tracing/kprobes: Avoid perf_trace_buf_*() if ->perf_events is empty

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().

Link: http://lkml.kernel.org/r/20130620173806.GA13151@redhat.com

Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 8 insertions, 4 deletions

diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
index f2374172ba7b..c35bebe53ffe 100644
--- a/kernel/trace/trace_kprobe.c
+++ b/kernel/trace/trace_kprobe.c
@@ -1156,6 +1156,10 @@ 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));
@@ -1171,8 +1175,6 @@ 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);
 }
@@ -1188,6 +1190,10 @@ 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));
@@ -1203,8 +1209,6 @@ 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);
 }