From: Will Deacon <will@kernel.org>
To: Changbin Du <changbin.du@gmail.com>
Cc: corbet@lwn.net, catalin.marinas@arm.com, x86@kernel.org,
linux-doc@vger.kernel.org, linux@armlinux.org.uk,
rostedt@goodmis.org, linux-kernel@vger.kernel.org,
mingo@redhat.com, bp@alien8.de, hpa@zytor.com,
tglx@linutronix.de, 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
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next prev parent reply other threads:[~2019-07-15 8:29 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-13 12:10 [PATCH] tracing/fgraph: support recording function return values Changbin Du
2019-07-15 8:29 ` Will Deacon [this message]
2019-07-15 10:12 ` Peter Zijlstra
2019-07-16 14:08 ` Changbin Du
2019-07-16 14:20 ` Peter Zijlstra
2019-07-16 15:24 ` Steven Rostedt
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