Re: [PATCH] perf/ftrace : Fix repetitious traces when specify a target task

[Peter Zijlstra]

On Tue, Oct 10, 2017 at 03:04:48PM +0200, Peter Zijlstra wrote:
> On Tue, Oct 10, 2017 at 01:33:21PM +0200, Peter Zijlstra wrote:
> > But now you've got me looking at 75e8387685f6, which also looks
> > completely insane.
> 
> The reason I insta stumbled on that patch is that it only addresses the
> ftrace situation and doesn't mention the other _5_ places that use this
> interface. It doesn't explain why those don't have the problem and if
> not, why their solution doesn't work for ftrace.
> 
> So all (well syscall and regular tracepoints, didn't check the others)
> avoid that problem by simply not registering multiple times at the
> tracepoint. Tracepoints use tp_event->perf_refcount and the syscall
> things use sys_perf_refcount_{enter,exit} for that.
> 
> Doing the same for function trace looks a little something like the
> below (after reverting 75e8387685f6)
> 
> Except the below doesn't compile because of
> kernel/trace/trace_event_filter.c, which is where I lost the plot.

OK, so that filter stuff was the entire reason for this trainwreck :/

Using ftrace_ops filters allows ftrace to patch less functions etc.. So
that requires an ftrace_ops per event. Still that then instantly
suggests we fix the whole hlist situation instead of making it worse.

See below; I now have 3 patches: revert, the below, kill
FTRACE_OPS_FL_PER_CPU.

How's this?

---
 kernel/trace/trace_event_perf.c |   68 +++++++++++++++++++++-------------------
 1 file changed, 36 insertions(+), 32 deletions(-)

--- a/kernel/trace/trace_event_perf.c
+++ b/kernel/trace/trace_event_perf.c
@@ -240,27 +240,31 @@ void perf_trace_destroy(struct perf_even
 int perf_trace_add(struct perf_event *p_event, int flags)
 {
 	struct trace_event_call *tp_event = p_event->tp_event;
-	struct hlist_head __percpu *pcpu_list;
-	struct hlist_head *list;
 
-	pcpu_list = tp_event->perf_events;
-	if (WARN_ON_ONCE(!pcpu_list))
-		return -EINVAL;
+	if (!tp_event->class->reg(tp_event, TRACE_REG_PERF_ADD, p_event)) {
+		struct hlist_head __percpu *pcpu_list;
+		struct hlist_head *list;
+
+		pcpu_list = tp_event->perf_events;
+		if (WARN_ON_ONCE(!pcpu_list))
+			return -EINVAL;
 
-	if (!(flags & PERF_EF_START))
-		p_event->hw.state = PERF_HES_STOPPED;
+		if (!(flags & PERF_EF_START))
+			p_event->hw.state = PERF_HES_STOPPED;
 
-	list = this_cpu_ptr(pcpu_list);
-	hlist_add_head_rcu(&p_event->hlist_entry, list);
+		list = this_cpu_ptr(pcpu_list);
+		hlist_add_head_rcu(&p_event->hlist_entry, list);
+	}
 
-	return tp_event->class->reg(tp_event, TRACE_REG_PERF_ADD, p_event);
+	return 0;
 }
 
 void perf_trace_del(struct perf_event *p_event, int flags)
 {
 	struct trace_event_call *tp_event = p_event->tp_event;
-	hlist_del_rcu(&p_event->hlist_entry);
-	tp_event->class->reg(tp_event, TRACE_REG_PERF_DEL, p_event);
+
+	if (!tp_event->class->reg(tp_event, TRACE_REG_PERF_DEL, p_event))
+		hlist_del_rcu(&p_event->hlist_entry);
 }
 
 void *perf_trace_buf_alloc(int size, struct pt_regs **regs, int *rctxp)
@@ -306,15 +310,19 @@ static void
 perf_ftrace_function_call(unsigned long ip, unsigned long parent_ip,
 			  struct ftrace_ops *ops, struct pt_regs *pt_regs)
 {
+	struct hlist_head head = HLIST_HEAD_INIT;
 	struct ftrace_entry *entry;
-	struct hlist_head *head;
+	struct perf_event *event;
 	struct pt_regs regs;
 	int rctx;
 
-	head = this_cpu_ptr(event_function.perf_events);
-	if (hlist_empty(head))
+	event = container_of(ops, struct perf_event, ftrace_ops);
+
+	if (!event->ftrace_ops.private)
 		return;
 
+	hlist_add_head(&event->hlist_entry, &head);
+
 #define ENTRY_SIZE (ALIGN(sizeof(struct ftrace_entry) + sizeof(u32), \
 		    sizeof(u64)) - sizeof(u32))
 
@@ -330,17 +338,21 @@ perf_ftrace_function_call(unsigned long
 	entry->ip = ip;
 	entry->parent_ip = parent_ip;
 	perf_trace_buf_submit(entry, ENTRY_SIZE, rctx, TRACE_FN,
-			      1, &regs, head, NULL);
+			      1, &regs, &head, NULL);
 
 #undef ENTRY_SIZE
+
+	hlist_del_init(&event->hlist_entry);
 }
 
 static int perf_ftrace_function_register(struct perf_event *event)
 {
 	struct ftrace_ops *ops = &event->ftrace_ops;
 
-	ops->flags |= FTRACE_OPS_FL_PER_CPU | FTRACE_OPS_FL_RCU;
-	ops->func = perf_ftrace_function_call;
+	ops->flags   = FTRACE_OPS_FL_RCU;
+	ops->func    = perf_ftrace_function_call;
+	ops->private = NULL;
+
 	return register_ftrace_function(ops);
 }
 
@@ -352,19 +364,11 @@ static int perf_ftrace_function_unregist
 	return ret;
 }
 
-static void perf_ftrace_function_enable(struct perf_event *event)
-{
-	ftrace_function_local_enable(&event->ftrace_ops);
-}
-
-static void perf_ftrace_function_disable(struct perf_event *event)
-{
-	ftrace_function_local_disable(&event->ftrace_ops);
-}
-
 int perf_ftrace_event_register(struct trace_event_call *call,
 			       enum trace_reg type, void *data)
 {
+	struct perf_event *event = data;
+
 	switch (type) {
 	case TRACE_REG_REGISTER:
 	case TRACE_REG_UNREGISTER:
@@ -377,11 +381,11 @@ int perf_ftrace_event_register(struct tr
 	case TRACE_REG_PERF_CLOSE:
 		return perf_ftrace_function_unregister(data);
 	case TRACE_REG_PERF_ADD:
-		perf_ftrace_function_enable(data);
-		return 0;
+		event->ftrace_ops.private = (void *)1UL;
+		return 1;
 	case TRACE_REG_PERF_DEL:
-		perf_ftrace_function_disable(data);
-		return 0;
+		event->ftrace_ops.private = (void *)0UL;
+		return 1;
 	}
 
 	return -EINVAL;