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Date:   Tue, 15 Jan 2019 08:40:02 +0800
From:   Changbin Du <changbin.du@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Changbin Du <changbin.du@...il.com>, linux@...linux.org.uk,
        x86@...nel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Matthew Wilcox <willy@...radead.org>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH] fgraph: record function return value

Hi Steven,
On Mon, Jan 14, 2019 at 11:21:15AM -0500, Steven Rostedt wrote:
> On Sat, 12 Jan 2019 14:57:01 +0800
> Changbin Du <changbin.du@...il.com> 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@...il.com>
> > ---
> >
> 
> 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.
> 
I cann't wait to see it! I can rebase my cheanges after your work. Thanks!

> Thanks!
> 
> -- Steve

-- 
Cheers,
Changbin Du

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