[PATCH 1/3] tracing/kprobes: Avoid perf_trace_buf_*() if ->perf_events is empty

[Oleg Nesterov <oleg@redhat.com>]

perf_trace_buf_prepare() + perf_trace_buf_submit() make no sense
if this task/CPU has no active counters. Change kprobe_perf_func()
and kretprobe_perf_func() to check call->perf_events beforehand
and return if this list is empty.

For example, "perf record -e some_probe -p1". Only /sbin/init will
report, all other threads which hit the same probe will do
perf_trace_buf_prepare/perf_trace_buf_submit just to realize that
nobody wants perf_swevent_event().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/trace/trace_kprobe.c |   12 ++++++++----
 1 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
index 9f46e98..c0af476 100644
--- a/kernel/trace/trace_kprobe.c
+++ b/kernel/trace/trace_kprobe.c
@@ -1157,6 +1157,10 @@ kprobe_perf_func(struct trace_probe *tp, struct pt_regs *regs)
 	int size, __size, dsize;
 	int rctx;
 
+	head = this_cpu_ptr(call->perf_events);
+	if (hlist_empty(head))
+		return;
+
 	dsize = __get_data_size(tp, regs);
 	__size = sizeof(*entry) + tp->size + dsize;
 	size = ALIGN(__size + sizeof(u32), sizeof(u64));
@@ -1172,8 +1176,6 @@ kprobe_perf_func(struct trace_probe *tp, struct pt_regs *regs)
 	entry->ip = (unsigned long)tp->rp.kp.addr;
 	memset(&entry[1], 0, dsize);
 	store_trace_args(sizeof(*entry), tp, regs, (u8 *)&entry[1], dsize);
-
-	head = this_cpu_ptr(call->perf_events);
 	perf_trace_buf_submit(entry, size, rctx,
 					entry->ip, 1, regs, head, NULL);
 }
@@ -1189,6 +1191,10 @@ kretprobe_perf_func(struct trace_probe *tp, struct kretprobe_instance *ri,
 	int size, __size, dsize;
 	int rctx;
 
+	head = this_cpu_ptr(call->perf_events);
+	if (hlist_empty(head))
+		return;
+
 	dsize = __get_data_size(tp, regs);
 	__size = sizeof(*entry) + tp->size + dsize;
 	size = ALIGN(__size + sizeof(u32), sizeof(u64));
@@ -1204,8 +1210,6 @@ kretprobe_perf_func(struct trace_probe *tp, struct kretprobe_instance *ri,
 	entry->func = (unsigned long)tp->rp.kp.addr;
 	entry->ret_ip = (unsigned long)ri->ret_addr;
 	store_trace_args(sizeof(*entry), tp, regs, (u8 *)&entry[1], dsize);
-
-	head = this_cpu_ptr(call->perf_events);
 	perf_trace_buf_submit(entry, size, rctx,
 					entry->ret_ip, 1, regs, head, NULL);
 }
-- 
1.5.5.1