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Message-ID: <ad0745b53e5046b79378bdc12ddfdd51@huawei.com>
Date: Wed, 18 Jun 2025 02:40:21 +0000
From: duchangbin <changbin.du@...wei.com>
To: Steven Rostedt <rostedt@...dmis.org>
CC: LKML <linux-kernel@...r.kernel.org>, Linux Trace Kernel
<linux-trace-kernel@...r.kernel.org>, Masami Hiramatsu <mhiramat@...nel.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Mark Rutland
<mark.rutland@....com>, duchangbin <changbin.du@...wei.com>
Subject: Re: [PATCH] fgraph: Do not enable function_graph tracer when setting
funcgraph-args
Hi, Steven,
On Tue, Jun 17, 2025 at 12:08:30PM -0400, Steven Rostedt wrote:
> From: Steven Rostedt <rostedt@...dmis.org>
>
> 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@...r.kernel.org
> Fixes: c7a60a733c373 ("ftrace: Have funcgraph-args take affect during tracing")
> Reported-by: duchangbin <changbin.du@...wei.com>
> Closes: https://lore.kernel.org/all/4ab1a7bdd0174ab09c7b0d68cdbff9a4@huawei.com/
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> ---
> kernel/trace/trace_functions_graph.c | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> diff --git a/kernel/trace/trace_functions_graph.c b/kernel/trace/trace_functions_graph.c
> index 9234e2c39abf..b6211c304c7f 100644
> --- a/kernel/trace/trace_functions_graph.c
> +++ b/kernel/trace/trace_functions_graph.c
> @@ -455,10 +455,16 @@ static int graph_trace_init(struct trace_array *tr)
> return 0;
> }
>
> +static struct tracer graph_trace;
> +
> static int ftrace_graph_trace_args(struct trace_array *tr, int set)
> {
> trace_func_graph_ent_t entry;
>
> + /* Do nothing if the current tracer is no this tracer */
typo: no -> not
> + if (tr->current_trace != &graph_trace)
> + return 0;
> +
This can fix the issue. But how are tr->current_trace and &graph_trace linked?
When are they equal?
> if (set)
> entry = trace_graph_entry_args;
> else
> --
> 2.47.2
>
>
--
Cheers,
Changbin Du
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