CPU(atomic_t, perf_cgroup_events);
> >
> > +static void perf_sched_events_inc(void)
> > +{
> > +	jump_label_inc(&perf_sched_events_out);
> > +	jump_label_inc(&perf_sched_events_in);
> > +}
> > +
> > +static void perf_sched_events_dec(void)
> > +{
> > +	jump_label_dec(&perf_sched_events_in);
> > +	JUMP_LABEL(&perf_sched_events_in, no_sync);
> > +	synchronize_sched();
> > +no_sync:
> > +	jump_label_dec(&perf_sched_events_out);
> > +}
> 
> Nice! I didn't realize we can simply use JUMP_LABEL() directly and then
> the code doesn't depend on HAVE_JUMP_LABEL.
> 
> Now, the problem is, after I read the comments I am not sure I understand
> what synchronize_sched() actually doe. Add Paul.
> 
> So. synchronize_sched() above should ensure that all CPUs do context
> switch at least once (ignoring idle). And I _thought_ that in practice
> this should work.
> 
> But, unles I misread the comment above synchronize_sched(), it seems that
> it only guarantees the end of "everything" which disables preemption,
> explicitly or not. IOW, say, in theory rcu_read_unlock_sched() could
> trigger ->passed_quiesc == T without reschedule.

For rcu_read_lock() in preemptible RCU, this is true.  But for
rcu_read_unlock_sched(), the only way rcu_note_context_switch() is called
is if the code is preempted, which ends up calling schedule().

> Oh, and this is not theoretical, afaics. run_ksoftirqd() does
> rcu_note_context_switch().

Interesting...  Color me confused.

Suppose the rcu_note_context_switch() in run_ksoftirqd() was replaced
with schedule().  This has to be OK, right?  But schedule() itself
invokes rcu_note_context_switch().  So if it is OK to call schedule(),
it should be OK to call rcu_note_context_switch() directly, right?

So, what am I missing here?

> So, I think we need something else :/

The thing that I would be more concerned about is the idle loop.
If a CPU is in the idle loop, then rcu_sched_qs() will be invoked
(and which is invoked by rcu_note_context_switch()).  So is it
illegal to use the above in the idle loop?

BTW, if it turns out that the idle loop is a problem, I could put
an explicit call to rcu_sched_qs() in the affected idle loops.
But currently anything in an idle thread is a quiescent state.

						Thanx, Paul
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