Re: [RFC][PATCH 11/14] function_graph: Convert ret_stack to a series of longs

[Joel Fernandes]

On Wed, Nov 21, 2018 at 08:27:19PM -0500, Steven Rostedt wrote:
> From: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx>
> 
> In order to make it possible to have multiple callbacks registered with the
> function_graph tracer, the retstack needs to be converted from an array of
> ftrace_ret_stack structures to an array of longs. This will allow to store
> the list of callbacks on the stack for the return side of the functions.
> 
> [ Note, this currently breaks architectures that access the ret_stack of a
>   task to handle unwinding when 'return_to_handler' is on the stack ]
> 
> Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
> ---
>  include/linux/sched.h |   2 +-
>  kernel/trace/fgraph.c | 123 +++++++++++++++++++++++-------------------
>  2 files changed, 70 insertions(+), 55 deletions(-)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index d6183a55e8eb..71a084a300da 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1119,7 +1119,7 @@ struct task_struct {
>  	int				curr_ret_depth;
>  
>  	/* Stack of return addresses for return function tracing: */
> -	struct ftrace_ret_stack		*ret_stack;
> +	unsigned long			*ret_stack;
>  
>  	/* Timestamp for last schedule: */
>  	unsigned long long		ftrace_timestamp;
> diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
> index 9b85638ecded..1389fe39f64c 100644
> --- a/kernel/trace/fgraph.c
> +++ b/kernel/trace/fgraph.c
> @@ -23,6 +23,17 @@
>  #define ASSIGN_OPS_HASH(opsname, val)
>  #endif
>  
> +#define FGRAPH_RET_SIZE (sizeof(struct ftrace_ret_stack))
> +#define FGRAPH_RET_INDEX (ALIGN(FGRAPH_RET_SIZE, sizeof(long)) / sizeof(long))
> +#define SHADOW_STACK_SIZE (FTRACE_RETFUNC_DEPTH * FGRAPH_RET_SIZE)
> +#define SHADOW_STACK_INDEX			\
> +	(ALIGN(SHADOW_STACK_SIZE, sizeof(long)) / sizeof(long))
> +#define SHADOW_STACK_MAX_INDEX (SHADOW_STACK_INDEX - FGRAPH_RET_INDEX)
> +
> +#define RET_STACK(t, index) ((struct ftrace_ret_stack *)(&(t)->ret_stack[index]))
> +#define RET_STACK_INC(c) ({ c += FGRAPH_RET_INDEX; })
> +#define RET_STACK_DEC(c) ({ c -= FGRAPH_RET_INDEX; })
> +
[...]
> @@ -514,7 +531,7 @@ void ftrace_graph_init_task(struct task_struct *t)
>  
>  void ftrace_graph_exit_task(struct task_struct *t)
>  {
> -	struct ftrace_ret_stack	*ret_stack = t->ret_stack;
> +	unsigned long *ret_stack = t->ret_stack;
>  
>  	t->ret_stack = NULL;
>  	/* NULL must become visible to IRQs before we free it: */
> @@ -526,12 +543,10 @@ void ftrace_graph_exit_task(struct task_struct *t)
>  /* Allocate a return stack for each task */
>  static int start_graph_tracing(void)
>  {
> -	struct ftrace_ret_stack **ret_stack_list;
> +	unsigned long **ret_stack_list;
>  	int ret, cpu;
>  
> -	ret_stack_list = kmalloc_array(FTRACE_RETSTACK_ALLOC_SIZE,
> -				       sizeof(struct ftrace_ret_stack *),
> -				       GFP_KERNEL);
> +	ret_stack_list = kmalloc(SHADOW_STACK_SIZE, GFP_KERNEL);
>  

I had dumped the fgraph size related macros to understand the patch better, I
got:
[    0.909528] val of FGRAPH_RET_SIZE is 40
[    0.910250] val of FGRAPH_RET_INDEX is 5
[    0.910866] val of FGRAPH_ARRAY_SIZE is 16
[    0.911488] val of FGRAPH_ARRAY_MASK is 255
[    0.912134] val of FGRAPH_MAX_INDEX is 16
[    0.912751] val of FGRAPH_INDEX_SHIFT is 8
[    0.913382] val of FGRAPH_FRAME_SIZE is 168
[    0.914033] val of FGRAPH_FRAME_INDEX is 21
                      FTRACE_RETFUNC_DEPTH is 50
[    0.914686] val of SHADOW_STACK_SIZE is 8400

I had a concern about memory overhead per-task. It seems the total memory
needed per task for the stack is 8400 bytes (with my configuration with
FUNCTION_PROFILE
turned off).

Where as before it would be 32 * 40 = 1280 bytes. That looks like ~7 times
more than before.

On my system with ~4000 threads, that becomes ~32MB which seems a bit
wasteful especially if there was only one or 2 function graph callbacks
registered and most of the callback array in the stack isn't used.

Could we make the array size configurable at compile time and start it with a
small number like 4 or 6?

Also for patches 1 through 10:
Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>

thanks,

 - Joel