Re: [PATCH] tracing/fgraph: support recording function return values

From: Will Deacon <will@kernel.org>
To: Changbin Du <changbin.du@gmail.com>
Cc: rostedt@goodmis.org, mingo@redhat.com, corbet@lwn.net,
	linux@armlinux.org.uk, catalin.marinas@arm.com,
	tglx@linutronix.de, bp@alien8.de, hpa@zytor.com, x86@kernel.org,
	linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] tracing/fgraph: support recording function return values
Date: Mon, 15 Jul 2019 09:29:30 +0100	[thread overview]
Message-ID: <20190715082930.uyxn2kklgw4yri5l@willie-the-truck> (raw)
In-Reply-To: <20190713121026.11030-1-changbin.du@gmail.com>

On Sat, Jul 13, 2019 at 08:10:26PM +0800, Changbin Du 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 the 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.

This seems like quite a significant drawback and I think it could be pretty
confusing if you have to filter out bogus return values from the trace.

For example, in your snippet:

>  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 */

mutex_unlock() is wrongly listed as returning something.

How much of this could be achieved from userspace by placing kretprobes on
non-void functions instead?

Will