Update README#24
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Signed-off-by: Salendarsingh Gaud <sgaud@qti.qualcomm.com>
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Jun 24, 2025
When setting the funcgraph-args option when function graph tracer is net
enabled, it incorrectly enables it. Worse, it unregisters itself when it
was never registered. Then when it gets enabled again, it will register
itself a second time causing a WARNing.
~# echo 1 > /sys/kernel/tracing/options/funcgraph-args
~# head -20 /sys/kernel/tracing/trace
# tracer: nop
#
# entries-in-buffer/entries-written: 813/26317372 #P:8
#
# _-----=> irqs-off/BH-disabled
# / _----=> need-resched
# | / _---=> hardirq/softirq
# || / _--=> preempt-depth
# ||| / _-=> migrate-disable
# |||| / delay
# TASK-PID CPU# ||||| TIMESTAMP FUNCTION
# | | | ||||| | |
<idle>-0 [007] d..4. 358.966010: 7) 1.692 us | fetch_next_timer_interrupt(basej=4294981640, basem=357956000000, base_local=0xffff88823c3ae040, base_global=0xffff88823c3af300, tevt=0xffff888100e47cb8);
<idle>-0 [007] d..4. 358.966012: 7) | tmigr_cpu_deactivate(nextexp=357988000000) {
<idle>-0 [007] d..4. 358.966013: 7) | _raw_spin_lock(lock=0xffff88823c3b2320) {
<idle>-0 [007] d..4. 358.966014: 7) 0.981 us | preempt_count_add(val=1);
<idle>-0 [007] d..5. 358.966017: 7) 1.058 us | do_raw_spin_lock(lock=0xffff88823c3b2320);
<idle>-0 [007] d..4. 358.966019: 7) 5.824 us | }
<idle>-0 [007] d..5. 358.966021: 7) | tmigr_inactive_up(group=0xffff888100cb9000, child=0x0, data=0xffff888100e47bc0) {
<idle>-0 [007] d..5. 358.966022: 7) | tmigr_update_events(group=0xffff888100cb9000, child=0x0, data=0xffff888100e47bc0) {
Notice the "tracer: nop" at the top there. The current tracer is the "nop"
tracer, but the content is obviously the function graph tracer.
Enabling function graph tracing will cause it to register again and
trigger a warning in the accounting:
~# echo function_graph > /sys/kernel/tracing/current_tracer
-bash: echo: write error: Device or resource busy
With the dmesg of:
------------[ cut here ]------------
WARNING: CPU: 7 PID: 1095 at kernel/trace/ftrace.c:3509 ftrace_startup_subops+0xc1e/0x1000
Modules linked in: kvm_intel kvm irqbypass
CPU: 7 UID: 0 PID: 1095 Comm: bash Not tainted 6.16.0-rc2-test-00006-gea03de4105d3 #24 PREEMPT
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
RIP: 0010:ftrace_startup_subops+0xc1e/0x1000
Code: 48 b8 22 01 00 00 00 00 ad de 49 89 84 24 88 01 00 00 8b 44 24 08 89 04 24 e9 c3 f7 ff ff c7 04 24 ed ff ff ff e9 b7 f7 ff ff <0f> 0b c7 04 24 f0 ff ff ff e9 a9 f7 ff ff c7 04 24 f4 ff ff ff e9
RSP: 0018:ffff888133cff948 EFLAGS: 00010202
RAX: 0000000000000001 RBX: 1ffff1102679ff31 RCX: 0000000000000000
RDX: 1ffffffff0b27a60 RSI: ffffffff8593d2f0 RDI: ffffffff85941140
RBP: 00000000000c2041 R08: ffffffffffffffff R09: ffffed1020240221
R10: ffff88810120110f R11: ffffed1020240214 R12: ffffffff8593d2f0
R13: ffffffff8593d300 R14: ffffffff85941140 R15: ffffffff85631100
FS: 00007f7ec6f28740(0000) GS:ffff8882b5251000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f7ec6f181c0 CR3: 000000012f1d0005 CR4: 0000000000172ef0
Call Trace:
<TASK>
? __pfx_ftrace_startup_subops+0x10/0x10
? find_held_lock+0x2b/0x80
? ftrace_stub_direct_tramp+0x10/0x10
? ftrace_stub_direct_tramp+0x10/0x10
? trace_preempt_on+0xd0/0x110
? __pfx_trace_graph_entry_args+0x10/0x10
register_ftrace_graph+0x4d2/0x1020
? tracing_reset_online_cpus+0x14b/0x1e0
? __pfx_register_ftrace_graph+0x10/0x10
? ring_buffer_record_enable+0x16/0x20
? tracing_reset_online_cpus+0x153/0x1e0
? __pfx_tracing_reset_online_cpus+0x10/0x10
? __pfx_trace_graph_return+0x10/0x10
graph_trace_init+0xfd/0x160
tracing_set_tracer+0x500/0xa80
? __pfx_tracing_set_tracer+0x10/0x10
? lock_release+0x181/0x2d0
? _copy_from_user+0x26/0xa0
tracing_set_trace_write+0x132/0x1e0
? __pfx_tracing_set_trace_write+0x10/0x10
? ftrace_graph_func+0xcc/0x140
? ftrace_stub_direct_tramp+0x10/0x10
? ftrace_stub_direct_tramp+0x10/0x10
? ftrace_stub_direct_tramp+0x10/0x10
vfs_write+0x1d0/0xe90
? __pfx_vfs_write+0x10/0x10
Have the setting of the funcgraph-args check if function_graph tracer is
the current tracer of the instance, and if not, do nothing, as there's
nothing to do (the option is checked when function_graph tracing starts).
Cc: stable@vger.kernel.org
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/20250618073801.057ea636@gandalf.local.home
Fixes: c7a60a7 ("ftrace: Have funcgraph-args take affect during tracing")
Closes: https://lore.kernel.org/all/4ab1a7bdd0174ab09c7b0d68cdbff9a4@huawei.com/
Reported-by: Changbin Du <changbin.du@huawei.com>
Tested-by: Changbin Du <changbin.du@huawei.com>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
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