4.4-stable review patch. If anyone has any objections, please let me know.
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[ Upstream commit be54f69c26193de31053190761e521903b89d098 ]
# echo 1 > options/stacktrace # echo 1 > events/sched/sched_switch/enable # cat trace <idle>-0 [002] d..2 1982.525169: <stack trace> => save_stack_trace => __ftrace_trace_stack => trace_buffer_unlock_commit_regs => event_trigger_unlock_commit => trace_event_buffer_commit => trace_event_raw_event_sched_switch => __schedule => schedule => schedule_preempt_disabled => cpu_startup_entry => start_secondary
The above shows that we are seeing 6 functions before ever making it to the caller of the sched_switch event.
# echo stacktrace > events/sched/sched_switch/trigger # cat trace <idle>-0 [002] d..3 2146.335208: <stack trace> => trace_event_buffer_commit => trace_event_raw_event_sched_switch => __schedule => schedule => schedule_preempt_disabled => cpu_startup_entry => start_secondary
The stacktrace trigger isn't as bad, because it adds its own skip to the stacktracing, but still has two events extra.
One issue is that if the stacktrace passes its own "regs" then there should be no addition to the skip, as the regs will not include the functions being called. This was an issue that was fixed by commit 7717c6be6999 ("tracing: Fix stacktrace skip depth in trace_buffer_unlock_commit_regs()" as adding the skip number for kprobes made the probes not have any stack at all.
But since this is only an issue when regs is being used, a skip should be added if regs is NULL. Now we have:
# echo 1 > options/stacktrace # echo 1 > events/sched/sched_switch/enable # cat trace <idle>-0 [000] d..2 1297.676333: <stack trace> => __schedule => schedule => schedule_preempt_disabled => cpu_startup_entry => rest_init => start_kernel => x86_64_start_reservations => x86_64_start_kernel
# echo stacktrace > events/sched/sched_switch/trigger # cat trace <idle>-0 [002] d..3 1370.759745: <stack trace> => __schedule => schedule => schedule_preempt_disabled => cpu_startup_entry => start_secondary
And kprobes are not touched.
Reported-by: Peter Zijlstra peterz@infradead.org Signed-off-by: Steven Rostedt rostedt@goodmis.org Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/trace/trace.c | 19 ++++++++++++++++++- 1 file changed, 18 insertions(+), 1 deletion(-)
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c index e409ddce8754..1a47a64d623f 100644 --- a/kernel/trace/trace.c +++ b/kernel/trace/trace.c @@ -1757,7 +1757,17 @@ void trace_buffer_unlock_commit_regs(struct trace_array *tr, { __buffer_unlock_commit(buffer, event);
- ftrace_trace_stack(tr, buffer, flags, 0, pc, regs); + /* + * If regs is not set, then skip the following callers: + * trace_buffer_unlock_commit_regs + * event_trigger_unlock_commit + * trace_event_buffer_commit + * trace_event_raw_event_sched_switch + * Note, we can still get here via blktrace, wakeup tracer + * and mmiotrace, but that's ok if they lose a function or + * two. They are that meaningful. + */ + ftrace_trace_stack(tr, buffer, flags, regs ? 0 : 4, pc, regs); ftrace_trace_userstack(buffer, flags, pc); } EXPORT_SYMBOL_GPL(trace_buffer_unlock_commit_regs); @@ -1815,6 +1825,13 @@ static void __ftrace_trace_stack(struct ring_buffer *buffer, trace.nr_entries = 0; trace.skip = skip;
+ /* + * Add two, for this function and the call to save_stack_trace() + * If regs is set, then these functions will not be in the way. + */ + if (!regs) + trace.skip += 2; + /* * Since events can happen in NMIs there's no safe way to * use the per cpu ftrace_stacks. We reserve it and if an interrupt