linux-kernel - [PATCH v2] perf trace: Fix missing handling of --call-graph dwarf

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Date:   Fri, 12 Jan 2018 11:03:55 +0100
From:   Thomas Richter <tmricht@...ux.vnet.ibm.com>
To:     linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        acme@...nel.org
Cc:     brueckner@...ux.vnet.ibm.com, schwidefsky@...ibm.com,
        heiko.carstens@...ibm.com,
        Thomas Richter <tmricht@...ux.vnet.ibm.com>
Subject: [PATCH v2] perf trace: Fix missing handling of --call-graph dwarf

On Intel platform when I execute test case
   probe libc's inet_pton & backtrace it with ping
I get this output which is correct:

[root@f27 perf]# ./perf trace --no-syscalls
		-e probe_libc:inet_pton/max-stack=3/ ping -6 -c 1 ::1
PING ::1(::1) 56 data bytes
64 bytes from ::1: icmp_seq=1 ttl=64 time=0.064 ms

 --- ::1 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.064/0.064/0.064/0.000 ms
     0.000 probe_libc:inet_pton:(7f26a88498a0))
	   __GI___inet_pton (/usr/lib64/libc-2.26.so)
	   getaddrinfo (/usr/lib64/libc-2.26.so)
	   main (/usr/bin/ping)
[root@f27 perf]#

However when I add the global --call-graph dwarf to the command
line I get the exactly same result which is wrong:

[root@f27 perf]# ./perf trace --no-syscalls --call-graph dwarf
		-e probe_libc:inet_pton/max-stack=3/ ping -6 -c 1 ::1

My understanding is that the '--call-graph dwarf' option should be global
and applies to all events. However it is ignored.
The issue is buried in function apply_config_terms() called from
trace__run()
 --> perf_evlist__config()
     --> perf_evsel__config()
           --> apply_config_terms()

This function iterates over all additional terms and handles the
/max-stack=<xx>/ case. However if there is no additional call-graph=dwarf
setting as in -e probe_libc:inet_pton/max-stack=3,call-graph=dwarf/
the function defaults to "fp" regardless of the value of --call-graph
global command line flag.

So when I add ,call-graph=dwarf to the event I get a good result:

[root@f27 perf]# ./perf trace --no-syscalls
	-e probe_libc:inet_pton/max-stack=3,call-graph=dwarf/
	ping -6 -c 1 ::1
PING ::1(::1) 56 data bytes
64 bytes from ::1: icmp_seq=1 ttl=64 time=0.084 ms

 --- ::1 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.084/0.084/0.084/0.000 ms
     0.000 probe_libc:inet_pton:(7f9372a988a0))
	   __inet_pton (inlined)
	   gaih_inet.constprop.7 (/usr/lib64/libc-2.26.so)
	   __GI_getaddrinfo (inlined)
	   main (/usr/bin/ping)
	   __libc_start_main (/usr/lib64/libc-2.26.so)
	   _start (/usr/bin/ping)
[root@f27 perf]#

In fact the looks pretty similar to the output on s390x which only has
dwarf support.
Ok the max-stack is not observed, but this time the global --max-stack <x>
comes to rescue and limits the output:

[root@f27 perf]# ./perf trace --no-syscalls --max-stack 4
	-e probe_libc:inet_pton/max-stack=3,call-graph=dwarf/ ping -6 -c 1 ::1
PING ::1(::1) 56 data bytes
64 bytes from ::1: icmp_seq=1 ttl=64 time=0.061 ms

 --- ::1 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.061/0.061/0.061/0.000 ms
     0.000 probe_libc:inet_pton:(7fc9f6c188a0))
	   __inet_pton (inlined)
	   gaih_inet.constprop.7 (/usr/lib64/libc-2.26.so)
	   __GI_getaddrinfo (inlined)
	   main (/usr/bin/ping)
[root@f27 perf]#

This patch applies the global --call-graph <xxx> setting to the individual
events.

With this patch applied:
[root@f27 perf]# ./perf trace --no-syscalls --call-graph dwarf
	-e probe_libc:inet_pton/max-stack=3/ ping -6 -c 1 ::1
PING ::1(::1) 56 data bytes
64 bytes from ::1: icmp_seq=1 ttl=64 time=0.066 ms

 --- ::1 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.066/0.066/0.066/0.000 ms
     0.000 probe_libc:inet_pton:(7fe74f8358a0))
	   __inet_pton (inlined)
	   gaih_inet.constprop.7 (/usr/lib64/libc-2.26.so)
	   __GI_getaddrinfo (inlined)
	   main (/usr/bin/ping)
	   __libc_start_main (/usr/lib64/libc-2.26.so)
	   _start (/usr/bin/ping)
[root@f27 perf]#

When using dwarf the /max-stack=<x>/ is not observed.
The reason is that for call-graph=fp the kernel stack unwinder
is used and it stops after unwinding x entries.
For dwarf the complete stack data is returned and the unwinding
is done by perf itself and it uses --max-stack <x>
command line flag.

Signed-off-by: Thomas Richter <tmricht@...ux.vnet.ibm.com>
Reviewed-by: Hendrick Brueckner <brueckner@...ux.vnet.ibm.com>
---
 tools/perf/util/evsel.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
index d5fbcf8c7aa7..d1c25aafb821 100644
--- a/tools/perf/util/evsel.c
+++ b/tools/perf/util/evsel.c
@@ -788,8 +788,24 @@ static void apply_config_terms(struct perf_evsel *evsel,
 	if ((callgraph_buf != NULL) || (dump_size > 0) || max_stack) {
 		if (max_stack) {
 			param.max_stack = max_stack;
-			if (callgraph_buf == NULL)
-				callgraph_buf = "fp";
+			if (callgraph_buf == NULL) {
+				switch (callchain_param.record_mode) {
+				case CALLCHAIN_DWARF:
+					callgraph_buf = "dwarf";
+					break;
+				case CALLCHAIN_NONE:
+				case CALLCHAIN_FP:
+					callgraph_buf = "fp";
+					break;
+				case CALLCHAIN_LBR:
+					callgraph_buf = "lbr";
+					break;
+				default:
+				case CALLCHAIN_MAX:
+					callgraph_buf = "no";
+					break;
+				}
+			}
 		}
 
 		/* parse callgraph parameters */
-- 
2.14.3