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authorFrederic Weisbecker <fweisbec@gmail.com>2009-06-20 05:45:14 +0200
committerFrederic Weisbecker <fweisbec@gmail.com>2009-06-20 06:28:46 +0200
commitc85a17e22695969aa24a7ffa40cf26d6e6fcfd50 (patch)
parent44ad18e0a65e296b2e68a1452509f6222cdce743 (diff)
tracing/urgent: fix unbalanced ftrace_start_up
Perfcounter reports the following stats for a wide system profiling: # # (2364 samples) # # Overhead Symbol # ........ ...... # 15.40% [k] mwait_idle_with_hints 8.29% [k] read_hpet 5.75% [k] ftrace_caller 3.60% [k] ftrace_call [...] This snapshot has been taken while neither the function tracer nor the function graph tracer was running. With dynamic ftrace, such results show a wrong ftrace behaviour because all calls to ftrace_caller or ftrace_graph_caller (the patched calls to mcount) are supposed to be patched into nop if none of those tracers are running. The problem occurs after the first run of the function tracer. Once we launch it a second time, the callsites will never be nopped back, unless you set custom filters. For example it happens during the self tests at boot time. The function tracer selftest runs, and then the dynamic tracing is tested too. After that, the callsites are left un-nopped. This is because the reset callback of the function tracer tries to unregister two ftrace callbacks in once: the common function tracer and the function tracer with stack backtrace, regardless of which one is currently in use. It then creates an unbalance on ftrace_start_up value which is expected to be zero when the last ftrace callback is unregistered. When it reaches zero, the FTRACE_DISABLE_CALLS is set on the next ftrace command, triggering the patching into nop. But since it becomes unbalanced, ie becomes lower than zero, if the kernel functions are patched again (as in every further function tracer runs), they won't ever be nopped back. Note that ftrace_call and ftrace_graph_call are still patched back to ftrace_stub in the off case, but not the callers of ftrace_call and ftrace_graph_caller. It means that the tracing is well deactivated but we waste a useless call into every kernel function. This patch just unregisters the right ftrace_ops for the function tracer on its reset callback and ignores the other one which is not registered, fixing the unbalance. The problem also happens is .30 Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com> Cc: Steven Rostedt <rostedt@goodmis.org> Cc: stable@kernel.org
1 files changed, 5 insertions, 3 deletions
diff --git a/kernel/trace/trace_functions.c b/kernel/trace/trace_functions.c
index c9a0b7df44f..90f13476483 100644
--- a/kernel/trace/trace_functions.c
+++ b/kernel/trace/trace_functions.c
@@ -193,9 +193,11 @@ static void tracing_start_function_trace(void)
static void tracing_stop_function_trace(void)
ftrace_function_enabled = 0;
- /* OK if they are not registered */
- unregister_ftrace_function(&trace_stack_ops);
- unregister_ftrace_function(&trace_ops);
+ if (func_flags.val & TRACE_FUNC_OPT_STACK)
+ unregister_ftrace_function(&trace_stack_ops);
+ else
+ unregister_ftrace_function(&trace_ops);
static int func_set_flag(u32 old_flags, u32 bit, int set)