Re: [PATCH v5 1/2] function_graph: Support recording and printing the return value of function

[Mark Rutland]

On Mon, Mar 20, 2023 at 06:16:49AM -0700, Donglin Peng wrote:
> When using the function_graph tracer to analyze system call failures,
> it can be time-consuming to analyze the trace logs and locate the kernel
> function that first returns an error. This change aims to simplify the
> process by recording the function return value to the 'retval' member of
> 'ftrace_graph_ent' and printing it when outputing the trace log.
> 
> New trace options are introduced: funcgraph-retval and graph_retval_hex.
> The former is used to control whether to display the return value, while
> the latter is used to control the display format of the reutrn value.
> 
> Note that even if a function's return type is void, a return value will
> still be printed, so just ignore it.
> 
> Currently, this modification supports the following commonly used
> processor architectures: x86_64, x86, arm64, arm, riscv.
> 
> Here is an example:
> 
> I want to attach the demo process to a cpu cgroup, but it failed:
> 
> echo `pidof demo` > /sys/fs/cgroup/cpu/test/tasks
> -bash: echo: write error: Invalid argument
> 
> The strace logs tells that the write system call returned -EINVAL(-22):
> ...
> write(1, "273\n", 4)                    = -1 EINVAL (Invalid argument)
> ...
> 
> Use the following commands to capture trace logs when calling the write
> system call:
> 
> cd /sys/kernel/debug/tracing/
> echo 0 > tracing_on
> echo > trace
> echo *sys_write > set_graph_function
> echo *spin* > set_graph_notrace
> echo *rcu* >> set_graph_notrace
> echo *alloc* >> set_graph_notrace
> echo preempt* >> set_graph_notrace
> echo kfree* >> set_graph_notrace
> echo $$ > set_ftrace_pid
> echo function_graph > current_tracer
> echo 1 > tracing_on
> echo `pidof demo` > /sys/fs/cgroup/cpu/test/tasks
> echo 0 > tracing_on
> echo 1 > options/funcgraph-retval
> cat trace > ~/trace.log
> 
> Search the error code -22 directly in the file trace.log and find the
> first function that return -22, then read the function code to get the
> root cause.
> 
> ...
> 
>  1)          | cgroup_migrate() {
>  1) 0.651 us |   cgroup_migrate_add_task(); /* = 0xffff93fcfd346c00 */
>  1)          |   cgroup_migrate_execute() {
>  1)          |     cpu_cgroup_can_attach() {
>  1)          |       cgroup_taskset_first() {
>  1) 0.732 us |         cgroup_taskset_next(); /* = 0xffff93fc8fb20000 */
>  1) 1.232 us |       } /* cgroup_taskset_first = 0xffff93fc8fb20000 */
>  1) 0.380 us |       sched_rt_can_attach(); /* = 0x0 */
>  1) 2.335 us |     } /* cpu_cgroup_can_attach = -22 */
>  1) 4.369 us |   } /* cgroup_migrate_execute = -22 */
>  1) 7.143 us | } /* cgroup_migrate = -22 */
> 
> ...
> 
> Signed-off-by: Donglin Peng <pengdonglin@xxxxxxxxxxxxxx>

> diff --git a/arch/arm64/kernel/entry-ftrace.S b/arch/arm64/kernel/entry-ftrace.S
> index 350ed81324ac..d1a5d76e6d72 100644
> --- a/arch/arm64/kernel/entry-ftrace.S
> +++ b/arch/arm64/kernel/entry-ftrace.S
> @@ -276,7 +276,15 @@ SYM_CODE_START(return_to_handler)
>  	stp x4, x5, [sp, #32]
>  	stp x6, x7, [sp, #48]
>  
> +#ifdef CONFIG_FUNCTION_GRAPH_RETVAL
> +	/*
> +	 * Pass both the function return values in the register x0 and x1
> +	 * to ftrace_return_to_handler.
> +	 */
> +	mov	x2, x29			//     parent's fp
> +#else
>  	mov	x0, x29			//     parent's fp
> +#endif
>  	bl	ftrace_return_to_handler// addr = ftrace_return_to_hander(fp);
>  	mov	x30, x0			// restore the original return address


Please don't make the calling convention of the asm change depending on a
selectable config option.

We already store the regs here; I'd be happy to make that a struct ftrace_regs
and pass a pointer to that to C code. Then it's be easy to acquire the value
you want in the exact same way as upon entry, and it'll work even if we decide
to return a structure by value somewhere (as that can use registers x2-x7 too).

Thanks,
Mark.