Messages in this thread | | | Date | Fri, 21 May 2010 12:13:48 +0200 | From | Frederic Weisbecker <> | Subject | Re: [PATCH 02/10] perf, trace: Use per-tracepoint-per-cpu hlist to track events |
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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.
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