[PATCH v2 0/4] ftrace: Add function arguments to function tracers

Steven Rostedt
2024-12-23T20:14:50Z

These patches add support for printing function arguments in ftrace.

Example usage:

function tracer:

~# cd /sys/kernel/tracing/
~# echo icmp_rcv >set_ftrace_filter
~# echo function >current_tracer
~# echo 1 >options/func-args
~# ping -c 10 127.0.0.1
[..]
~# cat trace
[..]
ping-1277 [030] ..s1. 39.120939: icmp_rcv(skb=0xa0ecab00) <-ip_protocol_deliver_rcu
ping-1277 [030] ..s1. 39.120946: icmp_rcv(skb=0xa0ecac00) <-ip_protocol_deliver_rcu
ping-1277 [030] ..s1. 40.179724: icmp_rcv(skb=0xa0ecab00) <-ip_protocol_deliver_rcu
ping-1277 [030] ..s1. 40.179730: icmp_rcv(skb=0xa0ecac00) <-ip_protocol_deliver_rcu
ping-1277 [030] ..s1. 41.219700: icmp_rcv(skb=0xa0ecab00) <-ip_protocol_deliver_rcu
ping-1277 [030] ..s1. 41.219706: icmp_rcv(skb=0xa0ecac00) <-ip_protocol_deliver_rcu
ping-1277 [030] ..s1. 42.259717: icmp_rcv(skb=0xa0ecab00) <-ip_protocol_deliver_rcu
ping-1277 [030] ..s1. 42.259725: icmp_rcv(skb=0xa0ecac00) <-ip_protocol_deliver_rcu
ping-1277 [030] ..s1. 43.299735: icmp_rcv(skb=0xa0ecab00) <-ip_protocol_deliver_rcu
ping-1277 [030] ..s1. 43.299742: icmp_rcv(skb=0xa0ecac00) <-ip_protocol_deliver_rcu

function graph:

~# cd /sys/kernel/tracing
~# echo icmp_rcv >set_graph_function
~# echo function_graph >current_tracer
~# echo 1 >options/funcgraph-args

~# ping -c 1 127.0.0.1

~# cat trace

30) | icmp_rcv(skb=0xa0ecab00) {
30) | __skb_checksum_complete(skb=0xa0ecab00) {
30) | skb_checksum(skb=0xa0ecab00, offset=0, len=64, csum=0) {
30) | __skb_checksum(skb=0xa0ecab00, offset=0, len=64, csum=0, ops=0x232e0327a88) {
30) 0.418 us | csum_partial(buff=0xa0d20924, len=64, sum=0)
30) 0.985 us | }
30) 1.463 us | }
30) 2.039 us | }
[..]

This was last posted by Sven Schnelle here:

https://lore.kernel.org/all/20240904065908.1009086-1-svens@linux.ibm.com/

As Sven hasn't worked on it since, I decided to continue to push it
through. I'm keeping Sven as original author and added myself as
"Co-developed-by".

The main changes are:

- Made the kconfig option unconditional if all the dependencies are set.

- Not save ftrace_regs in the ring buffer, as that is an abstract
descriptor defined by the architectures and should remain opaque from
generic code. Instead, the args are read at the time they are recorded
(with the ftrace_regs passed to the callback function), and saved into
the ring buffer. Then the print function only takes an array of elements.

This could allow archs to retrieve arguments that are on the stack where
as, post processing ftrace_regs could cause undesirable results.

- Made the function and function graph entry events dynamically sized
to allow the arguments to be appended to the event in the ring buffer.
The print function only looks to see if the event saved in the ring
buffer is big enough to hold all the arguments defined by the new
FTRACE_REGS_MAX_ARGS macro and if so, it will assume there are arguments
there and print them. This also means user space will not break on
reading these events as arguments will simply be ignored.

- The printing of the arguments has some more data when things are not
processed by BPF. Any unsupported argument will have the type printed
out in the ring buffer.

- Also removed the spaces around the '=' as that's more in line to how
trace events show their fields.

- One new patch I added to convert function graph tracing over to using
args as soon as the user sets the option even if function graph tracing
is enabled. Function tracer did this already by default.

Steven Rostedt (1):
ftrace: Have funcgraph-args take affect during tracing

Sven Schnelle (3):
ftrace: Add print_function_args()
ftrace: Add support for function argument to graph tracer
ftrace: Add arguments to function tracer

----
include/linux/ftrace_regs.h | 5 +
kernel/trace/Kconfig | 12 +++
kernel/trace/trace.c | 12 ++-
kernel/trace/trace.h | 4 +-
kernel/trace/trace_entries.h | 12 ++-
kernel/trace/trace_functions.c | 46 ++++++++-
kernel/trace/trace_functions_graph.c | 174 ++++++++++++++++++++++++++++-------
kernel/trace/trace_irqsoff.c | 4 +-
kernel/trace/trace_output.c | 96 ++++++++++++++++++-
kernel/trace/trace_output.h | 9 ++
kernel/trace/trace_sched_wakeup.c | 4 +-
11 files changed, 324 insertions(+), 54 deletions(-)