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

perf_trace_buf_prepare() + perf_trace_buf_submit(head, task => NULL)
make no sense if hlist_empty(head). Change perf_syscall_enter/exit()
to check sys_data->{enter,exit}_event->perf_events beforehand.

Link: http://lkml.kernel.org/r/20130617170207.GA19806@redhat.com
Acked-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

diff --git a/kernel/trace/trace_syscalls.c b/kernel/trace/trace_syscalls.c
index 0611562157210ea09b760fe14b9f18f038a35944..ac0085777fbd39188702028c2b624c58d58e59cd 100644 (file)
--- a/kernel/trace/trace_syscalls.c
+++ b/kernel/trace/trace_syscalls.c
@@ -566,6 +566,10 @@ static void perf_syscall_enter(void *ignore, struct pt_regs *regs, long id)
        if (!sys_data)
                return;
 
+       head = this_cpu_ptr(sys_data->enter_event->perf_events);
+       if (hlist_empty(head))
+               return;
+
        /* get the size after alignment with the u32 buffer size field */
        size = sizeof(unsigned long) * sys_data->nb_args + sizeof(*rec);
        size = ALIGN(size + sizeof(u32), sizeof(u64));
@@ -583,8 +587,6 @@ static void perf_syscall_enter(void *ignore, struct pt_regs *regs, long id)
        rec->nr = syscall_nr;
        syscall_get_arguments(current, regs, 0, sys_data->nb_args,
                               (unsigned long *)&rec->args);
-
-       head = this_cpu_ptr(sys_data->enter_event->perf_events);
        perf_trace_buf_submit(rec, size, rctx, 0, 1, regs, head, NULL);
 }
 
@@ -642,6 +644,10 @@ static void perf_syscall_exit(void *ignore, struct pt_regs *regs, long ret)
        if (!sys_data)
                return;
 
+       head = this_cpu_ptr(sys_data->exit_event->perf_events);
+       if (hlist_empty(head))
+               return;
+
        /* We can probably do that at build time */
        size = ALIGN(sizeof(*rec) + sizeof(u32), sizeof(u64));
        size -= sizeof(u32);
@@ -661,8 +667,6 @@ static void perf_syscall_exit(void *ignore, struct pt_regs *regs, long ret)
 
        rec->nr = syscall_nr;
        rec->ret = syscall_get_return_value(current, regs);
-
-       head = this_cpu_ptr(sys_data->exit_event->perf_events);
        perf_trace_buf_submit(rec, size, rctx, 0, 1, regs, head, NULL);
 }