Re: [PATCH] fgraph: record function return value

[Steven Rostedt]

On Sat, 12 Jan 2019 14:57:01 +0800
Changbin Du <changbin.du@xxxxxxxxx> wrote:

> This patch adds a new trace option 'funcgraph-retval' and is disabled by
> default. When this option is enabled, fgraph tracer will show the return
> value of each function. This is useful to find/analyze a original error
> source in a call graph.
> 
> One limitation is that kernel doesn't know the prototype of functions. So
> fgraph assumes all functions have a retvalue of type int. You must ignore
> the value of *void* function. And if the retvalue looks like an error code
> then both hexadecimal and decimal number are displayed.
> 
> In this patch, only x86 and ARM platforms are supported.
> 
> Here is example showing the error is caused by vmx_create_vcpu() and the
> error code is -5 (-EIO).
> 
> with echo 1 > /sys/kernel/debug/tracing/options/funcgraph-retval
> 
>  3)               |  kvm_vm_ioctl() {
>  3)               |    mutex_lock() {
>  3)               |      _cond_resched() {
>  3)   0.234 us    |        rcu_all_qs(); /* ret=0x80000000 */
>  3)   0.704 us    |      } /* ret=0x0 */
>  3)   1.226 us    |    } /* ret=0x0 */
>  3)   0.247 us    |    mutex_unlock(); /* ret=0xffff8880738ed040 */
>  3)               |    kvm_arch_vcpu_create() {
>  3)               |      vmx_create_vcpu() {
>  3) + 17.969 us   |        kmem_cache_alloc(); /* ret=0xffff88813a980040 */
>  3) + 15.948 us   |        kmem_cache_alloc(); /* ret=0xffff88813aa99200 */
>  3)   0.653 us    |        allocate_vpid.part.88(); /* ret=0x1 */
>  3)   6.964 us    |        kvm_vcpu_init(); /* ret=0xfffffffb */
>  3)   0.323 us    |        free_vpid.part.89(); /* ret=0x1 */
>  3)   9.985 us    |        kmem_cache_free(); /* ret=0x80000000 */
>  3)   9.491 us    |        kmem_cache_free(); /* ret=0x80000000 */
>  3) + 69.858 us   |      } /* ret=0xfffffffffffffffb/-5 */
>  3) + 70.631 us   |    } /* ret=0xfffffffffffffffb/-5 */
>  3)               |    mutex_lock() {
>  3)               |      _cond_resched() {
>  3)   0.199 us    |        rcu_all_qs(); /* ret=0x80000000 */
>  3)   0.594 us    |      } /* ret=0x0 */
>  3)   1.067 us    |    } /* ret=0x0 */
>  3)   0.337 us    |    mutex_unlock(); /* ret=0xffff8880738ed040 */
>  3) + 92.730 us   |  } /* ret=0xfffffffffffffffb/-5 */
> 
> Signed-off-by: Changbin Du <changbin.du@xxxxxxxxx>
> ---
>

Hi Changbin,

I'm rewriting a lot of the function graph tracer code to have
kretprobes be able to work on top of it. It's still a work in progress.
It would be easier to add something to that work when its done than to
do it now.

Thanks!

-- Steve