Re: [PATCH 02/10] perf, trace: Use per-tracepoint-per-cpu hlist to track events

Frederic Weisbecker
2010-05-21T10:13:43Z

On Fri, May 21, 2010 at 12:02:05PM +0200, Peter Zijlstra wrote:
> On Fri, 2010-05-21 at 11:40 +0200, Frederic Weisbecker wrote:
> > On Fri, May 21, 2010 at 11:02:03AM +0200, Peter Zijlstra wrote:
>
> > > Also, avoid conditionals on the fast path by ordering with probe unregister
> > > so that we should never get on the callback path without the data being there.
> > >
> > \
> > > + head = per_cpu_ptr(event_call->perf_events, smp_processor_id());\
>
> > Should be rcu_dereference_sched ?
>
> No, I removed all that rcu stuff and synchronized against the probe
> unregister.
>
> I assumed that after probe unregister a tracepoint callback doesn't
> happen, which then guarantees we should never get !head.

I'm not sure about this. The tracepoints are called under rcu_read_lock(),
but there is not synchronize_rcu() after we unregister a tracepoint, which
means you can have a pending preempted one somewhere.

There is a call_rcu that removes the callbacks, but that only protect
the callback themselves.

>
> > > + for_each_possible_cpu(cpu)
> > > + INIT_HLIST_HEAD(per_cpu_ptr(list, cpu));
> > > +
> > > + tp_event->perf_events = list;
> >
> >
> >
> > I suspect this must be rcu_assign_pointer.
>
> Same thing as above, I do this before probe register, so I see no need
> for RCU.
>
> > > + list = per_cpu_ptr(list, smp_processor_id());
> > > + hlist_add_head_rcu(&p_event->hlist_entry, list);
> >
> >
> >
> > Ah and may be small comment, because using the hlist api here
> > may puzzle more people than just me ;)
>
> What exactly is the puzzlement about?

The fact we use the hlist API not for hlist purpose but for a list.

> > > + if (--tp_event->perf_refcount > 0)
> > > + return;
> > > +
> > > + tp_event->perf_event_disable(tp_event);
> >
> >
> >
> > Don't we need a rcu_synchronize_sched() here?
>
> Doesn't probe unregister synchronize things against its own callback?

May be I missed it but it doesn't seem so.

> > > + raw_data = per_cpu_ptr(perf_trace_buf[*rctxp], smp_processor_id());
> >
> >
> >
> > Needs rcu_dereference_sched too. And this could be __this_cpu_var()
>
> Ahh! so that is what its called.

:)

> > > + preempt_disable_notrace();
> >
> >
> > Why is this needed. We have the recursion context protection already.
>
> Because:
>
> @@ -4094,7 +4087,7 @@ end:
>
> int perf_swevent_get_recursion_context(void)
> {
> - struct perf_cpu_context *cpuctx = &get_cpu_var(perf_cpu_context);
> + struct perf_cpu_context *cpuctx = &__get_cpu_var(perf_cpu_context);
> int rctx;
>
> if (in_nmi())
>

Right.

Thanks.