Commit 4df29712 authored by Steven Rostedt (Red Hat)'s avatar Steven Rostedt (Red Hat) Committed by Steven Rostedt

tracing: Remove most or all of stack tracer stack size from stack_max_size

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

 # 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: default avatarSteven Rostedt <>
parent d4ecbfc4
......@@ -20,27 +20,24 @@
* 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.
# define add_func 1
# define fentry 1
# define add_func 0
# define fentry 0
static unsigned long stack_dump_trace[STACK_TRACE_ENTRIES+1] =
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 long *stack)
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)
......@@ -73,6 +74,10 @@ check_stack(unsigned long ip, unsigned long *stack)
/* 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 long *stack)
* 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;
stack_dump_trace[0] = ip;
* Now find where in the stack these are.
......@@ -123,6 +125,18 @@ check_stack(unsigned long ip, unsigned long *stack)
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, unsigned long parent_ip,
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;
check_stack(ip, &stack);
per_cpu(trace_active, cpu)--;
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