[ 040/104] tracing: Remove most or all of stack tracer stack size from stack_max_size

Greg Kroah-Hartman
2013-05-06T23:32:27Z

From: "Steven Rostedt (Red Hat)" <rostedt@goodmis.org>

commit 4df297129f622bdc18935c856f42b9ddd18f9f28 upstream.

Currently, the depth reported in the stack tracer stack_trace file
does not match the stack_max_size file. This is because the stack_max_size
includes the overhead of stack tracer itself while the depth does not.

The first time a max is triggered, a calculation is not performed that
figures out the overhead of the stack tracer and subtracts it from
the stack_max_size variable. The overhead is stored and is subtracted
from the reported stack size for comparing for a new max.

Now the stack_max_size corresponds to the reported depth:

# cat stack_max_size
4640

# cat stack_trace
Depth Size Location (48 entries)
----- ---- --------
0) 4640 32 _raw_spin_lock+0x18/0x24
1) 4608 112 ____cache_alloc+0xb7/0x22d
2) 4496 80 kmem_cache_alloc+0x63/0x12f
3) 4416 16 mempool_alloc_slab+0x15/0x17
[...]

While testing against and older gcc on x86 that uses mcount instead
of fentry, I found that pasing in ip + MCOUNT_INSN_SIZE let the
stack trace show one more function deep which was missing before.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
kernel/trace/trace_stack.c | 75 ++++++++++++++++++++++++++++++++-------------
1 file changed, 54 insertions(+), 21 deletions(-)

--- a/kernel/trace/trace_stack.c
+++ b/kernel/trace/trace_stack.c
@@ -20,27 +20,24 @@

#define STACK_TRACE_ENTRIES 500

-/*
- * If fentry is used, then the function being traced will
- * jump to fentry directly before it sets up its stack frame.
- * We need to ignore that one and record the parent. Since
- * the stack frame for the traced function wasn't set up yet,
- * the stack_trace wont see the parent. That needs to be added
- * manually to stack_dump_trace[] as the first element.
- */
#ifdef CC_USING_FENTRY
-# define add_func 1
+# define fentry 1
#else
-# define add_func 0
+# define fentry 0
#endif

static unsigned long stack_dump_trace[STACK_TRACE_ENTRIES+1] =
{ [0 ... (STACK_TRACE_ENTRIES)] = ULONG_MAX };
static unsigned stack_dump_index[STACK_TRACE_ENTRIES];

+/*
+ * Reserve one entry for the passed in ip. This will allow
+ * us to remove most or all of the stack size overhead
+ * added by the stack tracer itself.
+ */
static struct stack_trace max_stack_trace = {
- .max_entries = STACK_TRACE_ENTRIES - add_func,
- .entries = &stack_dump_trace[add_func],
+ .max_entries = STACK_TRACE_ENTRIES - 1,
+ .entries = &stack_dump_trace[1],
};

static unsigned long max_stack_size;
@@ -58,10 +55,14 @@ check_stack(unsigned long ip, unsigned l
{
unsigned long this_size, flags;
unsigned long *p, *top, *start;
+ static int tracer_frame;
+ int frame_size = ACCESS_ONCE(tracer_frame);
int i;

this_size = ((unsigned long)stack) & (THREAD_SIZE-1);
this_size = THREAD_SIZE - this_size;
+ /* Remove the frame of the tracer */
+ this_size -= frame_size;

if (this_size <= max_stack_size)
return;
@@ -73,6 +74,10 @@ check_stack(unsigned long ip, unsigned l
local_irq_save(flags);
arch_spin_lock(&max_stack_lock);

+ /* In case another CPU set the tracer_frame on us */
+ if (unlikely(!frame_size))
+ this_size -= tracer_frame;
+
/* a race could have already updated it */
if (this_size <= max_stack_size)
goto out;
@@ -85,15 +90,12 @@ check_stack(unsigned long ip, unsigned l
save_stack_trace(&max_stack_trace);

/*
- * When fentry is used, the traced function does not get
- * its stack frame set up, and we lose the parent.
- * Add that one in manally. We set up save_stack_trace()
- * to not touch the first element in this case.
+ * Add the passed in ip from the function tracer.
+ * Searching for this on the stack will skip over
+ * most of the overhead from the stack tracer itself.
*/
- if (add_func) {
- stack_dump_trace[0] = ip;
- max_stack_trace.nr_entries++;
- }
+ stack_dump_trace[0] = ip;
+ max_stack_trace.nr_entries++;

/*
* Now find where in the stack these are.
@@ -123,6 +125,18 @@ check_stack(unsigned long ip, unsigned l
found = 1;
/* Start the search from here */
start = p + 1;
+ /*
+ * We do not want to show the overhead
+ * of the stack tracer stack in the
+ * max stack. If we haven't figured
+ * out what that is, then figure it out
+ * now.
+ */
+ if (unlikely(!tracer_frame) && i == 1) {
+ tracer_frame = (p - stack) *
+ sizeof(unsigned long);
+ max_stack_size -= tracer_frame;
+ }
}
}

@@ -149,7 +163,26 @@ stack_trace_call(unsigned long ip, unsig
if (per_cpu(trace_active, cpu)++ != 0)
goto out;

- check_stack(parent_ip, &stack);
+ /*
+ * When fentry is used, the traced function does not get
+ * its stack frame set up, and we lose the parent.
+ * The ip is pretty useless because the function tracer
+ * was called before that function set up its stack frame.
+ * In this case, we use the parent ip.
+ *
+ * By adding the return address of either the parent ip
+ * or the current ip we can disregard most of the stack usage
+ * caused by the stack tracer itself.
+ *
+ * The function tracer always reports the address of where the
+ * mcount call was, but the stack will hold the return address.
+ */
+ if (fentry)
+ ip = parent_ip;
+ else
+ ip += MCOUNT_INSN_SIZE;
+
+ check_stack(ip, &stack);

out:
per_cpu(trace_active, cpu)--;