Stack trace using ftrace

[Jungseok Lee]

Greetings,

As playing with ftrace to gather kernel stack data on ARM64 kernel,
I've found out data from /sys/kernel/debug/tracing/stack_trace looks odd.

One of example using 4.1 kernel + dragonboard410c is as follows.

        Depth    Size   Location    (50 entries)
        -----    ----   --------
  0)     5256       0   notifier_call_chain+0x30/0x94
  1)     5256       0   ftrace_call+0x0/0x4
  2)     5256       0   notifier_call_chain+0x2c/0x94
  3)     5256       0   raw_notifier_call_chain+0x34/0x44
  4)     5256       0   timekeeping_update.constprop.9+0xb8/0x114
  5)     5256       0   update_wall_time+0x408/0x6dc
  6)     5256       0   tick_do_update_jiffies64+0xd8/0x154
  7)     5256       0   tick_sched_do_timer+0x50/0x60
  8)     5256       0   tick_sched_timer+0x34/0x90
  9)     5256       0   __run_hrtimer+0x60/0x258
 10)     5256       0   hrtimer_interrupt+0xe8/0x260
 11)     5256       0   arch_timer_handler_virt+0x38/0x48
 12)     5256       0   handle_percpu_devid_irq+0x84/0x188
 13)     5256       0   generic_handle_irq+0x38/0x54
 14)     5256       0   __handle_domain_irq+0x68/0xbc
 15)     5256       0   gic_handle_irq+0x38/0x88
 16)     5256       0   el1_irq+0x64/0xd8
 17)     5256       0   kmem_cache_alloc+0x258/0x294
 18)     5256       0   __alloc_skb+0x48/0x180
 19)     5256       0   alloc_skb_with_frags+0x74/0x234
 20)     5256       0   sock_alloc_send_pskb+0x1d0/0x294
 21)     5256       0   sock_alloc_send_skb+0x44/0x54
 22)     5256       0   __ip_append_data.isra.40+0x78c/0xb48
 23)     5256       0   ip_append_data.part.42+0x98/0xe8
 24)     5256       0   ip_append_data+0x68/0x7c
 25)     5256       0   icmp_push_reply+0x7c/0x144
 26)     5256       0   icmp_send+0x3c0/0x3c8
 27)     5256       0   __udp4_lib_rcv+0x5b8/0x684
 28)     5256       0   udp_rcv+0x2c/0x3c
 29)     5256       0   ip_local_deliver+0xa0/0x224
 30)     5256       0   ip_rcv+0x360/0x57c
 31)     5256       0   __netif_receive_skb_core+0x4d0/0x80c
 32)     5256       0   __netif_receive_skb+0x24/0x84
 33)     5256       0   process_backlog+0x9c/0x15c
 34)     5256       0   net_rx_action+0x1ec/0x32c
 35)     5256       0   __do_softirq+0x114/0x2f0
 36)     5256       0   do_softirq+0x60/0x68
 37)     5256       0   __local_bh_enable_ip+0xb0/0xd4
 38)     5256       0   ip_finish_output+0x1f4/0xabc
 39)     5256       0   ip_output+0xf0/0x120
 40)     5256       0   ip_local_out_sk+0x44/0x54
 41)     5256       0   ip_send_skb+0x24/0xbc
 42)     5256       0   udp_send_skb+0x1b4/0x2f4
 43)     5256       0   udp_sendmsg+0x2a8/0x7a0
 44)     5256       0   inet_sendmsg+0xa0/0xd0
 45)     5256       0   sock_sendmsg+0x30/0x78
 46)     5256       0   ___sys_sendmsg+0x15c/0x26c
 47)     5256       0   __sys_sendmmsg+0x94/0x180
 48)     5256       0   SyS_sendmmsg+0x38/0x54
 49)     5256    5256   el0_svc_naked+0x20/0x28

I think size *0* does not make sense.
It does not match with Documentation/trace/ftrace.txt.

Am I missing something?
I attach additional information which might be helpful.

1) Kernel configuration
CONFIG_NOP_TRACER=y
CONFIG_HAVE_FUNCTION_TRACER=y
CONFIG_HAVE_FUNCTION_GRAPH_TRACER=y
CONFIG_HAVE_DYNAMIC_FTRACE=y
CONFIG_HAVE_FTRACE_MCOUNT_RECORD=y
CONFIG_HAVE_SYSCALL_TRACEPOINTS=y
CONFIG_HAVE_C_RECORDMCOUNT=y
CONFIG_TRACE_CLOCK=y
CONFIG_RING_BUFFER=y
CONFIG_EVENT_TRACING=y
CONFIG_CONTEXT_SWITCH_TRACER=y
CONFIG_TRACING=y
CONFIG_GENERIC_TRACER=y
CONFIG_TRACING_SUPPORT=y
CONFIG_FTRACE=y
CONFIG_FUNCTION_TRACER=y
CONFIG_FUNCTION_GRAPH_TRACER=y
# CONFIG_IRQSOFF_TRACER is not set
# CONFIG_PREEMPT_TRACER is not set
# CONFIG_SCHED_TRACER is not set
# CONFIG_FTRACE_SYSCALLS is not set
# CONFIG_TRACER_SNAPSHOT is not set
CONFIG_BRANCH_PROFILE_NONE=y
# CONFIG_PROFILE_ANNOTATED_BRANCHES is not set
# CONFIG_PROFILE_ALL_BRANCHES is not set
CONFIG_STACK_TRACER=y
# CONFIG_BLK_DEV_IO_TRACE is not set
# CONFIG_PROBE_EVENTS is not set
CONFIG_DYNAMIC_FTRACE=y
# CONFIG_FUNCTION_PROFILER is not set
CONFIG_FTRACE_MCOUNT_RECORD=y
CONFIG_FTRACE_SELFTEST=y
CONFIG_FTRACE_STARTUP_TEST=y

2) Ftrace selftest result
[  110.791609] Testing all events: OK
[  110.999956] Testing ftrace filter: OK

Thanks for any advice or help in advance!

Best Regards
Jungseok Lee